P1860 PCM ECM ECU (1999 Yukon)

Question

I have the P1860 code, when first started up and rolling the transmission locks up smoothly at around 35 mph (consistly, every day), once the engine temperature reaches around 180 degress the lockup will disengage and service engine soon light will appear. If I pull over and restart it will lock up again for a short period of time, and every time I restart it will lock up for a short period of time but eventually it will unlock and the service engine light will come on. If I drive under 35 mph for short trips the service engine light will eventually go off. My dealer said it was the PCM
ECM ECU (brain box) or what ever the proper term (name) is but others told me it was the transmission, the vehicle shifts as smoth as ever, no shuttler, just won't stay engaged. So I was first hesitant to rebuild the transmission, now I'm hestitant to replace the brain box, either repair is a major expense, I don't want any guessing going on, and if the dealer changes the brainbox and the symthoms exist, well it's unlikely theay will take it back (remove it).

Answer 1

P1860 TCC solenoid valve circuit

The only time the TCC can engnage is when the engine is at operating temperature, and the brake pedal or switch is not in being depressed. P1860 means there is a continous open or short  in the TCC  solenoid circuit. This particular transmission has a TSB dealing with P1860 always on TSB # 01-07-30-023B this is where 1-2 gear has a harsh upshift and the solution is to replace the valve body. You may be able to just replace the TCC. But if there is a harsh upshift replace the valve body.