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Automatic transmission not upshifting


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I have a '99 GMC Sierra (old style) 4x4 that is having shifting issues in regular drive. The truck has 220,000 miles and the transmission is original.

They seem like electrical issues is why I hesitate to just go with a rebuilt transmission.

Here is what is happening: At times the transmission does not seem like it wants to go through the gears. It is not too noticeable until about 45 MPH, then it is obvious the gear is too low and the rpm's climb to about 3,000 (2,000 is more normal when cruising). It has never self-corrected and "kicked-in" but I have noticed that if I stop, shut it off and restart it, then it might work fine. I sometimes have to do this 2-3 times, however.

Suddenly working after a restart seems like an electrical issue, but a solenoid was replaced (twice, on the possibility it was defective) but the problem keeps coming back and is becoming more frequent.

Do I need a new transmission, or is there another issue going on?

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This will prob sound crazy but hear me out.

 

Replace the ignition switch.

 

Everything runs through the ignition switch. Had a friend's 1999 C2500 GMT400 generation truck. All of the sudden wouldn't shift out of first. Pulled transmission, completely rebuilt (truck had 200k). Drove truck with rebuilt trans and same problem. Wouldn't shift past first. Called around and was told replace ignition switch.

 

Replace ignition switch (in steering column). Drove truck.

 

Wait for it....

 

Transmission shifts as it should!

 

(Drop mic, walk off stage) ;-)

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  • 1 month later...

Still having this intermittent problem. Solenoid replaced twice, just in case the first was bad. I then had the ignition switch replaced with an AC Delco part, as per the above post and the recommendations of a transmission shop. Still an intermittant issue with not upshifting. The next step I am planning is a new transmission wiring harness.

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The ignition switch is a valid possibility; don't know how to troubleshoot to be sure that's the problem, unfortunately. I guess if it were me, I'd hose down the inside of the ignition switch with a good contact cleaner, or at least WD-40, & see if anything changes.

 

Watch your tach closely. For lots of glitches, the tranny goes to limp mode, which means 2nd gear only: starts from a stop in 2nd & never upshifts.

 

Is the SES* lit? If it were me, I'd get someone with a code reader to see if there are any codes, whether the SES is lit or not. With at least some of the codes, if the same fault occurs in the same mode of driving twice, it will store a code in the PCM.

 

*Service Engine Soon

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