Transmission issue

JASON_PETERSEN

MEMBER
1999 CHEVROLET TAHOE
5.7L
V8
4WD
AUTOMATIC
240,000 MILES

Okay, my SUV will not up shift or down shift on its own. I can put it in reverse, first, second, and drive just fine with no hesitations and motor runs fine. ECM/PCM has been replaced with new one and has been properly flashed and coded to for vehicle. The motor was replaced by GM 40,000 miles ago, Transmission was replace about 30,000 miles ago. Sending unit and fuel pump was replaced two weeks ago because fuel gauge pegged to full. After replacing fuel pump and sending unit the gauge still is pegged on full. Ignition Module on column has been replaced. Still truck has to be shifted manually through gears and still will not go into overdrive and will not up shift or down shift on its own. Transmission shop that installed it says diagram shows no signal from transmission no damage to wiring harness has been found.

1 Reply

HMAC300

MECHANIC
48,601 POSTS

You should have a mechanic scan for codes because it can be a broken wire on O2 sensor or anything causing this. The TCM may not be the problem, but you are getting no signal from transmission so there has to be a problem there I would think. It may be a transmission wiring problem like broken inside. I will send a picture along as well. It may be the connector where wiring connects to transmission.

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