Don Oswald

Just got done rebuilding my son's 1996 XJ 4.0L after it sat in the driveway for a year. It started right up and ran smooth while breaking in the new hydraulic cam. After 20 minutes it Idled at 750 rpm and life was good. Took it for a short test drive, the trans initially shifted smoothly into all 4 gears but when I came to a stoplight I noticed the idle was near 1000 rpm. Accelerating easily up to speed the trans didn't want to shift into 3rd until 3000 rpm and then didn't want to go into overdrive. Got back home and when in Park the idle was 1200 rpm. Next day I reset the ECU using the battery disconnect, touching + cable to -, headlights on/off method per the manual, started it back up and the engine idled at 750 rpm again. Took it for a short drive and experienced the same symptoms as the previous day. Hooked up my scan tool and while it came on, it showed it wasn't connected. Removed the OBD II connector from the dash so I could check it out; had 12v to pin 16, good grounds on pins 4 and 5, but no voltage on pins 3 and 11. I replaced the IAC and removed the ECU from the Jeep, didn't see or smell any obvious issue, cleaned the connectors and pins with contact cleaner, and shined up the ears where they come in contact with the mounting plate and fender well just in case they are supposed to provide a ground, but I believe the nylon inserts probably are insulators. Reset the ECU once more and experienced the exact previous symptoms....initial good idle but then after driving I am back to square 1. Could I have a bad ECU? Is there a way I can test it or am I S.O.L. and have to swap it out to see if that fixes the issues?
OBTW the check engine light isn't on.
Any thoughts?

BruceB

Take a look at an experience I had with my 96 Jeep....it may help...

https://www.cherokeeforum.com/f2/sen...roblem-251669/

lawsoncl

Tested the TPS sensor yet? That's the usual cause for the shifting issue. Especially if you pressured washed it.

Don Oswald

Interesting thread. Not certain of all the things you tried since I'm not sure of what is meant by a cavity or cav. I just went out and started the Jeep after it had sat for a day and it idled fine, just as it did when I first started the Jeep a couple of days ago so I'm wondering if the electrical issue is heat related. I am going to try disconnecting the TCM to see if I can get my OBD II scanner to work. If that doesn't do it, I'll try disconnecting other sensors to see if any affect it.
I did go to a pick-a-part place today that had two 1996 Cherokees and both ECUs had been removed. Seems like others have had issues with them.

Don Oswald

Didn't pressure wash the engine but I will test the TPS to see if it is working.

Don Oswald

I've decided to pull the throttle body and clean it and all the sensor connections. I'll let you know.

Don Oswald

Resolved the issue(s). Turns out it was a combination of a bad TPS and the wiring leading up to the TPS connector.
I couldn't find a used ECU at the pick-your-part so decided to take the advice to check the TPS. It was bad so I ordered one off of Amazon and installed it after removing the throttle body and cleaning it. When I reinstalled the throttle body the idle problem was resolved but the automatic transmission wouldn't shift correctly. Decided to check the new TPS and it was fine but I noticed the connector had been twisted 360 degrees and the signal wire was twisted around the 5V wire. When I untwisted it I noticed bare wire showing through the insulation on the signal wire. Put some liquid electrical tape on the bare spot and plugged it back in. It solved all the driving issues and now I can read the ECU.
Electrical gremlins are a PITA.
Thankyou Lawsoncl and Bruce B. Without your help I would still be chasing my tail.