i seem to be having an issue with my trans. its a 4l80e in a 2000 gmc(classic body) 6.5td. i have searched the internet and read many articals on this trans, but none seem to address my problem. i will do my best to discribe the problem in detail.

1st 2nd and 3rd gear all seem fine. the problem seems to be with the lock up on the converter in 4th gear. when there is a load on the engine going up hill or on the gas it seems fine. as soon as the load comes off the engine the converter unlocks and won't relock until a load is added again. trans has only about 45000 miles on it and the truck has 135000 mostly orignal parts. does anybody know what could be causing the problem. i hate to take the truck in for service as everytime i do i get the run around. i had to bring the truck in 5 times for FSD before they would replaced it and 3 times for my trans going into limp mode before they would replace that.

thanks in advance for any help you can give me. i can't be the only one to ever have this problem.

http://www.transmissionspecialty.com/parts/DM/4L80-E.htm

Gary

i do belive it is a TCC apply/release problem. but i have no check engine light or codes. could it still be the TCC Regulator Valve or Actuator Feed Limit Valve without throwing a code?

Forgive me if I'm not understanding your problem correctly, but it is *normal* for the converter to unlock when you are coasting. In a gasser, it stays locked.

Most gas and diesel automatic transmissions will unlock the converter as soon as the throttle is released, this is to prevent any damage and stalling of the engine if a panic stop locks the wheels.

i am talking when you reach crusing speed (steady speed not accel or decel) the converter unlocks. the converter only locks up like it should when you are accelerating or have a heavy load on the motor(trailer or up hill).

to explain it another way

if i merge onto the highway and press on the accelerator the converter is unlocked (normal). as the truck is getting up to speed the trans shifts 3rd then 4th then the converter will lock. without moving my foot on the pedal the truck will eventually reach a point where it is not gaing speed. as soon as this happens the conveter unlocks and the rpms go up.(not normal)

at slow speeds(0-35mph) the truck seems normal and shifts fine.

i am going to drop the pan change the fluid and filter and take a look around to see if there is any chunks of metal in the pan. i am not holding my breath this is going to fix the problem.

changed the fluid and filter today still having the same problem. any more ideas?

Well, I can't help with the diagnosis, but I do know of a very reputable shop in Allentown, PA (depending on where you are in NJ, this might be an option). They are Fuel Injection Services and they work on 6.5's all the time. Ask for John.

i fixed the problem monday it goes to the dealer. already ordered my new 08 with the LMM motor. problem solved

I can't think of a better way to solve your problem! :D Enjoy the '08!

4-6 weeks up to 3 months the wait time is killing mel. it hasent even been two weeks yet and i cant wait for the new truck.

I have the same problem and was hoping for a less expensive fix.

If your not getting a TCC slip code then I'd suspect the PCM is shutting it off.
Your going to have to look for a control problem.
Brake switch, vaccum sensor, TPS sensor, Temp sensor.

They all check out good. Thats what is throwing me off.

I am having this problem now too, I believe. I can get up into 4th (OD) no problem, then into lockup, but after a bit it seems to come back out of lockup and the rpms go up. Can't get it to re-engage, tried playing with the throttle and tapping the brake pedal. I already replaced the brake switch when I was trying to fix another problem.

Here's what I've done so far-I lost OD altogether, was driving it in 3rd for a few months while I tried to fix it. I replaced the brake switch, ECT sensor, TCC solenoid, removed and cleaned the rear VSS and put it back, then I did the Sonnax Sure Cure valve body job. That got the transmission working perfectly for about 15 miles, then I lost OD again. Finally I replaced both shift solenoids, which I had convinced myself weren't bad. Wish I'd done that before spending the money on the Sonnax kit, because the solenoids got me my OD back and it's still working. Only problem is like I said the lockup issue. Throttle position sensor sounds like a likely culprit at this point, since the actual trans temp sensor checks out okay. I don't know about the vacuum sensor, didn't even know there was one.

I have had the pan down as well as the valve body a bunch of times, and the tranny is spotless inside. It has worked hard plowing, towing and the occasional burnout, but it also has been maintained carefully so I think it is still solid mechanically. I hope I can nail down this lockup problem because I was planning on putting some money into the truck this year; Heath ECM, hanging ATA and marine injectors. But if I don't have the tranny right I'm not going to.

I still think it's an electrical gremlin as opposed to mechanical failure, however I think it's possible I hurt the torque converter being stupid last winter...

DD is the trans even attempting to go into lockup, say after starting up the truck and heading directly for the highway, does it try to lock up even once? If it trys once it maybe a indication that the lockup clutch is bad as the computer will see to much slippage and unlock the clutch, if it just doen't lockup at all you may have a different problem. There is a lockup solinoid as well is that one of the solinoids you changed?

This may sound like an extremely unlikely thing to check, but check to see if your brake lights are functioning properly-that they come on when they should and go off when they should. The brake switch/fuse unlocks the TCC solenoid. Also, does the cruise work, if it doesn't, you probably have an issue with your brake light circuit

I maybe wrong but check to see if there isn't two switches one for the LU and one for the brake lights.

I've been driving it for a little while now, it seems that what it is doing is going up to OD, then to lockup, then after about a mile it comes out of lockup and the rpms go up some and it stays. I've tried manually downshifting to see if its OD or 3rd, I can't tell any difference. But it will drive in OD, before I had to manually go to 3rd because it would just slip like mad.
I did have occasional problems with the brake light switch so I replaced that. It used to occasionally cause me to not be able to get it out of park. I'd have to reach up under the dash and wiggle the wires. But that seems to be cleared up now. Pretty sure my brake lights are working right, I check all my lights fairly regularly, but I will check them again. Last time I used cruise control it worked. I'll doublecheck that again too.

1 more check DD leave your truck in 3rd (gear lever) over the the speed of 45 mph with 4.10 gears your truck should lockup see if there is any difference in how your lockup works, does it lockup and stay locked up properly or is it the same as in OD?

1 more check DD leave your truck in 3rd (gear lever) over the the speed of 45 mph with 4.10 gears your truck should lockup see if there is any difference in how your lockup works, does it lockup and stay locked up properly or is it the same as in OD?
Can't feel it lock up when I'm in 3rd. What else I notice is when I use OD, it takes a couple miles to come back out of lockup, then it feels just like 3rd. In fact I can manually pull it down to 3rd and feel absolutely no change. Maybe I'm not getting lockup at all, and I'm actually coming out of OD into 3rd...
Another thing I've been noticing is when I get off the main road and into my neighborhood, I'm doing about 15-20 mph, no idea what gear it's in, but I feel it "hunting". I think it's tranny, not engine related. It's kind of a surging that you can feel and hear. I think the tranny is trying to make up its mind what to do.
Another thing, I've ben having ABS trouble lately too. In fact I pulled the fuse because I got tired of the ABS action every time I come to a stop. I had a wheel speed sensor go a year or so ago, replaced both now it's doing the same thing. And sometimes I'm not sure if the trans is downshifting right, it seems a little harder to stop the truck (before pulling ABS fuse).
Man I'm getting frustrated.

Mine is doing this same thing, and its defiantly the converter locking as when I slightly press the break the behavior is 100% exactly the same..

I have a 93 its mechanical injection.. What sensors are they that will take the truck out of lock up on a 93?

I just replaced the temp sender on the heater coolant line on the front of the motor as it was broken off and disconnected by the previous owner. This fixed my no over drive when cold issue but now I have this converter lock issue.. It's got to be a sensor somewhere, as its defiantly the lock up disengaging..

I feel better knowing what it is but now I really want to fix it.. its annoying.

Bump.. Can anyone help us out here?

You need to help yourself a little, I gave you a link how to check codes. The computer may well be telling you what the problem is if you ask it. Heres that link again http://dieselplace.com/forum/showthread.php?t=16399