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Not shifting lost third and forth gear

Tiny
SCBURTON
  • MEMBER
  • 1996 CHEVROLET 1500
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 315,000 MILES
Lost third and forth gear had the transmission rebuilt third and forth burned up again in less than 5 miles. The repair shop is telling me the PCM is the problem. Because it is not the original PCM. I did a swap to a 0411 PCM with a 2001 van calibration, same motor same transmission has been running like this since 2012 no problems. They said the PCM isn't communicating with the transmission. If that were the case it would throw codes and go to limp mode but the only codes I got were related to no shift. I commanded the solenoids with scan tool and they clicked like they were supposed to. When commanding third gear nothing happened. Is it possible for a PCM issues to burn a transmission up that fast. Or something they did wrong. They cant tell me how they came to the conclusion that it's the PCM.
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Wednesday, March 10th, 2021 AT 3:07 PM

9 Replies

Tiny
STEVE W.
  • MECHANIC
  • 13,476 POSTS
If the PCM has failed it could cause the transmission to slip and burn up the clutches by not applying pressure at the right time or at the correct pressure to prevent damage. If the PCM was working okay but the transmission failed the same way twice I would also suspect it's the PCM, as it could easily have failed causing the first issue, then the rebuild failed the same way for the same reason. Sort of like replacing the engine because it shut off and won't run, then installing the replacement and it only starting and shutting right off, then discovering it was all from a bad fuel pump. In this case I suspect an internal PCM failure.
Just because the solenoids respond to the scan tool doesn't really mean there isn't an internal issue in the control circuitry itself causing the problem.
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Friday, May 14th, 2021 AT 11:22 AM
Tiny
SCBURTON
  • MEMBER
  • 5 POSTS
I asked them if the transmission checked out good after the rebuild they ensured me line PSI was good and no trouble codes. My son picked up the truck he got about two miles. And called me and said something was not right I had him go back to the shop. He said the techs were looking under the hood I called my son and ask to speak to the tech he said my transmission cooler may be bad lines were very hot. He watched them hook up to the scan tool and said no codes came up. I ask them several days later if they figured out the overheating issue they told me it was my thermostat. But I had no issues with overheating before hand and I drove the truck quite a bit trying to troubleshoot the no shift issue. How can I determine if the PCM and transmission aren’t communicating? I also pulled the fuse and truck started in first gear like normal. I was told if you pull the fuse the PCM would default to third gear. But it didn’t do that stayed in first and shifted to second no problem.
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Friday, May 14th, 2021 AT 11:22 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,476 POSTS
The transmission cooler would get hot if the fluid was overheating from internal slippage. Being a 1996 that transmission doesn't have as much monitoring in it as the later units so it may not set a code. What was the original problem you started with? No 3-4 shift? Anything else?
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Friday, May 14th, 2021 AT 11:22 AM
Tiny
SCBURTON
  • MEMBER
  • 5 POSTS
Yes 3 4 slipping happened pretty much with no warning signs.
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Friday, May 14th, 2021 AT 11:22 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,476 POSTS
Have seen a few like that where it was poor line pressure control or internal leakage causing low apply pressures. The 3-4 clutch packs are a weak point in those transmissions even under good conditions. Hard to know for sure what failed without having it in front of me, you could have a different shop take a look and test the pressures to see what they find.
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Friday, May 14th, 2021 AT 11:22 AM
Tiny
SCBURTON
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  • 5 POSTS
Okay, thanks for your help. They are rebuilding it again so I hope it doesn’t do it again.
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Friday, May 14th, 2021 AT 11:22 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,476 POSTS
Let us know what they find inside, who knows, it could be they had a defective part.
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Friday, May 14th, 2021 AT 11:22 AM
Tiny
SCBURTON
  • MEMBER
  • 5 POSTS
Okay, will do. But at this point I don’t think they would admit they did something wrong. Seems like they never even diagnosed the original problem. But then they said they won’t release a vehicle until everything was checked. Not sure how they missed this one.
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Friday, May 14th, 2021 AT 11:22 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,476 POSTS
Depends on the shop, the guy I use would tell you everything about what was found, repaired, replaced. The problem is that there are a lot of shops that don't. Even if you showed up with proof.
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Friday, May 14th, 2021 AT 11:22 AM

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