4L80E Shifts out of Overdrive/4th Gear

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02-09-2019, 20:54
mikeyg1975

4L80E Shifts out of Overdrive/4th Gear

Where are my 4L80E gurus on here?? I am having an issue where my truck will accelerate just fine through the gears, shift into overdrive properly, only to kick back out of overdrive once getting up to about 50-55 MPH, then no matter what you do will not shift back into overdrive on it's own. Now, if you pull over to the side of the road, shut the engine off and power cycle the ignition, it will operate just fine again, this is an intermittent issue, it doesn't do it all the time, and when it does do it, like I said, you power cycle the ignition, it corrects itself. I have changed the fluid and filter as part of the troubleshooting process with no noted change in operation at this point, any ideas??
02-10-2019, 06:51
Robyn

Check the brake light switch.

Are you loosing the lockup, or 4th gear altogether ???

The brake light switch can affect the lockup but not the OD

If 4th gear is the culprit then it is likely the shift solenoid on the valve body that is either failing or possibly the filter screen in the solenoid is plugged up.

When was the last time the tranny was serviced
02-10-2019, 10:22
Yukon6.2

My Buddys son was having trouble with his gasser EL60E and it turned out to be his throttle position sensors.
Shifting issues were his problem,but i can not tell you what exactly they were.He was looking for a replacement tranny and had his truck parked.Then he found out about the throttle position sensor issue.And the rest is history.
02-10-2019, 11:35
JohnC

If shutting it off and restarting it "cures" the problem than there's probably one or more codes being stored.
02-10-2019, 19:30
mikeyg1975

Quote:

Originally Posted by Robyn

Check the brake light switch.

Are you loosing the lockup, or 4th gear altogether ???

The brake light switch can affect the lockup but not the OD

If 4th gear is the culprit then it is likely the shift solenoid on the valve body that is either failing or possibly the filter screen in the solenoid is plugged up.

When was the last time the tranny was serviced

Hello Robyn, Thanks for all the good info, I will check all that out for sure. I just serviced the trans as the first step in troubleshooting this issue, fluid really wasn't that dirty and had very little debris on the magnetic drain plug of my aftermarket pan. The thing that is throwing me off is the fact that it is not setting a check engine light, and I will have to get a scan tool to see if it has set any codes, I'll keep you posted!!
02-10-2019, 19:32
mikeyg1975

Quote:

Originally Posted by Yukon6.2

My Buddys son was having trouble with his gasser EL60E and it turned out to be his throttle position sensors.
Shifting issues were his problem,but i can not tell you what exactly they were.He was looking for a replacement tranny and had his truck parked.Then he found out about the throttle position sensor issue.And the rest is history.

I thought about that, I'm going to get a scan tool and see if it's throwing any codes, and with TPS sensors being cheap and plentiful, maybe I'll throw one on there and see what happens.
02-10-2019, 19:33
mikeyg1975

Quote:

Originally Posted by JohnC

If shutting it off and restarting it "cures" the problem than there's probably one or more codes being stored.

It's not showing a check engine light but I will get a scan tool and check it for sure.
02-10-2019, 19:38
jggiedeman

Slipping

Does it only slip in 4th/OD or have you tried also in 3rd gear? It seems like you would get a code 68/39. I currently have the same issue on a 98 tranny. Just too cold to get the thing out and look it over. I believe the codes will set when the rpm difference or slippage is greater than 200 rpm.
02-10-2019, 20:11
mikeyg1975

Quote:

Originally Posted by jggiedeman

Does it only slip in 4th/OD or have you tried also in 3rd gear? It seems like you would get a code 68/39. I currently have the same issue on a 98 tranny. Just too cold to get the thing out and look it over. I believe the codes will set when the rpm difference or slippage is greater than 200 rpm.

Seems like it's only slipping in 4th, it actually doesn't even slip, it just kicks itself down to 3rd and stays there, after it kicks down it will not up-shift back to 4th until you power cycle the ignition.
02-11-2019, 07:53
56pan

My '93 C2500 6.5 diesel did the exact same thing. Only has a TCM and stored no codes. It was a bad ignition switch. Badly worn and with green corrosion. The clue is shutting it off, restarting, and the trans works right again. Hope this helps.
02-11-2019, 08:37
Robyn

What 56 pan said.

No codes or SES could certainly point to the switch.

Have you actually tried pulling the shift lever into 3rd when it does its thing ??

Dropping out of lockup will give the feeling that it's out of 4th gear as most of the 6.5 rigs hit lockup at 45 mph and it is a very firm shift and when the lockup is unlocked there is not much difference in feel between 3 and 4...

When it does it's thing try manually shifting to 3rd and see if the RPM go up.

If this is the case, then its the lockup and likely the stop light switch.
02-11-2019, 08:41
JohnC

I never owned a truck with a mechanical pump an an automatic transmission, but I'm pretty sure you can check for codes by putting a jumper (paperclip) in the DTC socket. There's an article on the member's section of the web site that discusses how to do it on a 94-95 model year.

Yeah, the ignition switch... There's a separate set of contacts that powers the instruments and the trans. If you have issues with the tach as well, Bob's your uncle... You can check the voltage on the instrument fuse to see if that's what's happening.
02-11-2019, 09:07
Robyn

92 and 93 had a tranny control module

Too bad GM did not stick with the mechanical injection...would have saved many of us a lot of excess pain and suffering. :( for sure...
02-11-2019, 10:38
Yukon6.2

Quote:

Originally Posted by mikeyg1975

I thought about that, I'm going to get a scan tool and see if it's throwing any codes, and with TPS sensors being cheap and plentiful, maybe I'll throw one on there and see what happens.

Hi No codes were thrown on his truck...
02-12-2019, 07:17
56pan

Quote:

Originally Posted by Robyn

92 and 93 had a tranny control module

Too bad GM did not stick with the mechanical injection...would have saved many of us a lot of excess pain and suffering. :( for sure...

Total agreement with the 2nd sentence.:)
02-13-2019, 15:08
Kennedy

Quote:

Originally Posted by 56pan

My '93 C2500 6.5 diesel did the exact same thing. Only has a TCM and stored no codes. It was a bad ignition switch. Badly worn and with green corrosion. The clue is shutting it off, restarting, and the trans works right again. Hope this helps.

Start with this ^^^

Those trucks of that era had poor switches especially when they got some age and miles on them. The trans requires stable voltage to do its electrical pwm thing.
02-19-2019, 19:17
mikeyg1975

Quote:

Originally Posted by 56pan

My '93 C2500 6.5 diesel did the exact same thing. Only has a TCM and stored no codes. It was a bad ignition switch. Badly worn and with green corrosion. The clue is shutting it off, restarting, and the trans works right again. Hope this helps.

I appreciate everyone's input and the winner goes to 56pan and all others who suggested the ignition switch!! I finally had time to work on it today and changed out the ignition switch with a genuine ACDelco NOS part, just test drove it tonight, works like a champ!! I'm glad I asked y'all because I never would have thought that!!
02-20-2019, 06:36
Robyn

Good deal...

Glad you are all fixed.
02-21-2019, 07:20
56pan

Quote:

Originally Posted by mikeyg1975

I appreciate everyone's input and the winner goes to 56pan .......

Even a blind hog gets an acorn every now and then. ;) Glad you're fixed.