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Transmission neutral shift when off throttle

Rich235

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Rich235 New Member

Rich235

Posted
Posted

Hi everyone,

New to this forum and hopefully all the knowledge here can put my mind at ease or suggest the remedy for my truck.

I own a 2012 GMC Sierra 4.8 4x4, 4 speed trans and 100,000kms. (4L60E)

I have only owned the truck 4 months, but have noticed something strange about how it drives. 

I place it in drive and start off. When I back off the throttle before it shifts to 2nd, the rpm will drop to idle. Then I press the accelerator pedal the rpms rise, seems like a slight slip and the gear will grab. The same applies to 2nd, 3rd and possibly 4th. But it is more noticeable in the lower gears. When I place it in manual, the truck will hold the gear, whichever gear it's placed in.

I took it to 2 transmission specialist. I finally bit the bullet and got one of them to investigate. They suspected a sprag starting to fail. Anyway, they stripped it down and found very little wrong with it. All clutches were in good condition, no signs of any leaks. The only real problem they found were some brass bits from the torque converter and one of the pistons had some scoring down one side and thought perhaps the trans was losing pressure and possibly dropping out of gear. Anyway, they installed a shift kit, torque converter, replaced the piston and rebuilt the transmission. I drove it home today and sure enough, I have the same problem. It seems to go into neutral when I back off the throttle. My buddies 2008 truck is identical and doesnt do it. I had a 2002 Silverado and that didnt do it. Does anyone have an idea what the issue could be? Or is this some kind of fuel saving feature?

Thanks in advance.

Rich

swathdiver Senior Enthusiast

swathdiver

Posted
Posted
20 hours ago, Rich235 said:

Hi everyone,

New to this forum and hopefully all the knowledge here can put my mind at ease or suggest the remedy for my truck.

I own a 2012 GMC Sierra 4.8 4x4, 4 speed trans and 100,000kms. (4L60E)

I have only owned the truck 4 months, but have noticed something strange about how it drives. 

I place it in drive and start off. When I back off the throttle before it shifts to 2nd, the rpm will drop to idle. Then I press the accelerator pedal the rpms rise, seems like a slight slip and the gear will grab. The same applies to 2nd, 3rd and possibly 4th. But it is more noticeable in the lower gears. When I place it in manual, the truck will hold the gear, whichever gear it's placed in.

I took it to 2 transmission specialist. I finally bit the bullet and got one of them to investigate. They suspected a sprag starting to fail. Anyway, they stripped it down and found very little wrong with it. All clutches were in good condition, no signs of any leaks. The only real problem they found were some brass bits from the torque converter and one of the pistons had some scoring down one side and thought perhaps the trans was losing pressure and possibly dropping out of gear. Anyway, they installed a shift kit, torque converter, replaced the piston and rebuilt the transmission. I drove it home today and sure enough, I have the same problem. It seems to go into neutral when I back off the throttle. My buddies 2008 truck is identical and doesnt do it. I had a 2002 Silverado and that didnt do it. Does anyone have an idea what the issue could be? Or is this some kind of fuel saving feature?

Thanks in advance.

Rich

Rich, you really need a scan tool to see what's going on inside.  Also, get yourself a subscription to AllDataDIY so you can run through the troubleshooting guide in the shop manual.  Could be the pump slide spring in the pump or a valve body problem.  I suspect the former but such a diagnosis comes as I sit in my chair thousands of miles away!

Rich235 New Member

Rich235

Posted
  • Author
Posted

Hi swathdiver,

Thank you for your thoughts and diagnoses. I do have a fairly basic scanner and no codes are present. The transmission shop also checked for codes and they couldnt find anything reported or logged either. I will look into that information to see if I can try to isolate the issue. I do tend to feel its pump related.

  • 2 weeks later...
Rich235 New Member

Rich235

Posted
  • Author
Posted

Everything has settled down and all fluids have been up to temperature. I have driven about 300kms since the transmission rebuild. I have isolated the issue to 1st and 2nd gear now. I believe the new torque converter has improved things and made it a smoother drive. However, the rpm's still drop when off the throttle and it appears to be in neutral when driving in 1st and 2nd in Drive and 3. I'm taking it back to the transmission shop tomorrow. Hopefully they will have a solution.

sefiroxx Enthusiast

sefiroxx

Posted
Posted

The torque converter allows for "neutral coasting" by allowing the transmission to remain in gear while engine remains at idle. Engine idle would be a little higher than normal as the disconnected converter would help the engine by moving fluid in the same direction as engine.

Two ideas:
1)During the rebuild, did they replace the valve body or enlargen the scored hole and replace with a larger valve. There is a 1-2 accumulator that might be going or check balls getting stuck in the separator plate.

2)Gear shift is done by the computer based on
- rpm
- speed
- current gear
- desired speed (tps)
Incorrect tps signal would cause computer to shift sooner or later than spec.

sefiroxx Enthusiast

sefiroxx

Posted
Posted

The torque converter allows for "neutral coasting" by allowing the transmission to remain in gear while engine remains at idle. Engine idle would be a little higher than normal as the disconnected converter would help the engine by moving fluid in the same direction as engine.

Two ideas:
1)During the rebuild, did they replace the valve body or enlargen the scored hole and replace with a larger valve. There is a 1-2 accumulator that might be going or check balls getting stuck in the separator plate.

2)Gear shift is done by the computer based on
- rpm
- speed
- current gear
- desired speed (tps)
Incorrect tps signal would cause computer to shift sooner or later than spec.

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