What's this?

[john]

Originally Posted by geezer#99

IIRC it's a tvs. Transmission vacuum solenoid. Had one on my chevelle. One wire is powered. The other goes down to a switch on the transmission on a 4 speed tranny.
When you finally shift to 4th the switch on the tranny provides a ground. The vacuum switch opens and allows vacuum to the vacuum advance on your distributor. Lots of high compression motors ran like that. Very aggressive timing curve in the distributor but vacuum advance only under cruise conditions.

Geezer is right, only I think it is called TCS solenoid (trans controlled spark)