Help identify this mystery plug on three spd

[1970-Old_Betsy]

Not sure what this plug connection is for on my 1970 C/10 with three on the tree. First photo shows transmission , next is the connector, the wiring harness and then the relay. I think harness was for maybe for air conditioner blower. The only connection that was plugged in was a keyed hot wire to the relay on the firewall and the connection on the transmission. I have truck all back together after restoring it and everything seems to be working fine, even the back-up light ( switch on column ) Anyways...........what is the plug on tranny??

Thanks!

[geezer#99]

Ezzy Pezzy!!
TCS
Transmission controlled spark (timing). That plug on the tranny is set up as a switch to ground. Used to control your vacuum advance until you're in 3rd gear. THere would have been a switch on the firewall (or engine) that had a vacuum line from your carb on one side and a vacuum line out of the switch to the vacuum pot on your distributor. The switch had 12v to it when your ignition key was in run position. THe switch would stay closed letting no vacuum to the distributor until you shifted to third and completed the electrical curcuit to ground. The switch would open and you'd have vacuum to your distributor.

[1970-Old_Betsy]

Thanks for the info. !
This truck was converted to HEI before I bought it and it also has a 350 instead of the original 6 cyl. I have a hopped up 350 in it now.

I drove the truck regularly for a few years without this harness hooked up, is this something I need to get back together??

The photo is an old one I took before I stripped everything down.

Thanks again,,,,,,,,,,,

[geezer#99]

If it's running now don't sweat it.
Pedal to the metal!!!!

[1970-Old_Betsy]

On the few trips I have made around the block terrorizing the neighborhood, petal to the metal is the only way it is running right! Torker II intake may have to go...........

Thanks again for the info.......