1996 GMC Sierra Transmission whinning in drive and reverse.

1996 GMC SIERRA
147,333 MILES • V8 • 4WD • AUTOMATIC
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ONEFASTFREDDIE
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I just this week purchased a 1996 GMC Z71, 147333 miles on it. The previous owner had changed the transmission not long before selling the truck. He installed a transmission from a 1998 (Best I can tell from its removable bell housing and hex 6 bolt output) The trans shifts and drives OK but it is making a pump whinning noise when you put it in drive or reverse. No noise in park or neutral. Also when driving at speeds above 20, when completly off throttle or coasting there is a much louder whinning noise that is variable with vehicle speed not engine RPM. (Like a bad bearing) this noise is not affected by slipping the trans into Neutral. Could this be caused by the difference of year model in the computer or is this trans going bad? I changed the filter and fliud but no difference. (Filter looked original but was not as dirty as I expected, not very much metal on the magnet either. Fluid did not smell burnt but was overfilled by at least a quart or more) Thanks Fred
Oct 19, 2010 at 6:38 PM
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SERVICE WRITER
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Not sure on the compatibilty issue, I shy away from problems caused by parts that were not meant to be installed. A scrapyard should be able to inform you of the compatibility.

Failure of the bearing on the input shaft housing was common, and may cause damage to the reverse input clutch and the gear sets caused by this failure.

Assuming this is in the trans an not the differential.

I have heard of a ground problem at the pcm. but I think that was for the 2wd.

I would put it up in the air and run it. Using a stethescope you may better determine if the noise is from the trans or diff.
Oct 21, 2010 at 5:49 AM