[VetteVet]

The TCS system uses thermal switches and relays to roll back the timing and downshift the transmission in a pathetic attempt to control emissions. The two terminal sensor controls a vacuum switch and a relay set to a warning light.
Most of those devices were removed just after clearing the dealers lot. I don't fully understand how they work and I don't really care too. If you are doing a full resto on a truck you can find plenty of info on the net.