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Q: heater on all the time on 1990 Mercedes-Benz 190E

my heater blows hot air all the time . when the a/c is on the defrost blows hot hot air! the a/c works but the heater blows so hot it can not keep up. thanks,. chad
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To correct the other responses, your problem has noting to do with the AC, more than likely it is not any of the expensive units. The heat shut off valve is vacuum actuated and not electric. Apply vacuum with a vacuum pump to see if your valve shuts off or not. Apply vacuum to the reservoir to see if it holds pressure or not. You can find system drawings on line forums’ etc
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