4l80e No Overdrive

[Rich84]

New Ls swap..When I drive the truck at initial startup it shifts fine. Park the truck for a few minutes and drive it again and it will not shift into 4th gear.

The fluid was not too clean when I dropped the pan to change the filter.

It also seems like it might be starting in 2nd gear sometimes.

P0753 shift solenoid is the only code.

Is it as simple as replacing the solenoid?

[Custom 68]

was the transmission originally from your swap vehicle? I ask because you cant just add a 4l80 in place of a 4l60. You may know that but just checking.

[Rich84]

Harness and ECM are setup for a 480

[YoungPup1977]

Check your TPS...5v...just tossing it out there...Good Luck.

[clinebarger]

Originally Posted by Rich84

New Ls swap..When I drive the truck at initial startup it shifts fine. Park the truck for a few minutes and drive it again and it will not shift into 4th gear.

The fluid was not too clean when I dropped the pan to change the filter.

It also seems like it might be starting in 2nd gear sometimes.

P0753 shift solenoid is the only code.

Is it as simple as replacing the solenoid?

P0753 is a electrical fault code for the 1-2 shift solenoid circuit. Check PCM Red Connector Pin 48 for the terminal being backed out, Same for the Trans Plug Pin A.

#1 Disconnect the PCM. Key on, Using a DVOM com lead hooked to ground, Probe Pin 48 of the Red connector with the key on. The DVOM should read close to battery voltage.
If power is present....Ground pin 48 with a 5 amp fused jumper. If it blows the jumper fuse, You have a short to B+.
On a good circuit, Grounding pin 48 will activate the solenoid & you will hear a audible click.

#2 If no voltage is present in step #1......Key off, Hook the com lead to a battery hot source, Probe Pin 48.....If it now has battery voltage (it will read negative voltage because the polarity is reversed, Not a concern), You have a short to ground somewhere in the 1-2 solenoid circuit.
Unplug the Trans.....
If you no longer have battery voltage.....The short is in the internal trans harness or the solenoid itself.
If you still have battery voltage.....The short is in the engine harness.

#3 If you can't get any feedback from the 1-2 circuit at the PCM connector in the above tests.....
Unplug the trans, Key off, Ground Pin A of the trans plug, With DVOM com lead still hooked to B+, Probe pin 48.
If the DVOM now reads battery voltage.....The wire between the PCM & Trans is good.
Check the trans connector for bent/laid over pins, If that checks good....Replace the internal harness & 1-2 shift solenoid (All solenoids would be recommended)

[Rich84]

Thanks Cline...
Do you think it's strange that it works fine before it gets hot?

[BR3W CITY]

Is there any leaking at the round connector on the trans side? They can leak from the seal and seep into the plug.

[Rich84]

No leaks..everything is clean and dry..

[ls1nova71]

Originally Posted by Rich84

Do you think it's strange that it works fine before it gets hot?

Probably not too strange, electrical connections and shorts can do some weird stuff. I once got a P0785 in my '33 when on a trip in Palm Desert California, about 2000 miles from home. Just got in it and went to take off and it didn't feel right, was taking off in 2nd and would only shift to 3rd. Not really knowing what to do, I bought a shift solenoid from the dealer and figured I'd put it in, but then it started working fine. Then it did it again. I drove the whole way home with it randomly throwing the code, and when it would, I would get the scanner and clear the code, on the fly no less, to get it to shift into OD. Once I got home it cleared up for a couple of months. I never did put the solenoid in it, but found there was a wire between the PCM and the trans plug that had continuity to ground, until you moved it a bit, then it wouldn't. I ended up running a separate new wire rather than trying to strip the harness in a completed car and haven't had an issue since.

[clinebarger]

Originally Posted by Rich84

Thanks Cline...
Do you think it's strange that it works fine before it gets hot?

The solenoid could go open once the unit if fully warmed up, You also need to verify the unit isn't overheating.

[clinebarger]

Originally Posted by BR3W CITY

Is there any leaking at the round connector on the trans side? They can leak from the seal and seep into the plug.

While 4L80E's are notorious for leaking connectors & the atf can eat through the wiring insulation causing shorts, ATF itself isn't a conductor.

Easy on me Brew

[BR3W CITY]

Meh, I only try to sh!t on people when I think they are wrong. I kinda stopped questioning you on trans related anything.

[Rich84]

Bad news..Transmission is toast. Black nasty residue on the bottom of the pan so it is coming out.

My cousin is a transmission builder so it is going to him. Not looking forward to taking it out though.

[BR3W CITY]

Bummer dude. Any DX as to the cause?

[Rich84]

He said something about a forward sprag?? And a bunch of other technical mumbo jumbo..

Lesson learned..I should not have been so anxious to get this truck moving again. Now I am waiting for the time to pull it out and get it rebuilt.

I own a construction business and I always preach "doing it right the first time"..I should have listened to my own advice.

[mongocanfly]

Wow....that's nasty

[YoungPup1977]

buy a good torque converter since you are rebuilding the trans....always replace the TC...and make sure you clean the cooler lines throughly ....Good Luck.

[Rich84]

Truck is back on the road...

Replaced the transmission with a good unit that I had in one of my other trucks. Immediately thru a code and was stuck in 2nd and 3rd again. After I calmed down, I started checking the wiring. I didn't see anything out of order at the transmission. So I went to the ECM to start tracing wires. What I found was that the large connector at the ECM was not fully seated properly. The only condition that it caused was the transmission problem I was experiencing. No other issues.
Transmission shifts beautifully and I couldn't be happier with the swap.