Brain buster - '92 XJ eating fuses

Okay, bear with me on this one. My buddy's '92 Laredo 4.0 a/t has been acting up pretty good as of late. Problem originated as a really bad misfire under a light load, no codes. Anyway, one day on his way home from work it dies at a light, and all it does is start/stall. So he panics, starts it up revs it to the moon and slams it into gear, bang. Drives fine after that EXCEPT, now it misfires only when making left hand turns...?

That brings us to yesterday. He calls me up, Jeep died just driving down the road, just cut out, nothing. So I go check it out, break out the Power Probe, Maxi-fuse 15 (engine management, 30a) in the PDC is blown. Give it power with the probe, starts right up, cool. Throw a new fuse in it it runs again, he goes to leave and the second he hits the gas leaving the driveway, dead. Same fuse blown. Throw one last fuse in it thinking maybe with any luck the last one was defective, no luck. Fires it up, and the second he throws it in drive, didn't even get a chance to take his foot off the brake, dead again. Obviously something is shorting out.

The strange part is, if you make it run and its just sitting there, you can shake, tug on, yank any wire or plug in the engine compartment including the wires near the PDC and nothing happens. You can rev it all you want, nothing, it runs fine when its not moving.

Now for my question(s), I noticed in the wiring diagram that the same fuse that keeps going bad also powers the TCU. I have to believe the problem is somewhere in the splice from the engine compartment to the TCU, since all the engine compartment wires seem okay. Is there a particular connector or wire chaffing point I should look for? I know some '94/'95s had a chaffing problem under the left kick panel somewhere, but that was related to power windows/locks. Could it be possible that the trans controller is shorting out internally?

I haven't had a chance to toy with it much other than towing him home yesterday.

TIA

This one has me stumped for a spot on answer. If it were my rig I would start pulling things and visually inspecting them such as any sensors, plug connections etc. I think you have ruled a ton of stuff out by just noticing that it will run without putting it in gear. Only if its put in gear does it blow the fuse. My guess is like yours, but I would do as I stated above by just checking things out visually, heck take the TCU apart if possible

Check this site out, it has some good info one what things correspond with the TCU, I learned quite a few things.

http://jeep.zerok.ru/index.php?page=52

You can disconnect the TCU and you should still be able to drive it by shifting manually. (I think.) Then you can see if the fuse still blows. Check the wires around the transmission. The engine/tranny could be moving around a little and a wire grounding out.

A good visual inspection might reveal the culprit. If not and you're at wits end, put inline fuses in each separate branch of the circuit and see which one goes, then follow that wire along.

My money is on winterbeater's tip. I think one of the switched outputs, like one of the solenoids has a shorted coil, or that feed wire is pinched. This would cause the fuse to blow when the TCU tried to energize that componant. I would unplug the TCU and shift it manually to test this theory.

My bet is that the cable from the CPS to the wiring harness has gotten next to the exhaust manifold and burned through allowing it to make intermittent contact. It will blow that fuse and can act up hitting bumps, going around corners, or when the engine flexes due to load.

I dont think you can manually control the shift of it. If I read it correctly the TCU sends a signal when you shift allowing a solenoid to open or close to allow transmission fluid to run the correct path to push you in the direction you want to go. Also by the OP when he revved it up and tossed it in gear that does a world of hurt to autos. He could have damaged one of the 2 solenoids or rocked the motor and trans hard enought to chafe a wire or pull something loose.

FiveNdime said:

I dont think you can manually control the shift of it. If I read it correctly the TCU sends a signal when you shift allowing a solenoid to open or close to allow transmission fluid to run the correct path to push you in the direction you want to go. Also by the OP when he revved it up and tossed it in gear that does a world of hurt to autos. He could have damaged one of the 2 solenoids or rocked the motor and trans hard enought to chafe a wire or pull something loose.

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You should do some research before you post your opinion that the AW4 can't be driven manually. this technique has been discussed here many times. With the shifer in 1-2, you have first gear only. The 3 and D work as expected. Old_Man is giving sound advice.

MoFo said:

You should do some research before you post your opinion that the AW4 can't be driven manually. this technique has been discussed here many times. With the shifer in 1-2, you have first gear only. The 3 and D work as expected. Old_Man is giving sound advice.

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Yep, 1st in 1-2; 3rd in 3, 4th in D, reverse, neutral and park--all without the TCU. In fact, the first test in the FSM for most driveability problems is to unplug the TCU to determine if it is transmission related.

Also, disconnect the battery, then unbolt and turn the PDC upside down--they like to corrode under there and play Hob with the electrical system.