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Jaguar V12 Temperature Gauge

Jaguar V12 Temperature Gauge

The temperature gauge on a Jaguar V12 is the most important item in the car!  Unfortunately, it's also one of the most misunderstood -- a fact that has resulted in untold numbers of V12's being scrapped and replaced with Chevy engines and the like.  Considering the importance, this page has been created to inform owners what they need to know as clearly as possible.

This picture shows the "vertical" gauge used on XJ-S's up until the early 1990's.

I guess a large part of the problem here is that the gauge doesn't really give a good indication of how hot is too hot; there's no red band or anything, nothing to tell the owner what's acceptable and what's not.  If you have the vertical style gauge, make a mental note: if the needle is high enough that it is no longer touching the letter N, it's too damn hot.  Pull over now, if possible use a garden hose on the radiator before shutting the engine off, shut the engine off, and do not start it again until the cooling system has been fixed.

The needle in this picture is about where it normally sits if you have 88°C (190°F) thermostats fitted.  In that case, this reading is fine.

If you have 82°C (180°F) thermostats fitted, the gauge shown above is indicating too hot; the needle should be reading below the N.  Please, please pay attention here: just because you have the colder thermostats fitted does not give you that much more range on the gauge before you need to be concerned.  If the gauge looks like the one in the picture, it's not because the 82°C thermostats have suddenly taken it upon themselves to start opening at 88°C.  It's because the car is overheating, and there are probably some spots in the engine -- notably in the left bank, since the temperature gauge sender is in the right bank and it's the left bank where trouble starts -- that are already much hotter than the gauge indicates.

James E. Teston:  "And then there was this great Ebay ad: 'Mechanically the car is fine and has been serviced only by authorized Jaguar mechanics.'"

Only 11,832 miles, too -- that's probably why this picture was included.  Doesn't matter a lick, that low-mileage engine is now useful for little more than a boat anchor.  If this engine is actually still running, it won't be for more than a few more minutes.  This temp gauge is reading severely overheating.  The owner should be taking action -- but I suppose he is, he's putting the car up for sale on EBay!

People, it is simply not possible to overreact.  Letting the needle get to where it's shown in this pic will cost you several thousand dollars at least, if not forcing you to unload the vehicle on EBay at a terrible loss.  Calling a tow truck long before this point is reached would be a very wise move.

There are variations on the same gauge.  In some countries and/or some models, the gauge is the same except that it's marked in degrees C instead of the simple C-N-H marking.  It makes no difference at all to the implications: if the needle is more than 1/8" above dead center, the engine is too damn hot.  If the car is fitted with 82°C thermostats, being at center is too damn hot.

"But, but, but... What if it's just the gauge malfunctioning?"  A valid concern, since this is Lucas we're talking about.  However, in most cases, a malfunctioning gauge is obvious -- it goes hard against the H end as soon as the key is turned on, it never moves at all, whatever.  They have been known to read a hair high or low due to corroded connections or the like, but for the most part if they're working at all they're pretty consistent.  But think about this: are you gonna risk it?  If you're driving down the road and your gauge starts reading like the pic above, are you just gonna tell yourself "Oh, the gauge must be actin' up."?  If so, I predict we'll be seeing your car for sale on EBay shortly thereafter.

In the early 1990's Jaguar/Ford changed the dashboard to round gauges.  Teston provided a picture of this dash, too, presumably from his own '92:

There are a couple of reports of nearly-new 6.0 litre cars running with this gauge a bit to the right of the N, but as of this writing I don't have enough reports to be able to describe exactly how hot is too hot on this gauge.  In general, the overheating problems show up on Jaguar V12's when they are about 10-12 years old; that's when the radiator gets plugged up, almost like clockwork, regardless of how many miles are on the car.  With the vertical gauges, there have been lots of cars that reached this age and overheated and dropped valve seats and crunched pistons and got their engines rebuilt or replaced with Chevy V8's, so we know full well that when the needle has cleared the N, it's time to finish this particular trip on the hook of a tow truck rather than continue driving.  But not enough cars with this later gauge have reached this point yet, so we don't know for sure.  Of course, it's a pretty safe bet that waiting for the needle to actually contact that red band would not be a good idea.

Now the other direction, timewise.  Paul Clarkson sends this pic of the coolant temp gauge in his 1974 Series II Daimler Double Six Vanden Plas:

Unfortunately, I don't have enough info at this time to advise on how hot is too hot on this gauge either.  That's largely because most of the discussions have been via e-mail and therefore text only, so one guy is talking about the needle being on "N" looking at a C-N-H gauge while another guy is thinking about a gauge like this, where N is not all that far from C.  Mucho confusion.  Hopefully, this page will help clear some of it up so we can achieve clarity on using this gauge.

A word about "Jaguar mechanics":  If you ask an "expert" about high temp gauge readings and he says "Don't worry about it; these cars always run a little hot", the man is a liar.  Ask him this:  "Does that mean that, if I continue driving it like this and the valve seats fall out and ruin the engine, you'll cover the cost of repairs?"  And while he's stammering for an answer, tell him to stuff his Authorized Jaguar Mechanic certificate where the sun don't shine and take your business elsewhere.

If all this is news to you and you have just become concerned about how hot your V12 is running, I'm sure you want to know what to do.  Short answer: if the radiator has been in the car for more than 10 years, pull it out and have it rodded, recored, or replaced.  Long answer: download the Book and read all about coolant filters, Bar's Leaks, foam surrounding the radiator, the fan clutch, electric fans, the uselessness of "flushing" or "boiling" a radiator, the one-and-a half-pass sideflow radiator, aftermarket aluminum radiators, drain plugs, incorrectly designed banjo fittings, jiggle pins, bleeding the air from the heater circuit, problems with the hose to the atmospheric catchment tank behind the LF wheel, and a host of other concerns regarding this cooling system.  Wrong answer: continue driving the car.


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