Chevy 4l80 lockup won't stay locked

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May 5, 2020
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I have a 2004 chevy 2500hd 6.0 with a 4l80. Just rebuilt the transmission. New tcc solenoid and torque converter. The truck runs ok till it warms. Then when you get to 50 it will lockup 3 or 4 seconds later it will come unlocked. Sometimes it will lock again for a few seconds but most times it won't at all. When on the interstate it gets to almost 200 degrees and gets worse. Usually it runs like 140 degrees. On the last 3 hr drive the truck tripped the check engine light ode p0894. I don't know what to do I've replaced everything except the pcm. Someone must have had the same issue and figured it out. Please help i don't know what else to do.
 
Have you tried adjusting the brake light switch down at the pedal? There was a time when a maladjusted switch would cancel the torque converter clutch. Check all the connectors everywhere. Almost sounds like it's slipping if you're getting that particular code. Did you blow out the valve body and all the passages in the case?
 
Old and worn out? The TC that is. Clinebarger would know the issue is...

Until fixed I'd shift out of OD and that should keep it much cooler.
 
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Did the problem show up after the rebuild and new converter, or has it been there all along?
 
When the converter locks the output rpm should exactly proportion the engine rpm, if it does not you get that code. Are all speed sensors working?
 
I had a OD lockup issue with a '04 4L80 in a 2500 also. Mine would lockup for a few miles then unlock and stay unlocked unless you stopped and cycled the ignition. Every so often I would get random misfire codes but only at highway speeds after the OD kicked out. I will save the aggravating part and just say the issue was the ecu would not keep the CKP Variation Learn. I could relearn the crank position and it would stay locked until you cycled the power. Then it would unlock just like before. Eventually replaced the ecu and did a final CKP Variation Learn and no more problems.

Doesn't sound exactly the same but it's not always the part acting up that is the root cause.
 
Is the lu solenoid doing as it is commanded? Or is there a hydraulic/mechanical issue somewhere?
A P0894 indicates the lu solenoid is commanded on but there is slip between the flexplate and the converter output.
 
Originally Posted by supton
Old and worn out? The TC that is. Clinebarger would know the issue is...

Until fixed I'd shift out of OD and that should keep it much cooler.

Bah, I see now that I didn't read closely, and that's a fresh rebuild with a new convertor. Disregard my comment, sorry.
 
Sounds like a wore out TCC Regulator Valve Bore, Assuming everything else is sound? Sonnax 34994-01K is a drop-in replacement & fixes 90% of worn valve bodies I've ran across.

A sipping 4th/OD clutch can also set P0894, The ISS sensor reads the Forward Drum NOT the Input/Turbine Shaft!.....So that leaves the TCC & the 4th Clutch between the Flexplate & Forward Drum. I've seen sheared off 4th Clutch Housing bolts cause this code. But your description doesn't lead me to believe you have this issue.

A PCM or Brake Switch failure is unlikely given the P0894, Checking the output of the TCC PWM Solenoid control circuit is never a bad strategy, But likely a waste of time.

When the TCC Regulator Valve wears out the bore.....The TCC Signal oil will leak to the exhaust porting (Arrow), The hotter the fluid gets, The worse the leak becomes!

[Linked Image from i.imgur.com]
 
thanks for the info. Yea the hotter it gets the worse. This sounds like it could be my problem. I've got a bunch of valve bodies ill try another one just to see what it does.
 
Originally Posted by Weldlife207
4l80s aren't a overdrive transmission. They are a 4 spd with lockup


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4th is 0.75 . That's overdrive....
 
The problem was there before the rebuild. I rebuilt the trans along with the valve body with new valves. Then put new torque converter in it.
 
Could fooled me. I don't see overdrive in the shift indicator. I see some trucks do but not mine.
 
Originally Posted by Weldlife207
4l80s aren't a overdrive transmission. They are a 4 spd with lockup


The 4L80 is the bones of the classic turbo 400 with a lockup converter and an added planetary for overdrive 4th gear and electronic controls.
 
Originally Posted by Weldlife207
Could fooled me. I don't see overdrive in the shift indicator. I see some trucks do but not mine.


I think they may of stopped marking that. Probably just states D, 3, 2, 1
 
it sounds like similar issue. Mine when you come to a stop and start going again it will lock up for accople seconds the same way then come out
 
I think getting rid of "OD" on the shifter was to reduce confusion among the average owner. A lot of people think that "D" is the proper selection for regular driving, and their gas mileage would suffer.

The converter is only supposed to lock at highway speed. If you're having strange bumps when taking off from a stop that is probably another something going on within this rebuilt transmission.
 
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