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199 GMC Sierra downshifting

Tiny
OGDEN72
  • MEMBER
  • 1999 GMC SIERRA
  • V8
  • 4WD
  • AUTOMATIC
  • 165,000 MILES
1999 gmc sierra 4 wheel drive 5.0 liter 160,000 miles. In overdrive on highway, transmission will down shift to 3 rd gear, then shift back to OD, and repeat. When coasting down a long steep hill will do the same without touching the accelerator. Replaced the TPS. No change.
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Saturday, April 16th, 2011 AT 11:49 AM

3 Replies

Tiny
KHLOW2008
  • MECHANIC
  • 41,815 POSTS
Did you try to retrieve for trouble codes?

Possible cause would be the OD solenoids failing or has poor circuit continuity.
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Saturday, April 16th, 2011 AT 12:15 PM
Tiny
OGDEN72
  • MEMBER
  • 2 POSTS
No fault light displayed. Where are the OD solenoids and how you change? How perform circuit test and what results should I see?
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Saturday, April 16th, 2011 AT 1:51 PM
Tiny
KHLOW2008
  • MECHANIC
  • 41,815 POSTS
Sometmes trouble codes do not trigger the CEL or OD light. Get a scan done to check if any trouble codes are available.

One of the possible cause of the prbem would be low line pressure at idle due to a weak pressure spring in the vaolve body.

You would have to perform a pressure test to verify.
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Saturday, April 16th, 2011 AT 4:39 PM

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