foreveryoung001

This is kind of long post, but really want to give everyone an idea of what is going on and what I have done so far to diagnose it. I really need some advice from the experts here.

Bought this 96 4.0 auto a couple of months ago. Been fixing it up, but can't get a handle on this one issue.

When I'm coasting at speeds over 45mph, like going down a steep hill, the torque converter will unlock, then lock, then unlock, making the rpms jump up and down.

So far, I have adjusted the kickdown cable, cleaned the throttle body, and replaced the TPS with a Echlin TPS from NAPA.

It shifts very smooth, and from what I can tell, it shifts at the correct speeds, but this torque converter problem just won't go away.

I also tested the switch on the brake pedal by touching the brakes while holding a steady speed at 55mph. As soon as I press on the brakes, the torque converter unlocked, and as soon as I stopped braking, it locked right back up.

The tranny fluid looks like its in great shape, and is at the proper level, but was going to drain and fill as soon as the weather warmed up a bit... but unless its something other than the recommended dextron3, I'm thinking the problem lies somewhere other than the fluid.

I'm wondering on a couple of things. The little plunger on the kickdown cable, when I adjust it, comes all the way out. Is it possible that the cable has stretched over time and needs to be replaced. Basically, I'm thinking the self adjuster just might not have enough range to get the cable where it needs to be. I'm wondering if this is the case, because when the torque converter is doing this, if I just lightly rest my foot on the gas pedal, it immediately locks... and I mean "lightly". Like its just enough pressure to take out any slack in the throttle cable, but isn't causing the Jeep to accelerate. I'm not sure how that whole system works on the AW4, but seems like an extra millimeter of tension in that cable is all it needs to behave... unless the TPS is reading that slight pressure... I just don't know enough about it to diagnose it.

My other line of thought would be a bad TCC solenoid, but again, from what I've experienced on other vehicles, a bad TCC solenoid would cause the torque converter to stay locked up or not lock up at all, but not jump between lock and unlock while coasting. I haven't climbed under the dash with a meter yet, but I'm really thinking it is something other than the solenoid.

Any thoughts would be really appreciated.

SeriousOffroad

TCC Lock-Up signal goes through the normally closed contacts in the brake light switch. Next time it happens keep your right foot on the gas and lift the brake pedal with your left foot. If the RPM's settle, you need to adjust the brake light switch.

foreveryoung001

Yeah, should have mentioned that I tried that as well. There is no difference in the symptoms when I pull back on the brake pedal, but thanks for the idea.

cruiser54

Why not LOOK at the switch that the brake pedal rests against when not applied?

93gc40

Have you adjusted the TPS yet.. You sure it the converter locking and unlocking and not the tranny going in and out of OD

SeriousOffroad

TPS is not adjustable on a 96.

PocketsEmpty

I have a similar issue on my '98. Very similar actually, except it happens when the tranny selector is in 3. I contributed it to my 31's and stock gearing, but maybe not...are you stock or running bigger tires, etc?

Subscribed for ideas.

1991Jeep_Man

This happens on my 91 as well. I always figured it was somehow normal. I will check my brake switch and also watch this thread like a hawk.

Same symptoms with an otherwise perfect performing AW4.

foreveryoung001

That was my first thought of what was happening when I first noticed it, but from how the RPMs act, I'm pretty sure its a lock up issue. When going downhill, if I'm already in OD and take my foot off the gas, the RPM's fall, which tells me that the torque converter just unlocked.

I just visually inspected the Brake switch out in the parking lot, here at work, but not 100% sure what I am supposed to be seeing. Its in tact, the wiring looks good, no corrosion or fraying for the length I could follow it.

The plunger is touching the brake pedal though. Should I back that off so there is slight gap?

I'll search the forum for some more info on it when I get some time this afternoon, but in my head, I'm questioning why, if the problem is the brake switch, would it only effect the torque convert while its going downhill? Seems like if this were the cause, it would give the symptoms all the time... but I'll read up on it, since I admit my knowledge on this issue is sorely lacking.

Thanks everyone for all the advice so far. I will get this figured out.... it is a moral imperative!

foreveryoung001

Everything is stock right now. We'll see what the summer brings. LOL. I just had a good conversation with a mechanic friend of mine. He gave me a couple of other ideas, but I won't have time to mess around with it until I get home form work this evening.

93gc40

Taking your foot off the gas SHOULD normally cause the rpms to drop, motor slows down, and if going downhill the TC might actually lock causing the rpms to drop faster.

foreveryoung001

Understood, but this is a jump down, not a normal, gradual slow down, and then it jumps back up. I don't how else to describe it, other than a jump. You can feel it, when it happens.

On the flip side, on a flat or uphill road, when I remove my foot from the gas pedal, there is gradual slow down in the RMPs, and it behaves like every other vehicle I have driven (well, the ones that worked correctly), but as soon as the road tilts downhill, it starts the jumping... the first jump in RPMs is always down. If I touch the gas pedal, you can feel the torque converter lock in again. I have experimented with this on every road in my town, trying different speeds, different grades of hill, and different techniques in lifting my foot.

The tranny shifts smoothly through all the gears and the torque converter usually locks between 40-44mph, which, from my understanding, is within the expected range. I do not mean to dismiss your argument, but I am as close to 100% sure, as I can be, that my torque converter is locked when this issue happens.

93gc40

It's the kicking in and out that leads me to suspect either the TPS or brake switch is causing the issue. Something is telling the solenoid to engage/disengage..

IDK, just asking the questions I would ask myself.. Based on my understanding of how the converter work, which could be wrong. I have no way of knowing whether or not you have already asked or answered them or your level of experience.... Not like I threw the, Is your fluid burned? question out there.

foreveryoung001

No worries. And my level of experience is relatively small, especially when it comes to Jeeps, but not quite a novice when it comes wrenching.

I just don't know enough about automatic transmissions to troubleshoot this logically. If I had a better understanding of how they work, how the different sensors work in harmony to control shift points, and all, I might be able to see the bigger picture. Right now, though, I feel like I'm just throwing darts at the wall, but since I've already gone through, my understanding, of the "usual" suspects, I don't know what else to do other than keep throwing the darts.

But that's why I posted up the question, to get other people thoughts, because I just don't feel like I'm thinking clearly on this one. I just posted the question yesterday, but I've been trying to diagnose this for the past couple of months. I guess the upside is, its not getting worse, and everything I've done has only improved how the jeep is running and shifting... it just hasn't affected this particular problem.

So please, if you have anymore thoughts, do not hesitate to ask.

kjc

I'd have to agree with cruiser.. check/adjust/replace the brake light switch.. On my '93 the piece holding the switch broke so it wouldn't stay in position. I couldn't get a new/used one, so i 'honky rigged' it and its been fine since.
When it was broken, the overdrive and torque converter lockup was fubar. Now it's good. If I'm say cruisin' at 60mph, w/o letting off the gas and touch the brake, the TC unlocks immediately with a noticeable rise in RPM.. I assume it also drops out of OD (cause the manual says so) but I have no proof of that. Coming out of OD (downshifting) also raises the RPM as I've shoved it down into 3rd on occasion while in OD with no change in throttle position. The RPM go up/down about 500 when the TC locks/unlocks, and about 1000 for the OD. I hope your issue ends up being something simple <--(cheap!)