Transmission kicks out of overdrive at 80+ mph

[Masoman]

I have a 1998 Tahoe with the 5.7. I had the 4L60E rebuilt around 4 thousand miles ago. I noticed that when I get it up to 80 MPH the transmission kicks out of overdrive, and the RPMS fluctuate based on how much pedal I’m giving it instead of being at a fixed rpm. It will only do this for a few minutes at a time every 15 or so miles. This causes me to have to slow down. I’ve noticed that it only does it at 80 and i haven’t noticed anything at 70 mph. Is there anything I can look at to try and fix this?

 

[NickTransmissions]

I have a 1998 Tahoe with the 5.7. I had the 4L60E rebuilt around 4 thousand miles ago. I noticed that when I get it up to 80 MPH the transmission kicks out of overdrive, and the RPMS fluctuate based on how much pedal I’m giving it instead of being at a fixed rpm. It will only do this for a few minutes at a time every 15 or so miles. This causes me to have to slow down. I’ve noticed that it only does it at 80 and i haven’t noticed anything at 70 mph. Is there anything I can look at to try and fix this?

You need to determine if it's falling out because it's being commanded to backshift to 3rd or if it's falling out due to an electrical or mechanical fault...The best way to do this is to plug in a bi-directional scan tool and take the vehicle for a test drive, over 80mph and see what the transmission is doing by looking at the shift and TCC apply data...Specifically, look for 'gear', 'shift speed' from 2-3 and 3-4 as well as TCC duty cycle and TCC slip speed.

If the slip speed is in excess of 5-7 mph when 4th at highway speeds and you don't see the PCM commanding a downshift from 4 to 3 prior to getting into TCC lock up then you have a mechanical problem in the trans, typically the 3-4 clutch pack is beginning to fail...You may also have an issue with the 2-4 servo as well...

But before spending too much time on any one specific cause, figure out what's happening between the PCM and transmission so you're starting at the right place with your diagnosis.

 

[Masoman]

You need to determine if it's falling out because it's being commanded to backshift to 3rd or if it's falling out due to an electrical or mechanical fault...The best way to do this is to plug in a bi-directional scan tool and take the vehicle for a test drive, over 80mph and see what the transmission is doing by looking at the shift and TCC apply data...Specifically, look for 'gear', 'shift speed' from 2-3 and 3-4 as well as TCC duty cycle and TCC slip speed.

If the slip speed is in excess of 5-7 mph when 4th at highway speeds and you don't see the PCM commanding a downshift from 4 to 3 prior to getting into TCC lock up then you have a mechanical problem in the trans, typically the 3-4 clutch pack is beginning to fail...You may also have an issue with the 2-4 servo as well...

But before spending too much time on any one specific cause, figure out what's happening between the PCM and transmission so you're starting at the right place with your diagnosis.

Thank you much!

 

[NickTransmissions]

Thank you much!

You're welcome, man.