makWS6

iTrader: (4)

Guys I have a unique issue with my transmission. First off it a 01 ZR2 Blazer with around 148,000 (reman tag on trans with no idea on mileage). I bought it last year cheap and did a motor swap so I've only put around 5,000 miles on it. For the first time out on the interstate about 30-40 mins of driving it would slip out of 4th and into 3rd. TC is staying locked and I can manually shift into 3rd with no RPM change. Only time it would slip out is under a very light load (downhill and cruise) and 65+MPH. As soon as it slipped out I let off the throttle and shifted right back into 4th, but about 20 seconds later same thing. If you stay in the gas it never slips out. I went ahead and changed the TPS because I had another, but no difference. I did notice that if I pressed the brake the TC did not unlock, but downshifts ok. Truck shifts great with no other issues. Onlt thing I've done to the trans since I've owned it are fluid/filter change and vette servo. Seems to only be an issue when the trans is warm. Any ideas? Thanks a lot!

mrvedit

You probably need to hook up a scanner that displays the PCMs commanded gear. I suspect that the PCM is commanding the downshift and there is nothing wrong with the trans itself.

Just a guess - I recall reading that the truck PCMs have logic to detect a too-heavy load (like towing a trailer) and then lockout 4th gear. This might be based on high trans temperature or converter slippage.
Could someone that knows this better please confirm or refute this, or make other suggestions.

makWS6

iTrader: (4)

I planned on doing some scanning Monday to do exactly what you said. I just thought it was strange that it happens exactly at 65 MPH. I'll let you know the outcome. I also have another good PCM coded for my VIN.

makWS6

iTrader: (4)

Well I did some data logging yesterday and the PCM isn't commanding it into 3rd. Trans temp was staying steady around 180. I noticed that it would only slip into 3rd under 10% on the TPS and has to be over 65 mph. Any ideas? Thanks

mrvedit

Did you mean to say "isn't commanding it into 4th"?
If not, I'm confused. If yes, please correct your post.

makWS6

iTrader: (4)

Sorry for the confusion. The truck is dropping into third without being commanded to do so. Shows im in 4th tho Im not. Better?

mrvedit

It does seem surprising that it would shift from 4th to 3rd without being commanded. I now suspect the 4th apply circuit has a leak. This could be at the 4th gear piston in the servo or a leak the 3/4 accumulator under the separator plate.
While those could be checked, I would first test line pressure to make sure it is up to spec.
Most 4th apply circuit problem can be fixed without removing the trans.

BTW - Are you aware that sponsor RPM Transmissions is in Anderson, IN?

makWS6

iTrader: (4)

I'll take your advise and start off with a pressure check. Yeah I do know the RPM boys and have had T56 work previously, but I have much cheaper options being as this is just an old DD not a performance vehicle. Thanks for your help

sefiro

Any luck on thus? I have the same problem.
Background/history: 2004 Sierra. Orig tranny made it to 110k before loosing 3rd/4th. Had it rebuilt (probably with stock parts). At 135K, the above problem started (may have occurred first time but I was not paying attention). Took it to the rebuilder who could not recreate problem. Symptom kept occurring when I took car on Hwy and drove at least 20miles (never occurred in stop and go around city). Lost 3rd/4th gear at 150K - just out of warranty. Took it to a new rebuilder and paid for upgrade rebuild - shift kit, vette servo, bands etc - 4 months ago. Drove around town w/o problem and an occasional - possible above symptom. Took it long trip 2 weeks ago and burned out 3/4 again. Took it back for another rebuild (warranty) - they upgraded clutches another notch. Symptom came back on the first trip above 15 miles on hwy. Same symptom - around 65mph and with just a touch on tps - it feels like its either slipping to 3rd or convertor let go. IDEAS?