[FHC1971]

I'm finishing up a rebuild of my 1971 YS motor. I just located a working 3 prong Temp. Switch. My question is "Why 3 prongs?" The switch must do more than just on/off. Help is needed.
Thanks
Frank

[OG68]

IIRC the switch is also used for distributor vacuum control.

[Kenth]

This TVS was used for the 1970-74 TCS-system (Transmission Control Spark):

[FHC1971]

I would never have thought about those usages. Thanks, Makes sense now.

[necdb3]

If you read the switch, it has temp readings on it. The switch allows vac advance on a cold engine and also an overheating engine. A normal operating temp requires either 38mph on an automatic or 4th gear on a 4spd from a trans switch/sensor. The temp switch is what provides controls vac adv. on hot and cold "extremes".

[gto4ben]

Originally Posted by necdb3

If you read the switch, it has temp readings on it. The switch allows vac advance on a cold engine and also an overheating engine. A normal operating temp requires either 38mph on an automatic or 4th gear on a 4spd from a trans switch/sensor. The temp switch is what provides controls vac adv. on hot and cold "extremes".

Here's an example. AC-95M-220B-253M.
I believe "M" stands for Make and "B" stands for Break.
Full advance below coolant temperature 95°F (Break)
Full advance above 220°F (Break)
TCS Between 95°F and 220°F (Make-contacts are closed)
Temperature Warning Lamp - (Make-grounded at 253°F)

Contacts Open means vacuum advance to distributor.

[FHC1971]

Mine AC 85M 220B 253M