1992 Chevy Silverado Slips out of Overdrive. Sometimes

Tiny
ZOFFINGER
  • MEMBER
  • 1992 CHEVROLET SILVERADO
  • V8
  • 2WD
  • AUTOMATIC
  • 170,000 MILES
My Silverado has a 4l80E transmission, and it works great. Except, recently it'll slip into 3rd after I drive it for a while. Especially if I'm heavy on the gas or if I'm going uphill. Once I lose overdrive, it won't come back unless I shut the engine off. After 30 seconds or so, I turn the motor over, get up to speed and BING! 4th gear is back!
So far, I've dropped the pan, replaced the filter and fluid and one of the two speed sensors (one looked as if the end of the cylinder that goes into the tranny had been chewed on or burnt)
The problem persists.
I figured it to be an electrical problem because there's no noise or vibration, but a local dealer's diagnostic computer says that electrically, it's fine.
Any suggestions?
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Monday, December 1st, 2008 AT 8:00 PM

4 Replies

Tiny
DOCFIXIT
  • EXPERT
The ECM must signal to lockup and unlock. If any slippage is noted between the two speed sensors it will drop out. Have you checked for codes by jumping between A & B on the ALDL?If you could get a scanner hooked up a drive and see if ECM is signaling unlock that would let you know if it is elect. Or mechanical.
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Tuesday, December 2nd, 2008 AT 10:05 AM
Tiny
ZOFFINGER
  • MEMBER
Hey Docfixit,

Thanks for the reply.
I don't have an Assembly Line Diagnostic Link. To get the tranny diagnosed, I waited six hours at a local double-A shop! Afterwards, they wanted $450 just to see IF they could find the problem, which they insisted was internal.
I remember the mechanic telling me that the computer WAS telling the tranny to shift.
Your thinking the ECM might act differently on the road verses on the lift where the diagnosis was done?
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Tuesday, December 2nd, 2008 AT 5:15 PM
Tiny
DOCFIXIT
  • EXPERT
Your truck have a one wire to ALDL? That was gm pre ODBII phase and only they have the Tech1 scaner to read truck. You might want to put a new solenoid for TCC. Need to pull pan in valve body not a big job. If ECM is signaling maybe switch is the fault.
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Wednesday, December 3rd, 2008 AT 7:21 AM
Tiny
ZOFFINGER
  • MEMBER
Hey Doc,
I'll try that solenoid and let you know.
And by the way, any meager donation this poor man would gladly give should this work couldn't possibly represent how grateful I am that there are folks out there willing to share knowledge!
Thank you. Your attempts to help mean a lot.
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Thursday, December 4th, 2008 AT 4:27 PM

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