4L80E transmission

I have a 2000 GMC with 5.7 engine & 4L80E transmission. Truck has 121000 miles, but it appears that the transmission and TC have been changed. I have had the truck about 14 months and it was done sometime before I got it.
TC is coming unlocked and it is dropping back to 3rd gear. Can pull off to the side of road , turn off switch and give it about 30 seconds, restart and it does fine.Then it may go 30 miles or it might go 130 before it does it again. Doesn't seem to be a connection to how hot the fluid may be.It always happens between 50 & 60 MPH and when you are only slightly pulling. (Service Engine Soon) light has never come on. It is throwing a 1870 code though
I have changed the fluid and filter and it looked good. Nothing in the filter or pan and the fluid looked and smelled like it had just come out of the bottle. The front of TC did not have any sign of hot spots on it.
I'm no mechanic, but I think it is an elec. problem, but don't know how to trace it down.
Anyone have any ideas?
charles
.
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That code indicates slippage errors, could be the TC itself or low line pressure or the usual problem with the aluminum valve bodies. It must be scanned by a pro with a bi-directional trans capable scanner to command and confirm trans solenoid operation and look at slippage and gear ratio errors to pinpoint the problem.

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I was under the impression the 5.7 came with the 4L60E. Is that wrong?
Al
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Per Big Al:

Mine did.
--
PeteCresswell

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My 2000 Chev C2500 came with the 4L80E I think as part of the towing package. Steve
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It is a 4L80E. It is also the 3500, maybe that is why it came with the 4L80E instead of the 4L60E.
I had it scanned at the GMC place like Shep said. They said TC and want $900.00 to install a new one with a 12 month or 12000 mile warranty.Guess that is about the going rate for a dealership.
How does it slip that bad, without putting grit in the pan & filter, or giving the fluid a burned smell?
Thanks charles

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This can be caused by low trans pressures also, but the scan would have seen a seperate code for that. The material in the TC would be trapped in the filter a, these are closed so it would be hard to see. An independent can do this cheaper using a reman TC. You would not usually generate enough heat with a failing TC clutch to generate that type of temps unless maybe towing in in o/d.

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Never did in a 2500 or 3500 series truck but 1500's always came with 700R4's or later 4L60's. THey have been using 4L80 behind 350's in HD truck since they stopped using the THM400 (about 92) because the 4L80 is basically a THM400 with OD and a locking converter added. If engine load is to high for speed due to tune of load of effective gear ratio (axle ratio and tire size) the tranny will unlock Personally I would be looking at engine performance and effective ratio issue before looking at tranny because a 4L80 behind a 350 is a pretty bullet proof combo ----------------- TheSnoMan.com
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Diagnostics for either trans is the same for this issue.

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