94 Grand Cherokee 4WD/AWD 4L won't go into overdrive

[jonathanstampf]

I've been trying to figure out why the jeep won't go into overdrive. 15 mpg is not doing me any good.

The O/D OFF switch is a pushbutton, with a light above it that lights when you opt for no overdrive. I pulled the wire harness off the back, and drove: no overdrive. I then connected the wires that the O/D OFF switch would close and drove: still no overdrive.

I put it all back together and noticed that the light stayed on no matter how many times I pressed the button. But interestingly, when the ignition is on the RUN position, but engine not running, the O/D OFF switch will turn that little light on and off. I think this is a key clue.

I've learned there's a little computer Transmission Control Module that tells the transmission electrically when to shift, and one fellow thinks the Throttle Position Sensor, one thing that feeds data to the TCM, could be the culprit. I checked the Throttle Position Sensor and it sends .2 - 5 volts to the TCM, depending on the position, and is $28. The TCM is $412. So you know what I'm hoping.

Any ideas? Why does the switch function not running but stays lit when running?

 

[jonathanstampf]

On more thing. I learned that when you replace the TCM, there's an electronic device that syncs it to your trans...something about optimizing shifting. This car had a recently replaced trans when I bought it...could not syncing it to the existing TCM when installed cause this?

 

[54cj3b]

overdrive in 4 lo or hi you dont run overdrive in 4wd hellooooooooooooooooo

 

[Bounty__Hunter]

It's a common problem, and I suspect it's one or both of the solenoids inside the transmission, accessible by removing the pan. Common to ZJ's and Dodge trucks.

 

[jonathanstampf]

That's what we're all looking for, isn't it : a common problem that's easy to fix. Thanks Bounty Hunter, I'm off to learn about solenoids.

 

[jonathanstampf]

Changed the two solenoids and got no difference. I'd like to re-consider the problem: not that the transmission won't go into overdrive; it seems it's not being directed to ever go into overdrive.

This has got to be the key symptom: the light over the O/D OFF switch always stays on when the engine is running...no matter how many times I press the button. However, the light will go off and on when I press that button if the key is in the on position but engine not running.

So I think some sensor on the engine is sending some signal when running that prevents the car's system from allowing overdrive.

Any ideas along that line?

 

[SteelHeadz]

Check for a 5volt supply between the PCM and the Trans. solenoid assembly. Also make sure that the fuse #17 on the PDC is getting a full battery voltage out to the solenoid pack.

 

[drudog1715]

Somebody please help. My '95 grand cherokee is doing the same thing and it is killing me. It is so frustrating because you can never control it.

Driving around town...on the freeway it is just a joke. Terrible gas mileage and slow acceleration.

Does anybody have a logical answer...please!!

 

[jonathanstampf]

I tested every contact point in the wire harness while connected to the TCM. I tested DC volts from the contact point to ground. Here's a diagram of the readings I got. #4 connector to throttle position is 1V, increases to about 4 when I press the gas pedal.
To reiterate: 42re auto trans, 1994 4L inline6, no trouble codes for engine or trans. OD/Off switch turns light off and on with key on/engine not running, but light stays on, no OD, when running. (Sometimes it will work for 5 minutes, about once a month, for no reason.)

 

[jonathanstampf]

I tested every contact point in the wire harness while connected to the TCM. I tested DC volts from the contact point to ground. Here's a diagram of the readings I got. #4 connector to throttle position is 1V, increases to about 4 when I press the gas pedal.
To reiterate: 42re auto trans, 1994 4L inline6, no trouble codes for engine or trans. OD/Off switch turns light off and on with key on/engine not running, but light stays on, no OD, when running. (Sometimes it will work for 5 minutes, about once a month, for no reason.)