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1995 plymouth grand voyager

TROUBLESHOOTING!!! I was driving my van with the heater on and all of a sudden it started blowing cold air, and the temp. gauge started to climb. Also the fan came on and would not turn off until I shut the van off. We replaced the thermostat and I drove it again, and it’s doing the same thing. But whats pretty funny is when I pull to the side of the road, the temp. will go down, as long as the heater is off. Looks like we have a weekend project ahead of us! Does anyone have a clue???

Was it physically overheated. Was the air from the radiator fan really hot? Hoses real hot etc.?
It could be an electrical gremlin. The heater controls and gauge cluster could share the same ground.

Make sure the water pump is spinning freely.