Hey everyone,
just purchased a 83 GMC High Sierra 1500 and trying to figure out what this thing is and supposed to do. It’s got wires connected to it but that’s it. Here is a pic.
thanks

 

Looks like a vacuum solenoid for something.

 

He calls it a tip in switch. 3:03 in this video

 

Tip In Vacuum switch.

 

For what though? Cold start?

 

It keeps the EGR from operating in open loop until the engine reaches operating temperature and goes into closed loop.

 

Tip-in vacuum switch is part of the Electronic Spark Control, ESC system. The switch is normally open and only closes on large vacuum decreases. As the name implys "tip-in". When it closes, it signals a pin on the ESC module. The ESC module then retards the timing ~10° for ~approximately 2 seconds.

I would trace your distributor wiring. Most of these trucks had the ESC bypassed when the ESC module failed. Others like my 83 G20 had a normal HEI installed, which is what I elected to do.

 

Looks like a vacuum switch maybe for use with a overdrive trans like a 700R4 when vacuum changes while slowing down it aids in un locking the TCC. Trace the wires down and see if it goes to the Trans electric plug connection on drivers side of trans. Tracing down wires gives you an indication for what it is used for.

 

I already clearly stated what that switch was and what it was for. The TCC switch is on the other side near the master cylinder and has a slightly different appearence. The Tip-In has a Yellow/Brown wire and the TCC has a Blue/Green wire combination. I already traced these back on my 83 G20 years ago when I EFI swapped it.