Mystery Connectors

If you’re restoring a GM muscle car produced after 1969, you may notice what appear to be mystery connectors on our Factory Fit engine and front light harnesses.

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These could be a mystery because the originals have most likely been removed by a previous owner. Cars built after model year 1970 originally came with a Transmission Controlled Spark (TCS) system to retarded ignition timing. This was accomplished by cutting off vacuum advance to the distributor, and was put in place to enhance emissions.

The system consisted of a relay, (connection pictured above) a solenoid (connection pictured below) a transmission pressure switch, (pictured below solenoid connection) and a temperature override switch. The relay in the system functioned on a time delay, and temporarily allowed distributor advance to prevent the engine from stalling during a cold start.

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In low gear, (or on cold start) the solenoid connection at the carburetor is energized in order to eliminate vacuum advance. Manual transmission cars used a connection on the shifter linkage for lower gears since line pressure is not an option as with the automatic. As the engine reached operating temperature and the transmission cycled into higher gears, the solenoid would gradually allow vacuum advance.

The TCS system saw changes from year to year, but the general concept was unchanged. Many users chose to remove or disconnect the components as they modified the car, so unless you’ve recently purchased a time capsule, or are the original owner, these parts may be long gone.

Since we build of Factory Fit harnesses to the original GM prints, we include these connections as they are necessary for a concours restoration. If you aren’t planning to use the TCS system, it’s safe to tape the extra connections back to the body of the harness.