I assume your speaking about the TCS system. (Transmission controlled spark) This is an early emission control device. It doesnt allow vac to the distributor for timing advance unless some conditions are met.
Conditions:
Cold eng or
overheating eng or
Trans in High Gear when at normal oper temp(auto 3rd gear, man 4th gear)
The solenoid doesnt allow vac to pass thru when it is energized. Unplugging the solenoid will DE energize the solenoid and allow adv.
Gm used this same system accross the car line.
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