Help Troubleshoot 4L80E TCC issue (kinda long)

[themaddhatter]

Alright, it all started like this:

I recently bought a 91 burban 3/4 ton 4x4 with a 4L80E. This is the third trans (complete reman 20K ago). Rig has 165K on engine.

When it gets up to 68 mph in OD, the torque converter goes into lockup, then it goes out of lockup like 10 seconds later. Then it repeats this many times, until I slow down fearing damage. Al the while, no codes are being thrown.

Put it on the snap on scan tool. Few things that I noticed:
1: the input and output VSS vary ever so slightly
2: the TPS at no pedal didn't go to zero (tweaked it and now it does)
3: no high pressures/odd temperatures/etc
4: at idle, torque converter slip varies around (jumps from 0 to 10%)
5: The computer says I am going 9 MPH slower than my speedo (factory everything, no odd tire size change, and I have verified the accuracy of the speedo: it is dead on)

Have changed the following as a result
-coolant temp sensor (for $hits and giggles)
-TPS
-Brake light switch (often a hidden culprit)
-TTech flushed the trans

After these, still does it. So, I went to a trans shop that has a computer to override the ECM (He called it a Transstar?). He plugs this into the ECM plug and the trans plug, and can manually control the works. He activates the solenoids, the TCC, and the pressure solenoid. Everything electrically works fine inside the tranny. If he tells the TCC to hold lock, it holds lock. so, this means that something is telling the ECM to not maintain the TCC lock, then the ECM takes it out of lock. Then the ECM tries to do it again, so on and so forth.

His feelings were this: the engine is gummed up, and it throws off everybody else. Too much carbon in the engine, cat may be bad (original cat), compression may be low (doesn't burn oil). He recommended doing a complete tune-up (was going to do anyway), get a good intake cleaning chemical (like hydro blast) to clean the intake and the valves, clean the TBI body and injectors, and start from there. Check compression ratio, etc.

Next, if that doesn't do it, replace the DRAC unit. I thought that my vehicle didn't even HAVE a DRAC (input and output VSS wired direct to ECM, where trans controlled).

If that doesn't do it: engine is too tired and replace. It runs rather well, and I don't accept that as an answer.
It idles well, has good pull, and never makes a peep. No lifter noise, nothing. sure you can't squeal the tires, but this is a 12 year old suburban.

So, I guess what I am getting to here is:
-has anyone had carbon problems give them issues like this?
-anyone know where the DRAC is located?
-Is it worthwile to replace the output VSS?
-Is there a way to check that the VSS is even bad?

Any help here would be appreciated. I am getting tired of chasing phantoms here.

 

[ratlover]

What throtle position are you running at when its unlocking?

 

[Blue85]

You've got to think about all of the inputs that feed the lockup output and look at each one. My guesses would be:
-VSS
-coolant temp
-Throttle position
-MAP
-??

It's also possible that you have a bad wire or connector somewhere that is making intermittent contact. Could also be the EMC itself has a bad output driver for the TCC

 

[themaddhatter]

Ratlover: Good question on the TPS location at the TCC lock/unlock. While I bet they are relatively the same, I think that they may vary slightly (depending if I am on a grade/accelerating, etc). It starts doing it at a relative RPM (not speed dictated, because I can do it at a slower speed with it in D not OD) and keeps doing it as I go faster.

For the sensors and wires, I was thinking along those lines as well. The scan tool will only scan at a set rate, and not necessarily give me if something drops out on occassion.

I have gone out and gotten prices on the VSS, but no one has been able to answer my question about the DRAC (or whether or not it even has one or if the computer itself does it).

Thanx

Shane

 

[Itali83]

When you adjusted your TPS to be a 0v at no throttle that is wrong. I'm almost positive the specs are .45-.75 volts at idle. It's in that range there. You don't want the tps voltage to be 0v at anytime. That could be your problem right there. I'd fix that first so you at least have the sensors where they belong before you trouble shoot anymore.
Ben 87 Jimmmy + 89 Blazer
/forums/images/graemlins/usaflag.gif

 

[dyeager535]

I find it hard to believe that a "worn out engine" can be the cause of your problem. If it was "worn out", and your tranny somehow knew, that would mean that A)GM hid compression ratio sensors in every cylinder /forums/images/graemlins/smile.gif or B) you were blowing so much oil smoke that it was throwing off your O2 sensor, in which case you'd throw a code because it would be constant.

Personally, sounds like he's making some of that up...not always reality, but theoretically any "gummed up" sensors should be showing either bad values when he scans it, or throwing codes.

Does the problem only occur over 68MPH, or is it any time the tranny tries to lock up? I'd have to *guess* its a problem of VSS coupled with TPS if not.

Pretty sure MJ has a 1991 suburban wiring diagram on his webshots album, might want to check there for how VSS is run, if it is even shown, it may not be.

 

[themaddhatter]

Yeah, it doesn't smoke (even on startup) but it is kinda gutless. I am going to check the inlet/outlet temps of the cat, and see if that is plugged up (still original).

It is definately an RPM event. In Drive and OverDrive, it happens at 2350 RPM or higher. We restored the TPS to what it was, replaced the air/fuel filter, have run injection cleaner through the tank and through the body. Am going to do the plugs/wires/cap/rotor this weekend, and see what happens. The scan tool shows a good engine vac, MAP looks good, EGR seems fine, O2 a.o.k., VSS (trans and transfer) seemed fine on the scan tool. Vehicle speed reads correct.

We went through the trans harness, and that is fine as well.

Oh, Who is MJ? Anyone have a link to their site?

Thanks

Shane

 

[dyeager535]

I don't know as I'd spend that money on all those tuneup items, but thats me. I'm kinda cheap that way, but unless they are unknown mileage, I wouldn't just replace them all.

Cap and rotor are prime examples, there is NO reason to replace them if you can't see the problem. There is nothing to them. If they are cracked, corroded to the point of lost contact material, or you see carbon tracking, they should be replaced. If not, they are fine, or can at least be "reconditioned" with a piece of sandpaper to the contacts. You can even tighten the spark plug terminals if they are loose on the cap.

I'm only saying this because adding those things into the equation means more money you will be spending that *won't* cure the problem. An ignition issue isn't related to TCC lockup. The sensors for lockup to function were already listed, and a problem in one of those circuits will surely be the cause of your problem.

When you have to replace or fix one of *those* components after spending good money on parts that weren't the problem, you might feel like you wasted the money.

I'm sorry I can't help with the lockup issue itself, if something has failed or is adjusted incorrectly, you'd *think* you just wouldn't have lockup, not on/off, especially when you know the TCC works. Perhaps the voltage to the TCC isn't great enough? Not a whiz with electrical stuff, but since it's a solenoid, I wonder if just enough voltage would allow it to lock, but under the increased pressure from higher RPM's, if the voltage is too low, it would kick back out?

 

[87GMCJimmy]

You need to look at why the Tcc locks unlocks. I don't remeber all of them off the top of my head but here are a few and this is based on a 700r4 knowledge but I imagine it's similar.

It will not lock under a certain speed, it's around 45 with stock gearing and tires.

It will not lock in 1st or 2nd I believe.

It will unlock with a TPS reading of idle, I don't think that the idle position should be 0 volts. I thought idle should be .08-.11 volts or something and WOT should be in the .40-.45 range.

I definitly don't think the motor will be the cause. Get the manual and check to make sure sensors are adjusted properly. I think this will help more than a tune up.

Good luck!
Mike