nctj

96XJ / AW4 / 4.0 / 130k miles

Jeep drives great and everything works normally on a trip. I go into a store, come back out to restart the Jeep and drive a bit, and the following issue will start: After getting up to speed, the torque-lockup will disengage (and may jump to 3rd as well?) when SLIGHTLY letting off the throttle. (and I do mean slightly!! An 1/8 of an inch! Speed or RPMs don't even change at all -- so only the TPS is "aware" that I've moved my foot). The torque converter will now be unlocked until I aggressively speed up or let off the gas fully -- almost like a "reset". Then I'll be back in OD and lock-up like normal. However, once that happens I can then reliably recreate this same symptom again and again by slightly letting off the throttle. If I pull over and restart the Jeep, the problem immediately goes away and everything acts normal. Sometimes for the rest of the trip. Sometimes symptoms reappear 10 minutes later.

In addition, on some occasions when this happens, the transmission will also shift to 3rd gear and get stuck there -- which is not fun when getting 3k+ RPMs on the highway -- or trying to accelerate in 3rd after coming to a stop. But again, if I restart the Jeep, it immediately goes away and performs as normal.

Some things to note:
1 - Not a mechanical transmission issue. These exact same issues happened with my prior transmission that I replaced for a 4x4 conversion. So I don't think it's the solenoids.
2 - TPS has been replaced a few times with new (Mopar) and previously working sensors
3 - TCU has been replaced (with a cheap eBay replacement, but would be unusual to have the exact same symptoms with a replaced TCU)
4 - Brake switch was replaced to test that out as well
5 - "kickdown" cable is adjusted

Because an immediate restart will clear any issues, I believe this to be a sensor or computer problem. And because I can recreate the issue with the slightest movement of the TPS, I believe it must be something linked to the TPS. Is is possible that the main PCM is going bad and corrupting the input from the TPS?

Very Red XJ

Yes, I replaced the engine management module on the driver's side inner fender with a rebuilt one and my transmission issues went away. Before you do check all wiring. Chrysler wire shields desperate and spell wires that ground out. Grounds that you do need can deteriorate. Check all wires to the transmission. If you can find an honest transmission shop they can find and repair it - but most likely they'd want the $$$ to rebuild the AW4 good luck

nctj

Thank you! Fortunately the transmission runs amazing typically - so I do not think I need to go down the rebuild route any time soon. And my wiring harnesses all look great - so I'm hoping the ECU solves this. I'm going to try and run over to the local LKQ today and grab one to test out.

lawsoncl

The AW4 is pretty robust and it's rarely a mechanical problem. The T/C unlocking could be a bad solenoid and I'd test it's resistance. Dropping from 4th to 3rd can't be solenoids because neither shift solenoid is engaged for 4th gear.

This smells like classic bad TPS to me, but you said you've changed it. Have you tested it with a meter, especially the ground connection? Maybe test continuity of the wires back to the TCU plug. Wiggle the wiring harness under the hood and see if that changes the output voltage from the sensor to the TCU. It's never a bad idea to clean up the ground connections on the passenger side of the block.

Very Red XJ

I hope that works out - it takes a little time to get a rebuilt one shipped - but you know that one has been tested
When you get that settled I'd highly recommenced replacing Chrysler's leaky fittings : https://www.cherokeeforum.com/f2/tra...d-help-264320/

lawsoncl

That makes no sense to me because the engine computer doesn't control the transmission. The transmission computer on a 96 is under the passenger side of the dash below the glove box. They rarely go bad.

Very Red XJ

Yes,, but Chrysler combined the transmission computer and engine management computer into one unit on later models - I have a 99. Both the engine computer and transmission computer are connected regardless of where they are located

nctj

My understanding is that while the ECU doesn't control the transmission directly, there are sensors that the ECU will receive (like the TPS) that will then go to the TCU to control the transmission. I don't believe the TPS sends a signal directly to the TCU - but maybe I'm wrong?

So in my case, my transmission issues present themselves at the SLIGHTEST release of the accelerator pedal. (the torque converter will unlock and sometimes the trans will kick down to 3rd gear and get stuck in third). But the pedal movement is so small, the RPMs nor the speed change at all -- literally the only sensor that is even aware of the fact I moved my foot would be the TPS. But after 3 different TPS sensors, the issue is still there (I have a dozen extra TPS sensors for all my Jeeps -- so I'm not just throwing parts at it). So I'm fairly certain the TPS has some sort of role in the issue even though I don't believe the actual sensor is bad.

Some assumptions I'm making that are helping me to diagnose this annoying issue (please correct me if I'm wrong):

1. The TPS sends its signal to the ECU to process before sending along to the TCU. So if the TPS was good, but the ECU was corrupting (or misinterpreting) that TPS signal, it would pass along bad information to the TCU.
2. Once the issue starts, if I pull over, turn off, and immediately restart the Jeep - all symptoms disappear. It's almost like a computer reset. (if it was a mechanical issue, or a heat issue, or if a wire was shorting out somewhere, it wouldn't "reset" like that)
3. The Jeep runs beautifully and smooth. No rough idle or other issues even when the transmission lock-up / "3rd gear stuck" issues start. If there was a bad TPS wire or ground issue somewhere, I would think that other functions would be affected - especially with so much relying on the TPS.

Unfortunately I have not been able to track down a junkyard ECU to test. I'll probably end up buying a Facebook marketplace one for "cheap" just to test to see if it solves my issue. Other than that, I'm not really sure how to diagnose an ECU issue like this.
The only other thing I can think of is that there is another sensor that works in conjunction with the TPS -- and that other sensor is going bad. (like the NSS?). That other bad sensor could be getting that signal from the TPS when I let off the gas and causing the issues. But the only two sensors that I can think of that weren't replaced with the 4x4 swap are the NSS and the VSS.

Very Red XJ

The TPS sends its signal to the ECU to process before sending along to the TCU. So if the TPS was good, but the ECU was corrupting (or misinterpreting) that TPS signal, it would pass along bad information to the TCU.

YES this is the modern version of the kick down linkage from the carburetor to the slush box from the iron age
Other loads are monitored for downshifting on hills etc...
I got my rebuilt unit from Computer Exchange, my rebuilt PCM cost only $219: https://carcomputerexchange.com/?cam...kaAtNNEALw_wcB

awg

As I understand it, the speed sensor (VSS) is also involved in gear changes. Its a long shot, but if you have a spare handy, may be worth swapping

nctj

The speed sensor on the transfer case? I’ve searched and couldn’t find any definitive info on what that sensor does beyond feeding the dash gauge. It’s one of the few parts that I reused from the previous transmission, and the connector does have some exposed wires from being stretched too far - so maybe I’ll swap that out with another one I have here as a test. But either way the wiring harness for it needs to be fixed anyway so maybe I’ll do that while I’m under there.

lawsoncl

Okay, I gotta clear some things up here.

- The Engine Computer does not control the transmission and the trans computer was never incorporated into the ECM. Some people think it did because the TCM was relocated to the drivers side under the dash in 1998. The Jeep Grand is another story though.

- For all years after Renix which had a dual output TPS, the TPS signal is connected directly to both the ECM and TCM. I believe the wiring itself splits off inside the ECM connector. They both get the same signal, it is not relayed or sent by the ECM. Renix had a dual output sensor, because the renix ECM wanted 5-0 volts and the TCM wanted 0-5 volts.

- The inputs wired to the trans computer are: throttle position sensor, NSS, input shaft speed (98 and newer, mostly used for diagnostic rather than shifting), and the transmission output speed sensor, and brake pedal switch. It is not connected to the VSS. A bad output speed sensor will result in not upshifting out of 1st gear.

- No they are not eating the cats and dogs.

nctj

Thank you for that information! That's what I've been trying to figure out. All I know is that these issues are directly or indirectly related to the TPS -- (they only happen when the TPS senses the slightest throttle movement (closing) - and once the issue starts, I can consistently repeat it by doing it again and again). I don't believe the actual TPS is bad, so my thought is that there is either a signal from the TPS getting messed up somewhere, or there is another sensor that works in conjunction with the TPS that is bad.

Very Red XJ

'i see" said the blind man