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Tranny problems

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M

maynard

Yoda
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My transmission light is on in my 94 XJS V12 and the tranny is in the limp home mode (third gear only). I suspect it is an electical problem since the light is always on as opposed to being off until shifted out or park), and when I shift manually, it drops into first, second and third, but not fourth.

Is there a way to pull codes without using the Jag Diagnostic system?
 
Unfortunately there are no transmission codes for 94 models since they are on OBD1. The most frequent source of the transmission limp home mode is the throttle potentiometer. If the resistance of the pot becomes non linear or interupted the limp home code will come on. Also the potentiometer can get out of calibration and must be reset. The technical documentation of the XJS covers calibrtation and replacement procedures. I think there is a transmission relay in the transmission electronics and its failure or dirty contacts can also cause a limp home mode. Beyond this, get the schematic of the transmission electronics and begin to test/substitute. The limp home mode is almost always caused by an electronic glitch.
 
Thanks for your reply John, but I beg to differ with you. My ROM lists codes for the transmission, but they only say they are from the Jaguar Diagnostic system.

If the pot was bad, wouldn't there be other symptons? The engine runs fine, no hesitation or roughness.

Also,wouldn't the light be reset if I disconnected the battery? I did, and it didn't.
 
Most of the limmp home modes that I have corrected were the throttle pot on the transmission side. It is very rare that you get a throttle pot that fails on the fuel injection side. The light will not reset if it is a hard failure. Sometimes the light will reset on its own but that is rare. On XJ6 sedans I have probably replaced 20 throttle pots for transmission limp home. I have replaced 2 throttle pots due to fuel injection fault codes. Since 92 through 95 V 12s are much rarer, I have had much more limited experience with those models. The Jaguar Diagnosstic System that you refer to is a dealer transmission diagnostic tool that applies to pre 95 models that do not have OBD2. The only way you can obtain one is from a Dealer. I have never used these codes since I have never had a case that could not be handled by normal diagnostic procedures and substitution of components. I would be willing to bet that you either have a faulty throttle pot (on the transmission side) or a transmission relay fault. The other culprit that I have encountered for the limp home mode is the tansmission decoder module (one case). Check your pot for not only operation but also the reference voltage supplied from the ECU. I think the reference voltage from tansmission ECU is 5 volts. Ground is supplied at the other end of the trottle pot.
 
Thanks for clearing that up. I will check the TPS. There are a bunch of wires (I think 6, but I'm not sure) from it. Do you know the colors of the transmission side?

Thanks in advance and Happy New Year!
 
I dont know the color code for the V12 pot. It is different from the 6. If you cant get a handle on them(three are for fuel injection and three are for transmission) let me know Monday and I will look it up on my CD at the business. Of the six pot leads, two pairs are gounded, two are reference voltage from the ECUs (5 volts) and two are signal to the ECUs (transmission and fuel injection). The test on each side is for smooth continuity through the range. You will also have to know the voltage or ohm reading at signal when at idle. Happy New Year and Happy Honika, Jack
 
Still working on the car. I am not sure how to test the pot, I used a digital multimeter and checkd for continuity and resistance. Should I be measuring voltage?

I did check for voltage on the incoming wires, and have 5 volts on both the w/b and the w/y wires. Does this mean the ECU is OK?

I would appreciate any more help I can get. Thanks
 
It means that the ECU is furnishing proper reference voltage to the throttle pot. You can test the throttlepot by removing it from base of the throttle butterfly valve. Dont romove it yet since thaat will uncalibrate its installation. Tomorrow I'll look up the color code of the throttle pot transmission leads and the proper voltage or resistance for calibration. I have found that very small variances in the trottle pot resistance can cause the limp home mode. I normally replace the throttle pot without a test and find the the problem goes away. It seems that the slightest corrosion on the pot windings can trigger the limp home mode.
 
Checked out the TPS today. AS you said, there are six wires going into it. The colors change at a plug the TPS connects to. One the ECU side of the plug, here are my results:

White/blue 5 volts
White/yellow 5 bolts
yellow/black 0 volts
yellow(maybe orange)/ white 0 volts
black/blue varied smoothly with rotation of pot
greeen/blue varfied smothly with rotaion of pot.

I assume this means the pot is good. Any further suggestions?
 
The pot has tested good but it still might be out of calibration or changed calibration, I will look up the value for the pot output at idle position.
 
Manyard, I am embarrassed about my response for color codes, etc., but this has been a bananas type week at Jacks Toy Store. I haved never experienced the need of working on British sports cars in January as it is now!! I will try to get back to the Toy Store computer and answer your questions about the throttle pot circuitry and calibration. If you have any further need for information, please respond tonight and I will also look that up tomorrow. Sorry for the delay(s) but I have to put business first. The Toy Store is closed on weekends and I should be able to get your information then. Jack Farrington
 
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