Chevrolet P/U 5.6L Lights Brake. Rebuilt transmission twice in 10,000 miles

Question = When I bought the truck,the torque converter wasn't locking up. After 2 rebuilds, I was made aware that there was no power going to the converter.I have 3 wires going to the trans (4L-60) one is tan and black,another is purple,and the third is blue.I hooked up the purple to 12V with a 15 amp fuse.I now have lock-up,but with any load,such as a trailer,it won't stay locked.I also have oversized
tires. By the way,it's a 2-wheel drive truck.

The transmission shop where I last had the tranny rebuilt said it was because I had oversize tires and that I might try putting a switched ground to manually lock -up. When I checked the other wires,they were both hot with the key on but not running. Any help would be appreciated.I just think I need the converter locked up so I don't burn up another transmission Thanks.
Answer:
It took me awhile to find the schematics i needed. OK. The purple wire is what feeds the tcc solenoid. The purple gets its power from  the cruise/tcc/brake switch. That brake switch gets its power from the 'gauges' fuse. You need to trace back from the transmission connector to the brake switch after making sure the gauges fuse is good. If the fuse is good, and power is going to the brake switch, then power should be going to the trans. (purple wire) .

The brake switch is normally closed, and should be feeding power to the trans. Since when you manually apply 12v to purple wire and tcc does work, then that means the PCM is grounding the solenoid. With the brake light on the dash, that may be the key to no power to purple wire. Either a faulty/misadjusted brake switch, or no power to the brake switch.

There are several reasons the tcc will go off. Faulty TPS on the engine, 4th gear pressure switch, weak tcc solenoid, etc. When the trans. Fluid temp gets too high, the PCM will command tcc on to try to reduce heat. Since no power to purple wire for tcc and the other switches in transmission that is the first thing to fix. The 4-3 downshifts switch will disengage tcc for a short period of time when accelerating, until a steady engine speed and load is attained again.

While towing and hitting gas, or going uphill etc. TCC coming off then back on is a normal condition. Oversized tires may just enhance that cycling. As stated above, need to get that purple wire problem solved before all else. SF
2002 chevy Silverado 1500. 4.8L V8.
Question: Transmissions slipping when getting off the highway and when coming to a stop then beginning to move again.
Answer:
First check the fluid level and condition. See if looks or smells burned. Try a transmission flush service along with a filter change. Just a filter change only removes about 4-5 quarts of fluid, but the flush service replaces all fluid. If that does not help, you may have a valve body problem internal to the trans. Sticking valves can cause the feeling that it is falling into neutral.
Chevy Truck Transmission Has No TCC Lock-Up

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