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Someone asked about this system a few days ago. I lost the thread, so I repy here.

The TCS is not as simple as transmission controlled spark advance, which is what TCS stands for. There are 2 major other components in the system, for it to work properly. I spent months figuring it out on my 1970. Here's what I learned.

The solenoid on the intake manifold, pass side, blocks vacuum from the carb to the distributor in neutral, reverse, first, and second gears. Vacuum is allowed to pass to the distributor in third and fourth. This is accomplished by the solenoid being energized by power from a blue wire coming into it, and a ground circuit, black wire, going down and away from the solenoid to a switch on the transmission which is activated by the gear shift levers.
The tranny switch completes the ground circuit in neutral, low, first, and reverse gears. No ground occurs in third and fourth.

If the system was as simple as the above, you would never have vacuum to the distributor in neutral, or the other low gears or reverse as I spoke about. However, there are 2 major exceptions or "overrides" to this general rule of operation.

There is a "dual temperature override switch" in the head, pass side, to read coolant temperature. When the engine is dead cold, the switch overrides the above system, to block power to the solenoid, so you get vacuum advance to the distribute during cold operation of the engine. The dual temp overrides comes from the fact that when the coolant approaches boiling hot temps, this overrides again operates. The result is, if you are sitting at idle in neutral, or driving in low gears in city traffic, where your engine is getting very hot, the override works to allow vacuum advance to operate, and help make the engine run more efficiently. I would think this is important for all big block owners.

The second override in the system, is a "twenty second time delay relay" mounted on the firewall. It intercepts the power going to the solenoid and stops it from getting there during engine cranking at start up, and for twenty seconds thereafter. The result is that in any temperature situation, you have full vacuum advance during start up, and for twenty seconds thereafter.

That's the system. I figured this out after reading, and re-reading the excellent schematic diagrams in the 1970 GM Chassis Service Manual, at section 6T-1 to 6T-2, and the Haynes Corvette Manual, 1968-82, at pages 84 and 85. Hope this helps someone.
 
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