QuestionQUESTION: I was driving my 97 LHS on the highway when I experienced what sounded like a grenade going off under the hood. The transmission immediately went into limp mode. I had the car at the dealer for over a month and they couldn't find the problem.
The transmission control relay doesn't engage. What I've determined is that the TCM is not energizing circuit T15. The dealer tried a new TCM but that didn't fix it. What causes T15 to energize? Or in my case what's missing that keeps T15 from energizing?
ANSWER: Hi Bob,
Limp mode is one and the same as a disengaged control relay. The TCM will do that for about 28 different reasons, and when it does so it stores a fault code in the TCM memory which is obtained with a fault code reader, at Chrysler called a DRB III. The code will tell you which of the 28 reasons is the cause. The TCM simply refuses to power up the actuation coil of the relay when it senses one of those problems. The only other reason would be if fuse 1 behind the left end cap of the dash or fuse Q in the power box in the engine compartment were defective.
So the dealer should be asked what fault codes they have found in the transmission controller memory. These are 4-digit numbers so ask for the specific number(s). If they say there are no numbers then ask if they have verified that power is getting to pins 11 and 15 of the TCM which is where the voltage that comes from fuse 1 and fuse Q is directed. Q is hot all the time, while 1 will be hot when the ignition is in the 'run' position. If the TCM is not provided with power it cannot store fault codes not power up the relay actuation coil if it wants to.
As for wiring, T15 should be connected to pin A of the relay socket while pin C should be connected to ground at the rear of the engine block on the left side. If power is on A and C is grounded the relay will activate.
Once you have a fault code we can go from there.
Roland
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QUESTION: I talked with the dealer techs and they couldn't give me a code. The reason why they thought the TCM was bad was because when they scanned it they would get different codes, nothing consistent. The conclusion they came to was that I had a broken wire or bad ground so I've spent the last couple of weeks doing continuity checks on the wiring harness and cleaning any ground connections i found. The only piece I have left is the harness that runs from the firewall to the transmission. I saved that part for last because it looks like it's going to be a pain. I'll let you know what I find. Assuming the harness checks out where should I go next?
ANSWER: Hi Bob,
It would be nice to know the various different codes they got. Why not try another shop for a tcm readout? On the harness from the firewall, the main wires that I see may be in that are a pink/light blue from pin 46 of the tcm to pin 14 of the data link connector, and violet yellow from pin 43 of the tcm to pin 7 of the white plug of the the body control module under the dash, and fuse 1 of the box in the dash to pin 11 at the tcm. I would get another readout before buying a new/replacement TCM and note all the code numbers.
Roland
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QUESTION: Hi Roland,
The dealer installed a new TCM and it still had the same problem which is why they felt it was a wiring harness issue.
Bob
AnswerHi Bob,
So do you have the old tcm or is the case that you paid for and have installed the new tcm?
In any case, if all the wiring checks out, and you have a new tcm, and a functing relay, then if you are still in limp mode there should be a relevant code that the tcm has logged in its memory.
The tcm wouldn't disable the relay without a reason so get another code readout. Presumably you verified the wiring to the relay.
The other thing you could try is to 'jump' across the relay socket's pins that the relay uses to power up the transmission and see how it drives that way. Jump from B to D, the two pins that are side by side. Drive it cautiously and of course don't leave it parked that way.
Roland
