Whacked Heater Control
 

Crazy problem here....

Hook up the vacuum on the heater contol hose, temperature guage become iratic, and wrong. Swings like a gate in the wind

Unhook vacuum, water circulates through heater core, guage indicates a true steady reading. Can follow thermostat opening after cold start and everything.

System appears correct according to pictures found from search.

1972 Blazer w/AC

Heater hose go as follows:

Intake-Heater Control Vacuum Switch-Heater core-Radiator