1997 Sierra PU 5.7L -auto transmission

1997 GMC SIERRA
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DIBS
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Over the last month or so I've noticed my transmission starting to be a little delayed in shifting from 1st to 2nd. Yesterday the truck actually jerked and popped (bucked) going into 2nd, 4 times in a row as I went down the street. Took the truck in to the mechanic and the fail code was P18 something (transmission related) and P141 (oxygen sensor).

Mechanic said he cleared the codes and if the codes come back in a day or so, it's rebuild time! But, if it takes a few days before it happens again, then he may have a fix for it by boring out one of the valves and replacing it?

Any thoughts?

This comes just two weeks after I had the transmission serviced (came back all clear and included a pressure and flow check), which was the done due to an overheat and blowout towing the boat home. The truck has 128K miles and I've never had any problems with it.

Thanks in advance.
Jun 17, 2007 at 11:48 AM
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DANIMALGMC
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I had a similar experience. The computer is sensing that the torque converter is not locking up. So after several attempts the tranny goes into protect mode (very hard shifting).
There is a valve that controls the locking up of the torque converter. The valve cylinder wall is aluminum and wears out. He will be boring it out and inserting a sleeve and a new piston device (Sonnax I believe).

Try this: As you drive down the road (in 3 not D), accelerate to 40mph and then slowly up to and settle at 45mph. You should notice the RPM should drop and you might feel a nudge when the TC locks up. At this point if you SLIGHTLY accelerate, you should not see a jump in the rpm. If the rpm rises (2000 to 2200ish) then the TC is not locked up. Keep in mind if you accelerate to hard, it will unlock the TC to meet the demand.
If you perform the action above below 40mph, you should notice the rpm increase as you slightly accelerate (just to give you an idea of what we're looking for).
Since I know what is happening, I always notice my rpm when highway or over 40 driving. Mine only acts up when it's hot or working hard.

Good luck
Jun 25, 2007 at 11:00 AM
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DIBS
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Thanks danimalgmc.

That was exactly what was happening. Mine was completely shot and not locking up at all. I had an overhaul done the transmission ~ $1900.
Jun 25, 2007 at 1:15 PM
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