No TCC lockup in high gear when warm

[FourEightZero]

93 K1500 with 4L60E.

Had this issue since day 1. It will drive fine for the first five or ten minutes then suddenly the torque converter will unlock in OD when I'm cruising at 50 and hit the throttle to accelerate around traffic.

Until I get to my destination and shut the truck off, 4th is pretty much useless and I even have a hard time getting the trans to stay locked in 3rd.

I've noticed a second brown wire on the valve body harness that is not connected to anything but has a butt splice on the end of it (see first pic).

The connected brown wire goes to terminal U on the connector. Except for that extra brown wire and butt splice, everything appears normal and there is no exposed wire. (I know the brown wires are for the TCC solenoid)

Second pic: It appears that I have the LD setup-
Pin S wire is white
Pin T wire is tan
Pin U wire is brown

I'm pretty sure I have an electrical issue. Trying to avoid the transmission shop. Any advice?

 

[Papablunt]

This sounds very much like the TCC regulator valve has worn, allowing fluid to seep by. As your trans heats up, it thins out, allowing fluid to seep by the valve upon acceleration (higher pressure placed on valve).

Sonnax makes an oversized valve, or you can get a replacement valve body.

This may not be your issue, but sounds identical to a problem I had in one of my trucks with a 4L60E. Hope this helps.

 

[stutaeng]

93 K1500 with 4L60E.

Had this issue since day 1. It will drive fine for the first five or ten minutes then suddenly the torque converter will unlock in OD when I'm cruising at 50 and hit the throttle to accelerate around traffic.

Until I get to my destination and shut the truck off, 4th is pretty much useless and I even have a hard time getting the trans to stay locked in 3rd.

I've noticed a second brown wire on the valve body harness that is not connected to anything but has a butt splice on the end of it (see first pic).

The connected brown wire goes to terminal U on the connector. Except for that extra brown wire and butt splice, everything appears normal and there is no exposed wire. (I know the brown wires are for the TCC solenoid)

Second pic: It appears that I have the LD setup-
Pin S wire is white
Pin T wire is tan
Pin U wire is brown

I'm pretty sure I have an electrical issue. Trying to avoid the transmission shop. Any advice?

Day 1 means when you bought the truck? Used, I assume? What is the history with it? Any idea on transmission service history? What's the mileage?

It looks like that brown wire is for PWM per the wiring image you posted. The PWM 4L60e transmission didn't show up until 1995, so that wire wouldn't go anywhere in your unit.

Some guys do run a switch for the TCC solenoid to the dash though to manually command lockup.

Agree with @Papablunt The TCC regulator valve bore wear is a common issue on these units. Here's a Sonnax solution: https://d2q1ebiag300ih.cloudfront.net/uploads/part/instructions/2458/77754-04K-IN.pdf?v=1554485625

I hope this helps.

I would drop the pan, and if there's no excessive clutch material, remove the valve body. Either get a remanufactured valve body or take apart all valve body valves, clean everything and install that Sonnax valve. Basically rebuild it yourself.

 

[FourEightZero]

Day 1 means when you bought the truck? Used, I assume? What is the history with it? Any idea on transmission service history? What's the mileage?

It looks like that brown wire is for PWM per the wiring image you posted. The PWM 4L60e transmission didn't show up until 1995, so that wire wouldn't go anywhere in your unit.

Some guys do run a switch for the TCC solenoid to the dash though to manually command lockup.

Agree with @Papablunt The TCC regulator valve bore wear is a common issue on these units. Here's a Sonnax solution: https://d2q1ebiag300ih.cloudfront.net/uploads/part/instructions/2458/77754-04K-IN.pdf?v=1554485625

I hope this helps.

I would drop the pan, and if there's no excessive clutch material, remove the valve body. Either get a remanufactured valve body or take apart all valve body valves, clean everything and install that Sonnax valve. Basically rebuild it yourself.

Bought it used a year ago. 260k on the odometer and the seller handed me paperwork showing the trans was recently rebuilt.

TCC unlocking happened on the way home. I considered the paperwork to be BS. Took the truck to a trans shop to be rebuilt and they said I had a bad seal on the back of it (it was dumping fluid). Seal replaced, it no longer dumps fluid but I still have the TCC issue and they probably didn't drive it enough to see it act up.

When I pulled the engine to rebuild it, torque converter looked brand new so now I'm wondering if that trans was rebuilt (just not very well?)

Thanks for the pointers, guys. Lessons learned the hard way... but I'm learning.

 

[Supercharged111]

I'd do a PWM delete on that, will require valve body removal but is a fairly easy task. And of course clean up that harness.