4L80E Shifting funny

[9365turbo]

My 93 K3500 with a 4L80E is shifting weird. It seems like the converter is not locking up,but it doesnt do it all the time. It seems if you drive it for a while then stop shut it off to go in to a store then come back start it up and drive off it wont shift into OD and the converter doesnt seem to lock up. If you turn of the truck it will go back to normal shifting.

[JohnC]

Check for codes. If it "wants" to shift but doesn't it'll set a code.

A faulty brake switch can keep the TCC from engaging.

[9365turbo]

How do you check the codes and the brake light switch? Also when it is acting up it shifts hard.

[Mainous]

Your problem is the Throttle Position Sensor. Classic symptoms.

[damork]

9365turbo,

Throttle position sensor is a good thing to check. I had the same symptoms with my truck for over a year when someone here recommended some "out of the box" thinking. They had experienced the problem and traced it to a wiring problem that originated in a trailer light plug in connector at the back of the truck.

Anyway, it was the beginning of looking very close at all grounds / wiring / and power to the transmission. The wiring harness to the trailer light connector was badly corroded where the wires attached inside of it. I replace the entire connector and sealed it with silicone waterproofing and it took care of the lockup problem. Corrosion and crossfeed seemed to be a big problem. Also changed out the brake light switch up under the dash which also can cause some problems with lockup (when you touch the brake pedal, the switch that turns your brake lights on also lets your lockup clutch know when to unlock). Changing it is not bad, just takes a boatload of patience.

I didn't stop there however, and added a power relay to feed +12V to the transmission voltage supply, so instead of the ignition switch feeding power to the transmission, I added another Bosch relay like I had used to power the lift pump instead of letting the oil pressure switch carry the load.

Another area that gets missed for strange 4L80E shifts, especially pulling away from stop signs with delayed or abrupt shifts is the input/output speed sensors on your transmission - especially if you have a 4x4. The output sensor in my 4x4 was in the transfer case and completely smothered with fine metal shavings that prevented the tip of the sensor from generating a proper speed signal for the transmission control. Cleaned it up and end of the problem.

Beyond that, there are some grounds in the wiring harness under the frame and near the rear of the truck (at least my truck had them) where the harness to the rear of the truck grounded - probably more to do with the fuel level sender in the tank, but I polished that up along with the under the hood grounds and all worked very well after that. I sold the truck last fall with 258000 miles on it - engine and trans never needed rebuilding and was still running strong.

[9365turbo]

Does the 93 mechanical injector pump have such a thing?

[9365turbo]

I check the speed senser and the trailer wiring adapter. I have had trouble with that before. When it is acting up it shifts really hard.

[9365turbo]

I also notice a couple of times on the freeway I had to let off the pedal and when I resumed speed it wouldnt lock up or go in to OD. Until I stopped the truck shut it off for a second and then it was back to normal shifting.