8.3 - Transmission Warning Light Troubleshooting ( John Ping,  )

GENERAL TRANSMISSION INFORMATION

The ZF 4HP24E9 transmission is a strong and reliable, electronically controlled, four speed automatic transmission with lock-up overdrive. It is of German design and manufacture, which is used by several automotive manufacturers. The electronic control system is by Bosch. Although Jaguar utilizes other transmissions on XJ40 lineup, this design is the mainstay for 90-94 model years. It was also carried over for use in the 4.0 X300 models of 1995-1997. The ZF 4HP22 is quite similar mechanically, but it does not utilize any electronics for control purposes. The ZF 4HP24E9 has approximately 30% greater engine torque handling capacity than the ZF 4HP22.

The ZF 4HP24E9 has the following gear ratios: 1st - 2.48 : 1, 2nd - 1.48 : 1, 3rd - 1.00 : 1, 4th - 0.73 : 1 and Reverse - 2.09 : 1. With a differential ratio of 3.58 on the 4.0-liter engine equipped 90-94 model years, the XJ40 has a road speed of 29.2 mph per 1000 rpm in overdrive. The transmission has an electrical "kick down" switch located behind the accelerator pedal for throttle-manipulated downshifts. The transmission sump capacity for a simple fluid and/or filter change is approximately 3.0 liters (3.2 quarts). The entire system holds approximately 9.5 liters (10 quarts) which is approximately one liter greater than the ZF 4HP22 models. The correct fluid is Dexron III or improved upgrade.


TRANSMISSION WARNING LIGHT TROUBLE-SHOOTING AND DIAGNOSTICS

The transmission warning light is designed to "self-check" illuminate when the ignition key is turned to the "ON" position. It should immediately extinguish (along with the other warning lights) a few seconds after the engine in started. Failure to turn "OFF" after an engine startup is an indication of an "electronic" problem with the transmission control system. It does not mean that a mechanical failure has occurred. If the transmission warning light illuminates while driving, pull over at the first reasonable opportunity, shut off the engine and immediately restart. If the light extinguishes on the restart, the problem was likely a control system malfunction (computer glitch). If the light remains illuminated, the transmission will likely default to "Limp Home" mode. In this operational mode, the transmission is electronically locked into third gear. It will not shift properly while in the "Drive" position or by manual J-Gate action. The vehicle is drivable in this condition, but the fault should be investigated and rectified as soon as reasonably achievable.

Note: This transmission has no pressure or temperature sensors. There are no low / high pressure and/or high temperature alarms. The transmission has only a single eight-pin connector providing the interface with internal solenoids or speed sensor.

The following is a step-by-step method for troubleshooting the problem. It's reasonably easy to perform and the cost is minimal. Copy and print this section of the manual so that you have the information at hand when you begin the troubleshooting process. Keep in mind that failure of the transmission control module (or transmission ECU by other names) should not be the first thought in your mind. Like most modern electronic control processors, they have a reputation for reliability and long-life. The fault is typically an ancillary component, but microprocessor failures are not impossible especially since these vehicles are well over ten years old. The following steps assume there is NO pre-existing check engine light illuminated.

Tools Required For Trouble-Shooting:

Typical hand tools for removal of upholstery / sheet metal panels to access the electronic rack and rotary position switch on the side of the transmission. Any good quality digital or analog multi-meter for performing voltage and resistance checks as suggested by trouble-shooting process.

Step #1

Re-boot the Transmission Control Module (microprocessor) by disconnecting the negative battery cable for a few minutes. Re-connect the cable, start the engine and check if the transmission warning light is extinguished. If not, go to Step #2.

Step #2

Check the Transmission Control Module battery power supply fuse. It's typically found in the Left Side Fuse Box in position C2. Remove the five-amp fuse and visually inspect. Replace with a known good fuse of the same amperage if you have any doubt as to its integrity.

Start the engine and check if the transmission warning light is extinguished. If not, go to Step #3.

Step #3

Check the transmission fluid level in accordance with the method described within the owner's manual. Ensure the fluid is at operating temperature by driving the vehicle for at least ten miles prior to the check (further in cold weather). Start the engine, move the gear selector through its range a couple of times to ensure the system is primed. With the engine idling in park, check the fluid level. The level should be between the minimum and maximum "hot" level indications. Add carefully (half liter at a time) to bring the level into the proper operating range. This transmission is reasonably tolerant of high fluid levels. If not certain as to the fluid level from dipstick observation, drain and refill the sump with the appropriate volume of Dexron III.

Re-start the engine and check for transmission warning light illumination, if still present, go to Step #4.

Step #4

Check the throttle position potentiometer. Throttle position is a major input for the control system. The throttle position sensor (located underneath the throttle assembly) provides this input value. On the 90-92 models, the TPS has its own potentiometer supplying a voltage signal to the transmission control module (TCM). On the 93-94 models, the engine management system provides the throttle position information to the TCM. In this case, only one of the two potentiometers is utilized within the TPS.

Locate the throttle position sensor electrical connector (in front and slightly below the throttle assembly) and remove it from its mount. While pressing inward on the side mounted barrel connector tabs, pull the connector apart to expose the male and female halves. Inspect the pins and sockets for corrosion and residue. Clean the pins and sockets with either a good quality electrical contact cleaner or a brake system/electrical parts cleaner. Thoroughly spray the pins and sockets to ensure they are clean and free of residue … do not use any petroleum-based solvent, which leaves a residue or could attack the connector seal.

If you have a wiring diagram available, connect a digital multi-meter across the "transmission" throttle position potentiometer power input and wiper to measure component resistance.

Note: For 90-92 models, the "white with blue tracer" conductor is the throttle potentiometer power input. The "green with blue tracer" is the throttle potentiometer wiper. A failure of the throttle potentiometer on the 93-94 models will result in a "check engine light" being illuminated.

With the throttle closed, the resistance should be approximately 5.5k ohms. With the throttle fully open, the resistance will be approximately 250 ohms. Ensure the resistance change is smooth while the throttle is operated. There should be no "abrupt" changes in resistance as the wiper moves. Re-connect the halves of the TPS connector while ensuring good alignment and seating of the pins.

Start the engine and check if the transmission warning light is extinguished. If not, go to Step #5.

Step #5

Inspect the transmission rotary switch located on the side of the transmission housing. Lift the vehicle in a safe manner; remove the debris shield located in front of the rotary switch (two 10 mm bolts). Initially, inspect the exterior of the rotary switch and its wiring for obvious defects. Remove the rotary switch cover (six T20 torx screws) to allow visual inspection. Do NOT touch the rotary switch mounting/locating bolts. Using a good quality electrical contact cleaner or a brake system/electrical parts cleaner, liberally spray the copper slides and runners to remove any corrosion film or dirt. Exercise the J-Gate shifter while cleaning to ensure all surfaces are sprayed. Re-install the rotary switch cover.

Further back (towards rear of car) on the transmission housing, locate the transmission electrical connector (round screw-on type) and inspect for any obvious damage to the connector or its wiring bundle. It is not necessary to remove this connector as it is quite difficult to re-install. Electrical checks can be performed on the transmission components from another more readily accessed connector.

Replace the rotary switch debris shield and lower the vehicle. Start the engine and check if the transmission warning light is extinguished. If not, go to Step #6.

Step #6

Check and/or Replace the "transmission relay". This relay is a simple four pin, 1" cube style design common to XJ40s. The relay is typically located in the front passenger footwell region. Consult your wiring diagrams, or post to the XJ40 List for location information. Its not usually necessary to purchase a new relay immediately as you can substitute any typical four pin relay for this application.

Disconnect the battery negative cable … do not forget this step as battery power is constantly supplied to many electrical / electronic devices. Remove the passenger side footwell upholstery cover. Remove the electronics and relay rack sheet metal cover.

Note: On early 90s models, the transmission control relay is mounted on the front of the passenger side climate control blower housing. You will need to remove the four (4) sheet metal screws that mount the "black" relay panel to the electronic rack. The lower two screws need not be completely removed as the panel has "slots" incorporated to slide over the partially fastened screws. Lower the relay panel to access the transmission control relay. It typically has a "purple" relay socket.

Re-connect the negative battery cable. But before replacing electronic rack panels, start the engine and check if the transmission warning light is extinguished. If not, go to Step #7.
Step #7

Again, disconnect the battery negative cable.

Inspect and/or replace the transmission decoder module. This module is a solid-state device containing two integrated circuits on a printed circuit board. Its purpose is to translate transmission gear shifter position from the rotary switch to the Transmission Control Module. It takes input from the transmission rotary switch and converts it to a two level signal (ground or open circuit) for the TCM.

The transmission decoder module is located on the rear side of the relay rack panel in the upper right side corner. It is directly behind the Door Lock Control Module. It's a rectangular "black" module with a "blue" multi-pin connector. It is mounted with two small machine screws secured with 7 mm locking nuts.

Remove the electrical connector and inspect for obvious problems. It is also possible to inspect the circuit board by removing the module cover and slipping out the board. If no defects are found, you may wish to simply replace this part as its reasonably inexpensive (new … $50 US, used … $25 US). Re-attach the electrical connector to the decoder module and re-mount to the relay rack panel.

Re-connect the negative battery cable. But before replacing electronic rack panels, start the engine and check if the transmission warning light is extinguished. If not, go to Step #8.

Step #8

Again, disconnect the battery negative cable.

Inspect and check the "Transmission Control Module (ECU). The TCM is located at the lower right region of the footwell area while the Engine Management ECU is located at the lower left. There is a schematic layout drawing on the electronic rack sheet metal cover providing component location. Typically, you will need to remove the front passenger side footwell climate control duct to access the TCM. There are no fasteners, just slip it off the black plastic outlet nozzle.

Disconnect the electrical connector to the TCM by lifting upward on it silver colored hasp. This action will pivot the connector off the TCM. Visually inspect and thoroughly clean both connector pins and sockets using a good quality electrical contact cleaner or a brake system/electrical parts cleaner.

The TCM electrical connector is a three-row type having 55 pins. Note that not all pin locations are utilized. Pin identifications are as follows: Viewing the connector in the horizontal plane with the wiring bundle on the right side, the top far right female pin female is #1 and continues across to the top far left female pin #19. The second row starts with #20 on the middle far right and continues across to the middle far left being # 37. The bottom far right starts with #38 and continues across the to the bottom far left being #55.

Electrical checks can be performed on the internal transmission components at this time if you have a quality digital or analog multi-meter. The three (3) internal solenoid valves should have an approximate resistance value of 34 ohms at 20 °C (68 °F). The resistance values can go as high as 60 ohms with the transmission at elevated temperatures. The main point is to verify a reasonable resistance value for each solenoid. The solenoids should have very similar resistance readings.

Resistance measurement points for solenoids MV1, MV2 and MV3 using the conductors provided by the TCM multi-lead connector are as follows:

Pin #19 (Solenoid Valve Power Supply) to Pin #5 (Solenoid Valve - Forward Drive)

Pin #19 (Solenoid Valve Power Supply) to Pin #24 (Solenoid Valve - 2nd)

Pin #19 (Solenoid Valve Power Supply) to Pin #42 (Solenoid Valve - Lockup)

Again, all three solenoids should have approximately the same resistance although this value will change as a function of temperature.

Resistance measurement points for the Pressure Regulating Solenoid using the conductors provided by the TCM multi-lead connector are as follows:

Pin #19 (Solenoid Valve Power Supply) to Pin #6 (Pressure Regulator Valve Solenoid)

This solenoid valve should have an approximate resistance value of 5-7 ohms. The value may be slightly higher with the transmission at elevated temperatures.

Resistance measurement points for Output Shaft Sensor using the conductors provided by the TCM multi-lead connector are as follows:

Pin #2 (Output Shaft Speed Sensor) to Pin #38 (Output Shaft Speed Sensor)

This sensor should have an approximate resistance value of 300 ohms (+/- 10%).

Check for proper TCM ground conductors by checking for minimal resistance between Pin #7, Pin #9, and Pin #26. There should be not significant resistance between any of these pins and a solid chassis ground.

Check for proper TCM power supply by performing the following voltage checks at the connector. Constant 12 VDC (battery power) should be supplied at Pin #39. Ignition switched power should be available at Pin # 1 when the ignition switch is in the "ON" position.

If any of the above resistance or voltage readings are abnormal or non-existent, problems are likely in the wiring harness or the solenoid(s) have failed.

Re-connect the TCM multi-lead electrical connector. Re-install all previously removed electronic components and replace the metal shield and upholstery panel.

Re-connect the negative battery cable. Start the engine and check if the transmission warning light is extinguished. If not, go to Step #9.

Note: The TCM also interfaces with the Engine Management ECU, but this type of diagnostics is best left to a franchised Jaguar dealer that has the necessary (proprietary) test equipment.

Step #9

Take your XJ40 to the Jaguar dealer of your choice and have the Service Department run a diagnostics check on the TCM. Franchised automatic transmission repair shops are NOT likely to have the necessary diagnostics equipment for your XJ40. If the dealer diagnostics indicates a major component fault, instead of purchasing a new transmission control module (ECU) from Jaguar (high expense) consider purchasing a good used component from a salvager or reputable used parts supplier.