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

Asset 21
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
this is a vacuum problem just fixed the same exact problem with mine.under the hood their is 2 vacuum lines up front on top around were windshield fluid runs,above battery mine is on opposite side their is 2 elbows (rubber) both of mine were bad fixed them with a different hose and solved this same exact problem.good luck!
a/c prob needs to be re charge 1st you drain it from the high end then you put 2 cans of 134a in it while put the 134a in make sure you have a/c turned on to high while the car is running
Is more than likely a heater control valve that failed, thus allowing coolant to enter the heater core all the time. My 190E has this problem; and it's 95 degrees outside. I'm thinking of putting a shutoff valve between the secondary coolant pump and the heater core.
If there is a problem with the climate control unit in your center console/dash it defaults to hot/defrost. You can replace this unit by removing it from the center dash just a few plugs in back and two screws holding the zebrano faceplate on. They're available on ebay not too expensive. This is just one possible solution, though.
This could be your problem . The fan is not comming on at the radiator and not cooling the engine off soon the head gasket will go .
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