4l80e kicking back to 3rd then no up shift

[TonyO454]

Transmission shifts all gears smoothly, problem is when you get on the highway and go to pass it kicks back into 3rd and won’t shift back to 4th unless you pull over shut the truck down, then it will run through all of gears perfect again, I replaced the ect sensor and the ignition starter switch (both ac Delco) is there anything else I can check before I start throwing parts fluid is full and red and this is in a 93 c3500 454 2wd any help would be appreciated

 

[stutaeng]

Do you have any engine codes? I'm not too familiar with TBIs, and those early 4L80e models had some different ECUs. I think the TBIs still have TPS, and the ECU commands fluid pressure based on this input, so that's also something you need to check that is operating correctly.

I wonder if your shift solenoid is acting up? I'd probably drop the pan and check resistance with a DVOM. They are pretty inexpensive and easy to order.

Otherwise get yourself a transmission pressure gauge and a service manual and start driving around monitoring the fluid pressure. There's a little 1/4" threaded port on the driver's side you can hook it up to.

 

[TonyO454]

Do you have any engine codes? I'm not too familiar with TBIs, and those early 4L80e models had some different ECUs. I think the TBIs still have TPS, and the ECU commands fluid pressure based on this input, so that's also something you need to check that is operating correctly.

I wonder if your shift solenoid is acting up? I'd probably drop the pan and check resistance with a DVOM. They are pretty inexpensive and easy to order.

Otherwise get yourself a transmission pressure gauge and a service manual and start driving around monitoring the fluid pressure. There's a little 1/4" threaded port on the driver's side you can hook it up to.

I will pull codes and go from there thanks for your reply

 

[TonyO454]

Ended up with code 68...transmission component slipping

 

[stutaeng]

Ok. I would check all of your fuses next. Read through here:

https://www.chevytalk.org/fusionbb/...ext=DTC 68 is likely a,or 1-2 shift solenoids.

A couple of causes could be the problem...

 

[TonyO454]

Thank you I will

 

[TonyO454]

Well it seems I figured it out. I checked fuses harnesses unplugged sensors and harness on transmission rechecked fluid level as well, then I saw cruise control switch on the turn signal was stuck on accel turned off cruise control and the truck is running awesome!!! Not sure exactly which one did it but at this point it does not matter....again thanks for your responses!

 

[stutaeng]

Well, that was easy! LOL Why would that put a CEL code though?

 

[Supercharged111]

Well, that was easy! LOL Why would that put a CEL code though?

I'm not convinced this was his problem. A component slipping sounds like a death sentence. Remember the 4L80 has an input and output speed sensor to measure this stuff.

 

[TonyO454]

It wasn’t slipping it was shifting perfect through all gears it would kick down to third when trying to pass and would not upshift back to 4th/od