The car Is a 442 with a transplanted M 21 transmission. On the drivers side of the transmission case where the first and second gear control rod goes into the case I have a broke off switch/relay or something. It is a mystery to my mechanic and I what the function of this switch etc is. Hoping someone can shed some light on this for me.

 

It is probably a backup light switch.

 

Let me guess. Your trans came out of a 1970-72 car, right? If so, it has a high gear switch that was used as part of the Transmission Controlled Spark system. It is NOT a backup-light switch - those were external on the Muncie and did not screw into the case (and they were mounted on the tail housing, not the main case). I'm assuming your "transplanted" installation does not use TCS, in which case you can ignore that switch. Just be sure to plug the hole.

 

Joe. Thank you. Your reply seems spot on. I will investigate the TCS aspect as I’ve never heard of it nor has my mechanic friend. Is there a way to find out the year the transmission was manufactured.

 

The second character of the VIN derivative stamp will tell you the model year. Olds only used TCS for the 1970-72 model years, but I think other GM divisions may have used it into the 1973 model year. TCS is an early emissions control system that inhibited vacuum advance in lower gears in an attempt to reduce NOx emissions.