I am restoring the TCS system on my '72, and I think I understand how it works from studying the wiring diagram. But, I'm not sure if it was an emissions-enhancing or performance-enhancing system. (I'm putting it on, anyway, for originality sake). Also, not sure how/why it interfaces with the carb idle stop solenoid.

Any experts out there?!

From our Nation's Capitol

I am restoring the TCS system on my '72, and I think I understand how it works from studying the wiring diagram. But, I'm not sure if it was an emissions-enhancing or performance-enhancing system. (I'm putting it on, anyway, for originality sake). Also, not sure how/why it interfaces with the carb idle stop solenoid.

The Transmission Controlled Spark system on your 72 is an early attempt at emissions control. The inputs are: engine temperature, transmission position, and a timer. The engine vacuum is controlled by all of these. If your engine is warmed up, you are in fourth gear (3rd or 4th for 71 & 70), and you have been in 4th gear for at least 30 seconds you will have an increased idle rpm. A lot of folks disabled it to return the car to pre-emissions type idle and running. There is a very good discussion of it in the 72 GM Service Manaul, under the Emissions section. The carb idle solenoid I think is a separate function. It is designed to slowly allow the idle to return to the typical 800 rpm rather than allowing it to just jump back from whatever rpm you were cruising at. Again, another attempt to prevent unwanted emissions. This was supposed to reduce the amount of unburned gas dumped in the exhaust, when you let off of the throttle.
Gary

The TCS is purly a emission device. Any time the timing is retarded (as when the TCS is engaged) the emissions drop but at the cost of performance. also by restricting the vacunn advance the torque/power curve go up and fuel mileage drops. When i lived in New Jersey, I had many previous cars hooked up to emission machines, while tuning for max performance and still trying to pass the tough emission tests for inspection. I currently have mine bypassed with no ill effects and run 12 degrees initial advance but future plans call for installing a modified TCS valve that allows the vacuum to work reguardless of its position. just have to locate an old unit as new ones are $$$

|UPDATED|1/19/2003 1:59:48 PM|/UPDATED|

GaryS and Garry; Thanks for the feedback! One more input for the dilemma of Original vs. Modified.

I am hooking the carb idle stop solenoid up to the power feed line for the A/C compressor. This keeps the idle steady at 650 RPM when the A/C is on. Not sure if that's how GM intended it, but it works!!

Bill

the idle stop should be engerized when ever the engine is running reguardless whether the air is on or not. is is supposed to be engerized when the key is turned, wired to the ignition switch.

The idle stop solenoid (also know as the anti-diesel solenoid) is used to protect the engine from dieseling when you shut it off. It is suppose to hold the carb in the normal running idle. They started using them because of the motors running hotter for emission reasons.

THANKS! That explains the portion of the wiring diagram I couldn't understand, then...because the Idle Stop Solenoid is wired directly to 12VDC all the time. At least now I understand how it SHOULD be!

Here's how I have been running it...idle set directly on the carb with the carb idle stop solenoid disengaged from the cam on the accelerator hook-up. When the AC is turned on, the carb idle stop gets 12VDC which engages it and keeps the idle from dropping as a result of the compressor being on.

For whatever it's worth.

Bill