drivability problem

heavyequipment · #1 04-18-2008, 05:56 PM

My brother-in-law's '87 Cherokee 4.0L surges or misses (he says it bucks) around 2500-2800 RPM just cruising or slowing down. The throttle position sensor has been replaced with a new one, and the MAP sensor was swapped with one from my own Cherokee that runs perfectly. I haven't driven it myself yet (he lives a couple of hours away) so I'm not quite sure if it's engine related or if the (auto) trans is doing something odd.

If anyone has seen something like this, I'd like to hear about it...

Thanks

heavyequipment · #2 04-25-2008, 11:48 AM

I finally got to test drive it yesterday. It seems to be shifting in and out of lockup between 2000-2200 RPM at part throttle.

It's got me stumped, so far. But I got a factory shop manual delivered just yesterday...

NIX · #3 04-25-2008, 04:23 PM

Sounds to me like a TCU problem.

heavyequipment · #4 04-25-2008, 05:33 PM

Thanks, NIX...

I was just thinking that after seeing another post about a Grand Cherokee. Someone also suggested looking at the engine and trans mounts.

Luckily, I have a spare TCU if the one from a 92 2-wheel drive will fit. I'm going to try that at the first opportunity.

NIX · #5 04-26-2008, 02:24 AM

If they are both AW4 trannies chances it might work.

heavyequipment · #6 05-23-2008, 03:31 PM

They are both AW4's, but the TCU is different. The plug won't fit. Studying the factory manual, I noticed that there is a brake switch that disables lockup when the brake is applied. I told my brother-in-law to test it by slightly depressing the brake pedal when it starts acting up. He did, and it quit jerking. I think this isolates the lockup control as the problem area. Could be the brake switch, the TCU, or maybe the speed sensor. Next time I see it I'll put in a temporary override switch to eliminate (or confirm) the brake switch as the problem.

JeepDawg · #7 06-04-2008, 08:19 PM

I was just about to suggest the brake light switch....

Two possible problems....

1. Out of adjustment
2. Brake pedel spring is broken.

You should have about 1" of travel before the brake light comes on.

BTW...The reason for that is so that the computer does not keep the converter locked up as you brake and slow down. With the converter locked, it would kill the engine when you came to a stop.

heavyequipment · #8 06-04-2008, 11:05 PM

I told my brother-in-law to depress the brake pedal a little when the surging starts. He did, and the surging stopped, proving that the transmission had been going in and out of lockup.

I had plans to test whether the brake light switch, the speed sensor, or the TCU was acting up, but now he tells me it doesn't do it anymore. If it sstarts acting up again, maybe we'll learn something new. I've learned a lot so far...