97 XJR with 85,000 miles.

My transmission warning light has been going on sporadically about once a day. The car will then hesitate. I can throttle back and the light goes off. Never stays on long enough to get a reading. Also got a P1777 code yesterday when the check engine light went on.

I checked the coils and they seem to be fine.

Is this a bad tcm? Is this a bad electrical connection between the tcm and ecm?

Where should I begin to troubleshoot?

Thanks

 

The manuals suggest a bad TCM or a wiring/connector problem between ECM and TCM.

But......

It's fairly common to have a wiring fault right where the harness connector plugs into the transmission itself. Excessive transmission movement due to a weak mount spring causes a tugging of wiring harness.

Might wanna look there first


Cheers
DD

 

Curious, if the harness has been "tugged", what is the fix?

Can the connector simply be plugged back in securely, or does the harness get damaged?

 

Harness gets damaged...usually wires torn/pulled out of the connector

Cheers
DD

 

Sounds like an opportunity for someone to make a short "extension" harness to avoid the problem. They'd just need the two connectors and some wire.

Interesting...

.

 

My 97 XJ6's had a transmission warning light on and a jerky motion during shifting. I immediately replaced the filter and transmission oil since. That was 20K miles ago and no issue since.

 

Before you get too bogged down with all the complications of the loom the TCM etc check the very basics first, as already suggested if you are unsure when last carried out change the filter and fluid, if the fluid seems particularly old change it several times as each time you change it you only get out a certain percentage of the fluid as the torque convertor cannot be drained. Secondly it may sound unrelated but make sure the spark plugs are in good order as a slight misfire can also cause 'transmission faults'.

 

I agree with the last two posts, new filter & fluid is a good place to start and can really smooth things out. I wasn't getting a warning light on, but a full flush then new filter and refill with synthetic ATF cured mine of some curious "scratching" sounds on up-shifts and general sluggishness. The shifts are now silent, quicker and very smooth.

 

P1777 indicates a very specific voltage level problem with the torque reduction signal that the TCM sends to the ECM. I doubt VERY seriously if oil changes, filters, or howling at the moon will fix it. It is the signal the tranny sends to the engine to tell it to "lay off so I can shift!"

A few months ago, someone had this problem and reported it was fixed by changing the TCM. You can check the signal with a scope or data probe connected to the signal on ECM pin PI105-26. It should give a pulse to ground at shift. I suspect you will find it near zero volts without a shift, indicating either a short to ground in the harness, a bad connection, or a bad TCM.

 

At 85k miles, unless its been changed in the recent past, I'd suggest as above as a course of action anyway, if you're keeping the car for a while.

 

May not be the most expert advice, or the solution in this case, but most aging transmissions are grateful for a fresh filter & fluid - it never hurts. Haven't tried howling at the moon yet...

 

Granted... but it is not associated with code P1777.

And... notice I did not say howling at the moon wouldn't work, just that I doubted it!