Hey there mates...I figured I would turn to my brother for this mystery. I have a 1994 Jaguar XJ12 87k miles..the check engine light was because of a failed distributor cap. We gave the car a full tune-up with all fresh parts. Local garage indicated in order for me to turn the check engine light on I must then disconnect the battery..which we did. NOOOOOOOOW the bloody transmission limp mode light is on...I have attempted to reset by disconnecting battery to no avail. I guess I am first wondering why that is on now.what would have caused the car to go into limp mode..and what steps shall I take to make it go off. Thank you in advance..I am forever grateful!

 

Hi Crownewourth,

The setup on the XJ12 is a little different, but the most common cause of Limp Home Mode on our '93 XJ6 has been oil contamination of the electrical connector for the Throttle Position Sensor (TPS). Both the ECM and TCM use the signal from the TPS for determining gearshift points, shift timing retard, etc. You might check to ensure that during your tuneup the connector wasn't disconnected or jarred loose. It's also well worth disconnecting the connector and flushing both halves with zero-residue electrical contact cleaner and allowing it to dry before reconnecting it.

If cleaning the connector doesn't work, another common cause of LHM is contamination of the TPS itself. Yours may be a little different from ours, but the photos below show how I and many others have drilled drain holes in the TPS case to enable flushing it out with contact cleaner.

Finally, the most recent cause of LHM on our '93 was a failed cold solder joint on one of the circuit boards in the TCM, but before we go there why don't you check the TPS connector and we'll go from there.

Cheers,

Don

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Don B..thanks mate! I will have a look at suggested problem spots. What has me puzzled...all was fine before we disconnected the battery cables....would it just simply go out like that???

 

I can't think of a reason disconnecting the battery cables would cause Limp Home Mode, unless a something was accidentally shorted and blew a fuse.

One question that comes to mind is, what was the fault code that caused the CEL to illuminate (that you traced to the distributor cap)? I'm wondering if a related fault could be causing the LHM....?

Cheers,

Don

 

what fuse would check Don if a fuse were to blow in a situation like this?

 

I'd first check any fuses related to transmission operation. If you don't have the electrical manual, I believe it's available in the download area of this forum. If I get a chance later tonight I'll take a look at the schematics.

Do you know what transmission you have? The late 6-cylinder cars use a ZF 4HP24 electronically-controlled gearbox, but I seem to recall that the 12-cylinder cars used a Getrag or GM gearbox. If you know the model, there may be a troubleshooting guide available as there is for the ZF gearbox (there's an excellent one in the XJ40 eBook at the Jag-Lovers forum).

Cheers,

Don

 

Hi Don..I will be back in Seattle late tonight so I will be working on the car. I will keep you posted on what I find..I am just baffled by this ...all was fine before I unplugged the battery....thanks so much for your help.

 

Thinking about disconnecting the battery, I assume the battery on your '94 XJ12 is in the trunk/boot as is the one in our '93 XJ6? If so, the ground point for every Logic Ground circuit in the car is behind the battery on the trunk bulkhead below the fuel tank. I don't know how you could have bumped any wires connected to that ground, but it might be worth removing the nut on that ground stud and thoroughly cleaning the threaded stud, all the eyelet terminals on ever wire, and the nut. Use a small brass-bristle brush and flush every part with zero-residue electronic contact cleaner. Allow to dry, then reassemble. The Logic Ground is the common point in the Serial Communications bus by which the ECM, TCM, CPU, ABS ECU and other electronic controllers communicate with one another. This may be a long shot, but it's the only thing I can think of near the battery that has anything to do with the transmission. The other thing in the boot/trunk that could be related to LHM is the ABS ECU, which is mounted in the left rear fender near the fuel filler neck. You didn't do any work in that area, did you?

And I wouldn't forget about oil contamination of the TPS electrical connector or the TPS itself - it's possible the battery disconnection was not actually the cause of the LHM.

Don