my 96 sub sometimes wont produce heat, i found that i can remove the vvacuum line from the valve and i get heat, i assume there is some type of vacuum valve controlled by the knob on the dash? i think i saw something on the firewall that the vacuum line led to but it was too cold to mess with it at the time
When you select the “max” cold detent on the temperature dial, the solenoid valve on the firewall energizes and vacuum is applied to the heater valve. Feel the bottom of the valve, there is a spring loaded rod. When vacuum is applied, the rod goes in (up) and closes the valve to block coolant flow into the heater core.
This is meant to maximize a/c performance. If pulling the vacuum line from the valve lets it open, then the heater valve is good. That means that the solenoid valve is either constantly energized or is stuck open and is applying vacuum regardless of the panel dial setting.
It’s either a bad solenoid or an issue with the AC panel. The solenoid connector has two wires. Disconnect it, put a meter across the two pins and see if you get 12V with the dash panel -not- on max. If so, then it’s probably the panel. If not, then the solenoid valve is stuck open.
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