4L80E torque lockup problem

[45467]

The transmission used is a 1991 model, could it be wired differently than a 94 & up ? I checked the wiring harness from the TCU to trans at the TCU connector & the 4L80E /4L60E connectors are on the right color wire as shown on the supplied wiring diagram The wirinharness could still be wrong . I will trace each wire to the plugs at each end to make sure they emd up on the right pins.

[Jake's Performance]

Yes,
A 1991 is wired differently than a 1994-up.
Let me get you some info to help you out, you'll likely need to drop the pan.
Someone may have updated the wiring if it was rebuilt though.

[Jake's Performance]

Sorry 1993 up is the break, 1994 they went to a different force motor and torque signal circuit.

Here's the info:

[45467]

Jake, correct me if I am wrong but the early trans had a different connector altogether. My connector is as shown on the wiring diagram for the 1993 model. Also would the rest of the transmisions performance be affected ? Everything but lockup works really well , the transmission was rebuilt & as far as I know the wiring was not changed. One last thing if the wiring was wrong I would probably get the wrong resistance reading when I check the TCC solenoid , when I checked it last Thusday it checked out at 10.7 ohms and it should be between 10 & 15 ohms.

[Jake's Performance]

Yes you are correct, the early connector used a small bolt to hold the harness to it. You must have the newer style harness and connector installed.

So you SHOULD be good. I posted this as a reference for you.

The first rule of actually repairing something is to isolate the problem.
I suspect you have a TCM or tuning issue, but if you can isolate it then you'll know for sure.

The internal harness splits the 12v Ign + to three different solenoids, the A and B shift solenoids and the TCC solenoid.
These all see constant 12v Ign power, they operate on the ground side. So you can ground the TCC solenoid wire and force it to operate independent of the TCM. This will isolate any TCC hydraulics out of the equation.
Then you can use a power probe tool or a LED to watch what the TCM is commanding and see if it matches what you feel.

I just recently did this exact diagnostic procedure on a Furd Exploder that had a weird issue that I THOUGHT was TCC related.

Another rule of thumb for trans work, the one thing you KNOW is OK and are being hard headed about, is what the problem is.

Maybe the diagrams will help, maybe not.

I would at least check the pinout of the TCM harness at the trans end because it differs from the 80 to the 60 and most would ship in the 60 configuration.

[MonzaRacer]

Oh god the horror I remember replacing 4L80E internal harnesses and outer ones too. I can still smell the shrinky butts glue getting hot, laying under buses!.
Yeah the harnesses has fluid leak issues too, see if you have fluid in the connector that will drive the trans NUTS!!!!
Good Luck
Lee Abel
AFTERMARKET PERFORMANCE

[turbo kid]

Have you fix the lock up problem?What was it?

[terrainman1]

I have a 1991 GMC 2500 Sierra.
The problem I have is that at 110km/hr the Transmission starts downshifting from overdrive then right back in and over and over. I had it recently into the transmission shop and after a full scan they said it was because the TPS is sending the driver demand downshift signal to the computer to shift the transmission. So I replaced both the TPS and the Computer and the problem persists.
This truck has been lifted 6 inches and has 35" tires on it now. Additionally the Speedometer was adjusted to match proper speed after the lift and tires were done.
Anyone have any suggestions what could be done to finally fix this problem. I am at my wits end and am tired of spending money with 0 results.