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XJ-S Cooling System

// JagWeb // XJ-S Help // Contents //

 

XJ-S Cooling System

Overheating

Believe it or not, the XJ-S does not overheat when it's running right -- and this from an owner who lives in Florida!

Probably the most notorious cause of damage to the Jaguar V-12 is overheating damage. In any aluminum-block engine, severe overheating can result in a warped block or warped heads, which in turn normally call for an engine replacement.

Do not continue to drive when the car is overheating. If no other options are available, drive it short distances at a time, shutting it off and allowing it to cool before starting again.

Dropped Valve Seats

In the Jaguar V-12, a more common symptom of an overheated engine is a dropped valve seat. Since the heads are aluminum, the valve seats are steel rings that are pressed into the aluminum. Since steel and aluminum have different coefficients of thermal expansion, overheating will cause a loose fit and the seat can just fall out. After that, it holds the valve part way open and bangs around in there.

Valve and Piston Damage

Amazingly, reports of broken valves are rare; more often, the owner who continues driving despite the annoying ticking under the hood allows the valve to beat the seat to pieces, which in turn bang up the piston, the other valve, and the head.

Troubleshooting

If yours tends to overheat, don't ignore it; overheating can cause warping in an aluminum block engine, as well as dropped valve seats. Check the following, all of which are described further in this book:

  1. Check the radiator for blockage or sludge. Crud in the system will cause overheating under all conditions, but usually more at speed than at idle.
  2. Suspend the thermostats in a pan of water on the stove and bring them to a boil. Do not let them contact the bottom of the pan. If the thermostats are not visibly wide open by the time the water boils, replace them. Their usual failure mode is to open only slightly, which will cause overheating under power more than at idle.
  3. Retarded timing will cause overheating under all operating conditions. See the sections on ignition timing.
  4. A seized centrifugal advance mechanism can cause the timing to be retarded at speed while correct near idle, so the car would overheat more while driving.
  5. The oil cooler and A/C condenser coil, both of which are in front of the radiator, have a relatively coarse fin pattern. The radiator has a very fine fin pattern. Dirt goes right through the oil cooler and A/C coil and plugs up the radiator. Remove the spoiler, the bottom panel, and then remove the piece of bodywork directly below the oil cooler. This will provide a limited access to the lower front of the radiator.
  6. A bad fan clutch causes overheating ONLY in stop-and-go traffic or other conditions where motion of the car doesn't provide enough air flow. The stock fan clutch is a thermostatic type, meaning it engages more to blow more air when the air coming through the radiator is hot. When the engine is hot, rev the engine to 2,000 rpm or so. The fan should be blowing hard. If it is just blowing gently, replace the clutch.
  7. The fan shroud flaps should be intact and free to flap as intended. If they are missing, overheating when stopped and idling is likely.
  8. Front spoiler -- it must be there, and it must be properly mounted.

Leak Sealers

Rumors persist that some Jaguar repair manuals specify the use of leak sealers in all cooling systems, whether they have ever leaked or not. Many Jaguar mechanics appreciate this, since a high percentage of their work is traced to this stuff -- or similar products -- plugging up the bottom half of the radiator and contributing to Jaguar's reputation for overheating problems. In theory, leak sealers will not solidify until they come in contact with air; however, there is always a little air INSIDE a cooling system, and in the case of the XJ-S there is apparently enough to cause trouble.

Please, do not use any leak-sealing substance within the V-12 cooling system. If the system leaks, fix it.

If the car is more than a few years old and having overheating problems, it's not a bad idea to just take the radiator to a shop and have it boiled or rodded out to restore its effectiveness -- especially if you're not the original owner, and the previous owner may have been using leak sealers.

Radiator

The XJ-S radiator is a side-flow radiator divided into a top third and a bottom two-thirds. The coolant coming from the left bank, via the left side thermostat, enters at the top left and flows left-to-right through the top third (it cannot go directly down to the outlet because there is an internal baffle in the radiator). Then the coolant from the right bank comes in, mixing with this already-cooled fluid. The mixture then flows right-to-left through the bottom two thirds of the core and back to the pump.

Because the mixed fluid goes through the pump and to both banks, and the thermostats divert flow rather than stopping it, both banks are always seeing the same coolant temperature and the same flow rate. One thermostat failing would not cause unequal cooling; rather, it would have an equal effect on both banks, and the effect would be less serious than a thermostat failure in a single-thermostat system. Although it is probable two thermostats were used to reduce piping, they do provide some redundancy.

John's Cars offers "super-duty" radiators to fit the XJ-S, including a 5-row unit for the pre-H.E. car. The additional cooling is probably unnecessary unless you're towing a heavy trailer up mountain roads, but if your radiator needs replacing anyway, it's worth considering.

Radiator Drain

Up until somewhere around 1988, the XJ-S was fitted with one of the most obnoxiously overdesigned radiator drain cocks in automotive history. A metal drain valve at the bottom right corner of the radiator, operated by a lever that extended up to just below the upper hose fitting. Evidently, the image of luxury is supposed to include being able to drain your coolant while wearing a tuxedo. For all this effort and expense, Jaguar couldn't bring themselves to provide an outlet out the bottom of the car, so opening the drain valve causes coolant to pour all over the structures in the area and dribble out wherever the catch pan isn't. It's not too difficult to tie a piece of hose to the valve, however, to make the job of changing coolant less messy.

Somewhere around 1988, Jaguar went from the overdesigned drain cock to no drain at all. On later cars, it is necessary to disconnect the lower radiator hose to drain the coolant. Wearing a tux is not recommended.

Even if you have the earlier system with the drain cock, there may be wisdom in disconnecting the hose to drain anyway -- or at least to flush. Plugged up radiators are a staple of Jaguar ownership, and perhaps opening the big hole in the bottom will flush out more junk than using the tiny drain. Since the drain cock is on one side and the bottom radiator hose is on the other, perhaps the best policy is to use both openings to get as much crud out as possible. Use the drain cock to neatly drain the coolant into a container, and then remove the radiator hose to flush water through.

Removal

Both the official Jaguar manual and the Haynes manual state that removing the radiator requires discharging the air conditioner freon circuit. They lie. In fact, as Jim Isbell reports,

In the Haynes manual there are 21 steps under section #21 that describe the removal. Steps 1, 3, 5, 9, 10, and 11 are all unnecessary.

Most are merely extra work, but discharging the freon is a waste of serious money.

Both manuals also provide the same illustration which shows the radiator sitting on top of the oil cooler. This may be the case for cars that weren't equipped with air conditioning, but for the rest of us the oil cooler is in front of the radiator, and the condenser is on top of it.

Forget the manuals and just dive in. The radiator comes out vertically upward, leaving the A/C condenser and oil cooler in place. Drain the coolant and disconnect the hoses, remove the air purge system tubing from the top of the rail over the radiator, unbolt the A/C dryer from the rail and leave it hanging, unbolt the fan shroud, and remove the rail. Either remove the hood, or simply remove the grille, disconnect the struts, and tilt it forward until it rests on the bumper. Disconnect the hoses from the transmission cooler and the wire from the coolant level sensor (early models only) and whatever other little things are hanging on, and pull the radiator out straight up.

Radiator Mount Bushings

The radiator is mounted on four rubber bushings, two at the bottom (C43577) and two at the top (C38333). If you wish, you may replace these by visiting your local discount auto parts store and looking through the selection of PCV valve grommets that are usually on a display rack in bubble packs. There is one intended for a Toyota that will serve quite nicely in both positions. It doesn't have as large a weight-bearing surface as the Jaguar originals, but it's not made of British rubber either.

You might wonder why the radiator, which has no moving parts, needs to be mounted in rubber. As any good mechanical engineer will tell you, taking any large mass and mounting it with damping material will go a long way toward reducing vibrations caused by OTHER sources. Rubber mounts render the water-filled radiator a large vibration absorber.

H.E. Radiator Caps

The XJ-S H.E. has two radiator caps, but only the one on the header tank (left side of the engine compartment) is actually meant to operate as a conventional radiator cap; namely, to control the pressure in the system. The one on the bypass pipe (at the top right of the engine) is really just a place to add coolant, using a standard radiator cap because they're available. If one or both of the caps go bad, they may be replaced with standard coolant-recovery radiator caps. However, you must make sure that the cap on the bypass pipe has a higher pressure rating than the one on the header tank. This is to prevent the bypass pipe cap from actually relieving any pressure.

Header Tank

The header tank is susceptible to rust perforation. The good news is that the later and cheaper tank fits better. The filler is curved to clear the air cleaner that is set forward to clear the ABS unit. The newer tank is about half the price of the older one and has studs instead of bolts to mount it.

Thermostats

The Jaguar engine requires little wiggle pins in the thermostats to bleed air out of the system. If you buy aftermarket thermostats and they have no wiggle pins, drill a 1/8" hole in the flange. Install thermostats with the wiggle pins or holes at the top.

NOTE: Don't try to operate this engine without thermostats. The thermostats must be in place to prevent the coolant from taking a short circuit and bypassing the radiator. Incidentally, it's not really a good idea to operate any vehicle without a thermostat.

Thermostat Seats

Jan Wikström reports that:

The seat of the thermostat bypass (supposed to close when the thermostat opens) in the thermostat housings is subject to erosion. Inspect and fit a bronze seat if necessary.

It's not known how common this erosion is, but if significant erosion is present it will definitely reduce cooling efficiency. Jan made a bronze seat by machining a pipe fitting and then machining a suitable recess in the thermostat housing to press it into. Other options would include building up with weld material and remachining or simply replacing the thermostat housings.

Air Bleed/Coolant Recovery System

The diagrams in the manuals show the return line from the heater to the bottom hose fitting on the radiator to go through a tee into the bottom of the header tank. The XJ-S H.E. does not have such a tee; the heater return line goes uninterrupted to the fitting on the radiator. Flow through the header tank (necessary to cause air to bleed into the header tank) is achieved by a line from the tank to the bypass pipe, directly at the water pump inlet.

Normally, each time an engine heats up and cools down, the expansion and contraction draws water back from a recovery tank through a line into the cooling system. However, the coolant return line from the pressure cap to the atmospheric tank behind the left front wheel is quite long. Since the expansion/contraction of an engine only moves a little water at a time, it requires several thermal cycles to bleed the air out of the hose (unless you overheat and blow steam). Each time you open the pressure cap, you allow the water to drain into the atmospheric tank and the line to fill with air. If you keep opening the pressure cap to check the level, it will never get a chance to work properly.

Coolant Overflow Tank

The overflow container is located directly behind the left front wheel, within the bodywork. To get to it, remove the left front wheel and remove the sheet metal panel at the rear of the wheel well.

The vent on the overflow container is somewhat unusual. On most cars, the container is within the engine compartment, and when it overflows (like, when your car is overheating big time), the fluid coming out the vent just dribbles out onto the ground. In the XJ-S, however, such leakage would result in antifreeze throughout the bodywork -- unacceptable. So, the container has a vent line that is routed out the bottom of the car. For this vent to work as intended, the container must be airtight. The design is really lousy, however, and it is likely to leak throughout the bodywork when overheating anyway.

Antifreeze

Don't operate the Jaguar or any car without antifreeze in the cooling system. The name "antifreeze" is an unfortunate misnomer, and pure water is a totally unacceptable coolant -- even in Hawaii. Antifreeze not only prevents freezing, it also retards corrosion and crud buildup, helps prevent boilover, and most importantly, serves as a water pump seal conditioner. Running pure water will result in early water pump seal failure. Also, replace the antifreeze annually, because the inhibitors in it wear out and it becomes corrosive.

Scott Fisher sends the following wisdom:

In the context of the automotive cooling system ethylene glycol is not an anticorrosive agent; it is in fact corrosive. To offset this fact, manufacturers add anticorrosives (inhibitors) to the glycol. These preparations, while in good condition, perform well in both minimizing corrosion and preventing freezing of the coolant. However, over the life of the coolant the anticorrosion properties of the inhibitors are depleted.

Water aids corrosion in three main ways: 1) bringing free oxygen in close contact with the metals so that corrosion (oxidation) can occur. 2) Water is conductive. Once water has been flowing in your cooling system for some time, its conductivity will rise as it picks up metal ions. The water may serve to promote electrical activity which may erode metals by galvanic action. 3) Some of the metal ions in the water may also react directly with the metal surfaces.

Apart from supporting the above three processes, ethylene glycol has the added unfortunate property that it oxidizes through several stages to oxalic acid. The products of ethylene glycol oxidation by oxygen and subsequent reactions include: aldehydes, carboxylic acid, nitric acid, glycolic acid, glyoxylic acid, oxalic acid, formaldehyde and formic acid. Most of the series of oxidation products to and including oxalic acid are directly corrosive to metals. Added to this, oxalic acid is highly toxic.

To combat the above acids and other corrosion activity, antioxidants and alkaline formulations are added to the glycol mix. These include many compounds which are used in cooling systems where antifreeze properties are not required and include primary, secondary and tertiary amines; organic and inorganic phosphates, silicates cresols and other phenolic substances; a wide variety of sulfur compounds; soaps; alkali metal salts; and borates.

These inhibitors slow down the corrosion process caused by the glycol and the water. They may coat the metal surfaces and prevent corrosion by passivation. Passivation is the process where the a protective film forms on the metal which prevents further contact with the solution. Unfortunately, in all coolant preparations (with or without glycol) the inhibitor system (during engine operation) is being continuously depleted in the performance of these actions. For this reason, proper cooling system maintenance is critical.

One aspect of cooling system maintenance that we can all easily follow is to minimize "aeration" of your coolant. Aerating accelerates the uptake of free oxygen from the atmosphere. As free oxygen is one of the essential ingredients for corrosion, the importance of minimizing it's uptake is clear. To this end you should make sure all your hoses are in good condition and clamped tightly. "Closed systems", where an expansion tank and recovery system closed to the atmosphere is used, also help in this regard.

If you overheat (boil) glycol-based coolants they must be replaced immediately as this accelerates the oxidization process of the glycol to acids.

Coolant Level Sensor

On early cars, it's at the front right side of the radiator, where it's very hard to find unless you have the hood off. Sometime in the early 1980's, it was relocated to the expansion tank, where it's a lot easier to get to.

Coolant Connecting Pipe

On top of each head there is a coolant pipe, C42595, that connects a fitting at the rear of the head to the thermostat housing at the front. It is a straight steel pipe with a small shoulder at each end to hold itself and the sealing bush in place.

Of course, being steel it is subject to rust and corrosion. But if you feel like it, there is an easy way to make a nice replacement. Drop by an air conditioning repair shop or supply house, and pick up a length of 5/8" copper tubing as well as a fitting or two. Note: air conditioning systems typically use the odd eighths sizes of tubing to differentiate them from water piping.

Cut the tubing the same length as the original. Cut the fittings to make rings and use a propane torch to solder them onto the tubing to form shoulders. If you take the effort to polish it up a little, having the copper tube across the top of each head looks really snazzy. Since this tube is mounted in rubber at both ends and has no direct contact with aluminum parts, galvanic corrosion is no more of a concern than in the copper radiator.

Radiator Hoses

The hoses in the Jag are not significantly different than any other car. For locations where the shape of the hose is not too critical, go to the local parts shop and ask to look over their selection of molded hoses. Find one with the right diameter, and with a section that will fit where you want it to. It is helpful to have the car there, and a shop that will let you take the hoses out to the car and look at them. Buy the hose and cut it to the length and shape you need and discard the rest. This method is usually cheaper than either buying the Jag hoses or using flex hose, and is very aesthetically pleasing.

Note: You will probably not find a hose with the exact same shape as the original. All that is important is that the two ends will connect properly, and that the hose doesn't run into anything in between. Also keep in mind that the engine moves around a little on its mounts, while the radiator stands still; a little room for flexibility in the radiator hoses is helpful.

Peter Smith advises that the top left hose is the same shape as (in Australia) a Holden 186 or Mazda 929 late 1980s.

Water Pump

On the top of the water pump is a setscrew with a locknut on it. This setscrew is to prevent the outer race from rotating in the housing, and either inserts into a hole in the bearing or tightens onto a flat.

Lubrication

If it inserts into a hole, you can remove the setscrew and screw in a zerk fitting (available at any hardware or auto parts store -- yes, it's even the right thread), and then you can grease the bearings with a grease gun. Be sure to reinstall the setscrew when you're done.

Removal/Crossover Pipe Installation

The repair manuals indicate that the crank pulley must be removed to remove the water pump. However, according to Jim Isbell,

The water pump will come out and go back in without removing the crank pulley. But the pump will not go back on with the crossover pipe connected if the pulley is still on as you have to snake it in over the pulley and the crossover pipe would be a big impediment.

So having said that and realizing that you are now going to replace the crossover pipe with the water pump already installed, there is only one way:

Lubricate all three of the pipe connections (on the two thermostat housings and the water pump) liberally with 3M water hose sealer. Then lubricate the three matching pipe ends on the crossover pipe with the 3M stuff. Now slip the new, cut to the proper length, hoses onto the crossover pipe. Put two clamps onto each of the three hoses, not tight, just enough so they don't slide off. The two clamps on the right side should be placed so the screw is on top when installed and slightly back. The clamp on the thermostat housing on the left side should be so that it is on the bottom and slightly back so there is enough room for a screwdriver angled down below the header tank. The two clamps on the water pump hose should be on the left side of the hose slightly back so the screwdriver is angled to the left side of the car. If you set the clamps up this way you will save a lot of grief later on.

Now push the hoses up onto the crossover pipe as far as they will go. Next place the center (water pump), hose onto the water pump tilting it and the crossover into position. It will take a little pushing and prying with a screwdriver, but it's not too bad and the hardest part is done.

Now, pull all the three hoses into approximately the correct position and lightly tighten the clamps. Now make sure the small pipe on the top of the crossover is clear of the big bolt on the block so that the hose to the overflow tank can be put on without being in a position to rub a hole in it. Tighten all six clamps and you are done.

Pump Rebuild Parts

The Jag water pump seal is an industrial standard; it can be found in any industrial equipment supply store, such as Grainger, as a type 68 shaft seal.

The bearing is also a fairly standard item, similar to those used in many common water pumps. Finding a bearing supplier may be difficult, however. The easiest way to get parts may be to purchase a rebuilt water pump for another type car from a discount auto parts store and remove the new bearing (and perhaps the seal as well) from it. After rebuilding your water pump, return the disassembled pump along with your old bearings for the core refund.

It is possible to order rebuild kits for this pump for reasonable prices.

Water Pump Rebuilding

Some, but apparently not all, water pumps have a single countersunk Phillips screw. According to Thomas Alberts, it is a common mistake to overtighten this screw, resulting in a fracture of the aluminum casting surrounding it. Apparently the casting was designed for a non-countersunk bolt, and adding the countersinking makes the metal too thin for serious tightening. If you wanna make sure the pump doesn't leak, use a good sealing compound, don't overtighten this bolt.

Fan Tip Rub

If the tips of the blades on your fan show signs of rubbing, the problem may be in the transmission mount. If you have a metal fan, you can hear it happen: you nail it from a standing start, and get a deafening screech from under the hood like all hell broke loose. A bad, or an incorrectly assembled, transmission mount allows the engine to pivot around on the engine mounts, causing the fan to rub.

A fan tip rub may also be caused by a failure of the left side motor mount. When stomping it in low gear, a lot of torque is applied to the drive shaft. According to Newton, this means that the same amount of torque is applied to the engine/transmission assembly in the opposite direction. The engine tries to tilt to the right, applying tension to the left motor mount that was really designed for compression only. If this rubber mount is torn, the entire engine will lift right up off its mount, causing the fan to rub.

Plastic Fan Cracking

According to issue 68 (June 1996) of Australian Jaguar Magazine:

Graham Cummins has recently found that the main plastic fan on the H.E. is prone to cracking and breaking up which can cause immense damage under the bonnet.

Any guesses as to how Mr. Cummins discovered this problem? Are YOU gonna find it the same way?

Jim Isbell ordered a replacement fan, and reports that the fan he was shipped did not look like the original.

It is black and has a flat center metal piece. The old one was white (now yellow) and the center piece was dished. The old one had a lower aspect ratio (short and fat) to the blades while the new one has the higher aspect ratio (long and skinny). The black flat one makes up for the "dishing" by offsetting the plastic instead.

With any luck at all, this means that Jaguar has recognized the problem and redesigned the fan, and this new one won't have cracking problems.

Fan Clutch

If it is determined that the fan clutch is a problem, there are several possible courses of action: the fan clutch can be replaced with a new one; it can be replaced with a substitute; or the entire belt-driven fan scheme can be chucked and electric fans installed. Your local parts shop is unlikely to carry a Jaguar fan clutch, so you will have to consult a Jaguar parts supplier (and spend some serious cash) to exercise the first option. The second and third options are discussed below.

Of course, you could bolt on a fixed or flex-blade fan and eliminate the fan clutch altogether. However, this results in slightly worse fuel economy and a considerable amount of noise (whine). Most would consider the noise unacceptable in a car such as the XJ-S.

The electric fan is probably the best overall solution, and will result in better fuel economy and more power. There will be slightly more noise at idle, but much less noise at higher rpm. It is unknown why Jaguar doesn't use this system to begin with; perhaps they don't like the sound an electric fan makes. Or, perhaps they want to minimize the use of Lucas parts.

Fan Clutch Substitution

The early XJ-S was fitted with an unusual fan clutch with a single nut on the front to hold it on. Later cars used a more conventional four-bolt mount on the front of the drive pulley. The following description generally applies to the later type.

There are several factors to consider to determine if an alternative fan clutch will fit:

  • The pilot hole in the center of the shaft must fit snugly around the stub in the center of the mounting flange. A hole too small won't go on, and a hole too big won't center the shaft properly.
  • The mounting bolt pattern must be workable. This generally isn't too critical, because they all seem to use four bolts and the aftermarket clutches provide radial slots to fit nearly any pattern.
  • The shaft must be of comparable length. Too long will press the fan into the back of the radiator. This dimension doesn't need to be exact, merely close enough to prevent interferences and keep the fan within the shroud for maximum efficiency.
  • The bolt pattern for mounting the fan to the clutch must match.

The auto parts houses normally have a cross-reference chart that lists the above dimensions for the fan clutches available. If you compare the clutch from your car to their chart, you can determine what can be used. Using this method, one clutch was found that would fit perfectly (from a GMC diesel), but it costs as much as the Jaguar original! It still may be the preferable choice, since it likely engages more forcefully than the stock Jaguar fan clutch, and would therefore provide a more reliable airflow.

If you find a clutch that is suitable except the pilot hole is too big, it would be a simple matter for a machine shop to fabricate a bushing to adapt. Some Jaguars come with such a bushing, EAC4382; perhaps this bushing can be used to adapt an aftermarket clutch. Its ID is 5/8 inch, OD is 3/4 inch. Michael Neal suggests you be sure the bushing is correct and necessary before pressing it into the clutch, since it can be difficult to remove.

If you find a clutch that is correct except for the fan mounting, you can replace the fan along with the fan clutch. A procedure is described below.

  1. Go to your local junkyard and buy a fan that fits the clutch, basic Ford or Chevy; preferably with unequally-spaced blades (reduces whine) and preferably with aluminum blades (easier to cut).
  2. Trim the tips of the new fan until it is the same diameter as the original. After cutting, round the corners and file the edges for safety.
  3. Bolt the sucker together and check for interferences. Noted possible interferences include the water pump pulley and an oil line across the bottom. The oil line may be bent and repositioned, or both interferences may be corrected by trimming or notching the blades. Obviously, trim all blades exactly the same way. It's helpful to cut out a cardboard template the shape of one blade and use it to mark each blade for trimming.
  4. Remove the fan from the clutch. Set the fan on razor blades centered on opposing bolt holes to check the balance. Trim a little metal from the blades on the heavy side until it balances. Rotate 90° and balance the other way. Make sure it balances both ways when completed.

A fan clutch that has the same pilot hole diameter, a slightly longer shaft length, and a different fan bolt pattern was found at AutoZone. It is made by Imperial, part number 215038. Since the offset of the mounting flange of the junkyard-purchased fan was less, the fan mounted in exactly the same place. The longer shaft also makes it a lot easier to get the mounting bolts in. It is believed this clutch, with a modified fan from a junkyard, will fit all the XJ-S's from 8/79 on.

Since the aftermarket clutch was designed to turn a 19" fan while the original turned a 17", the aftermarket clutch engages more forcefully than the original. This assembly will therefore make more noise (whoosh) than the original. But it will reliably keep the engine cool.

Some aftermarket fan clutches come with a lifetime warranty. But even if yours doesn't, you still can replace it much more easily next time, since you will already have a suitable fan and/or bushing.

If you have the earlier fan clutch that is retained by a single bolt through the front, replacement with an aftermarket unit will probably require much more effort. Perhaps replacing the bearing and pulley with the later versions will allow the scheme described above. If you develop a method, please write.

Electric Fan Substitution For Belt-Driven Fan

A better solution for fan clutch problems is to install a large electric fan (or two large electric fans) and remove the fan, clutch, mounting assembly (shaft & bearings), belt, and idler pulley altogether -- and the little stock electric fan, as well as the entire metal fan shroud assembly. Electric fans can sometimes be mounted on the front of the radiator, opening up considerable working room in the engine compartment. Keep in mind that the biggest electric fans that will fit should be used; a single 16" or dual 14" fans at least.

The electric fans can be controlled by either of several mechanisms. If the scheme used allows any possibility that the fans will be off when the car is idling, be sure to incorporate circuitry to run at least one fan whenever the air conditioner is operating (similar to the present wiring for the small stock electric fan).

The simplest method is to wire the fans to run whenever the ignition is on. This is wasteful, however, since the fans are only needed when the car is standing still or moving slowly. It also may cause the engine to run cold, or take too long to warm up, in cold weather.

The electric fans could also be connected to the existing electric fan control system, which automatically operates when the engine is hot or when the air conditioner is operating. Note that replacing the single tiny fan with a couple huge ones requires more electrical work than simply installing a larger fuse in the #1 position in the headlight fusebox; the stock wiring, even if it doesn't burn up, will provide too much resistance and the fans won't run as fast as they should. Some suitably heavy wiring should be run from the buss on the firewall to the new fans, using a separate relay for each fan.

Yet another possibility is to add another fan switch into the coolant system. Jaguar makes a suitable housing for a switch for the Mk III E-type, C34005, that fits into a radiator hose. With this arrangement, one of two fans can be connected to the stock wiring and the other to a separate switch. This would result in the two fans operating separately, and only one running when only a small amount of airflow is needed. One of two large fans running would be enough for the air conditioner operation, so there would be no need to add a similar circuit to the second fan. The dual circuit also provides a measure of redundancy, since one of the fans would provide some cooling in the event of the failure of the other circuit.

The fans could also be controlled via an air temperature sensor within the engine compartment. This method is often used by the aftermarket fans, providing a switch that mounts right on the fan housing. Note that if the fans are mounted in front of the radiator, the sensor must be moved to behind the radiator to work properly.

Yet another control system would be to provide a "paddle" switch that shuts the fans off when the airflow due to car motion is adequate. There don't appear to be any such items commercially available, but making one would not be difficult. A pivot with a paddle on one side and a tiny counterweight on the other, balanced to eliminate inertia effects, could be installed in the area behind the front grille. The arrangement could be rigged to operate a conventional microswitch with contact ratings sufficient to operate the fans directly. Using an ohmmeter, the car can be tested and the switch adjusted until it operates at a suitable speed, about 30 mph. This system would still operate the fans when the engine was cold, but would function properly with the air conditioner; the air conditioner requires airflow when running even when the engine is cold, but the motion of the car above 30 mph will be adequate; turning the fans off is OK.

The stock electric fan is discussed in the Electrical section.

Electric Cooling Fan Bootstrap Circuit

The electric fan includes a "bootstrap" circuit, so that if the coolant is hot enough for the fan to be running when the engine is shut off, the fan will continue to run until the coolant temperature switch shuts it off. Once off, the bootstrap circuit drops out and the fan cannot start again, no matter what.

What may be the worst heat problem on the V-12 Jaguars is what happens after the engine is shut off. The mechanical fan is stopped and the electric fan quits as soon as the coolant in the water pump inlet gets below its set point (just under boiling), and the heat from the engine rises and cooks the distributor, ignition amp, fuel rail, hoses, etc.

Other cars notably Japanese and other FWD 4-bangers have cooling fans that cycle on and off for quite a while after the engine is shut off. Clearly, they are wired so that the fan will run if the thermostat calls for it, regardless of whether the ignition is on or not. It is also evident that they COME on indicating that once they shut off the first time, they may still be needed again. Probably the engine heat soaking through the compartment.

Why did Jaguar provide this bootstrap circuit, rather than just wiring the fan to run when needed like the Jap cars? There would be two possible results if the bootstrap weren't in there: 1) The fan would never come on after shutting off the first time meaning that the bootstrap circuit was unnecessary; or 2) the fan WOULD come on after shutting down, which means it NEEDS to come on and the bootstrap circuit is contributing to the cooking of the engine parts!

The only plausible explanations: A) Jaguar was afraid the Lucas thermostat would fail in the on position and kill the battery; or B) they didn't feel that a fan kicking on and off in the parking lot was in keeping with the proper Jaguar image. Stuart Barnes adds C):

Many car alarms are voltage sensing and although they can cope with a voltage rise (to allow an electric fan to run and then stop) a lot of the aftermarket varieties can't cope with the voltage drop that would occur when the fan came back on.

For those of us who are more concerned with the life of our machines and don't have such alarm system concerns, it might be a good idea to rewire that circuit to run the fan whenever necessary. It's easy to do: On the left side of the engine compartment is a small blue box that looks like a relay, but it's the diode pack. Pull the LG wire off of terminal 4 and connect it to a 12V power supply. There is a 12V supply available at the solid brown wires at the headlight fusebox just a few inches forward of the diode pack.

Going one step further might involve adding an air temperature thermostat within the engine compartment preferably high and rearward, where the postshutdown cooking problems are the worst. This thermostat could be set much cooler than the one in the coolant and still wouldn't come into play while the car is moving or the enginedriven fan is circulating air. But if the air starts getting hotter after shutdown, it can come on even if the coolant in the water pump isn't that hot and flow some cool air through the engine compartment.

Fan Bearing

The fan on an XJ-S is mounted on a dedicated bearing instead of on the water pump as in most cars. The bearing appears to be similar to those found in several types of water pump, except the shaft on one end is too short to mount anything on. Perhaps a suitable water pump bearing can be found and the unused portion of shaft cut off (be careful not to get the shaft too hot and damage the seals!) Since finding the bearing itself may be hard, perhaps the most expedient procurement method would be to buy a suitable rebuilt pump, remove the bearing, throw the remains of the new pump along with the shot Jag bearing back in the box and return it for the core refund.

A better alternative is to chuck the engine-driven fan and install an electric fan.

Fan Shroud Flaps

At the lower left corner of the fan shroud are a couple of rubber flaps. These are designed to allow air to flow rearward through the openings in the shroud but not forward. At speed, the air coming in the front of the car and through the radiator merely blows these flaps open. At a standstill, when the fans are trying to draw air through the radiator, these flaps shut to prevent the fans from drawing air from the engine compartment instead.

They're as simple as they look. If they are damaged or missing, it is easy enough to make replacement flaps from an old inner tube, or even old shoe leather.

Front Spoiler

It's important to have the front spoiler in place. Engine cooling relies on air coming in through the radiator, and it must have a place to go. The XJ-S does not have vents through the hood or out the sides, so all this air must go out the bottom. The front spoiler is designed to direct air either into the radiator or around the sides of the car, and to restrict air from going under the nose of the car as much as possible. This results in low pressure under the car, so the air going through the engine compartment can easily flow out that way.

However, if the spoiler is missing, air can flow right under the nose of the car unimpeded. This increases the pressure under the front of the car, which in turn resists the outflow from the engine compartment. The air coming in through the radiator cannot escape as easily, and builds up pressure in the engine compartment. The resulting backpressure prevents as much air flowing through the radiator. It also tends to cause significant lift on the front end (try multiplying a very small pressure over the entire area of the front half of the car; the total lift can be very large indeed) and can cause the car to "wander" at high speed.

Keeping the Ignition System Cool

Possibly the worst area for heat problems is within the "V" on top of the engine. Early XJ-S's had so much trouble with cooking the ignition amp that Jaguar created a relocation kit to move it out of this area. Cracked distributor caps have been a problem. Seized centrifugal advance mechanisms are a problem. The wiring harnesses within the V always seem brittle. All of these are symptoms of excessive heat.

Maintaining a good airflow through the engine compartment does wonders for minimizing such heat-related problems on components. However, airflow to the V is largely blocked -- not by the A/C compressor so much as by the plate supporting the front of the compressor. See the section on the Air Conditioner & Heater for notes on correct installation of this plate.

One simple way to improve things would be to cut a big hole in this plate. Be careful to leave enough metal to properly support the compressor, but this will still allow a substantial opening. Since this area is directly behind the main fan, the hole should allow some airflow under the compressor and throughout the V area.

Heater Control Valve

The heater control valve is covered in the Air Conditioner & Heater section.

 

On to the Fuel System

 


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