Heater stops working when car is moving

1989 MERCEDES BENZ 300SE
191,000 MILES • 3.0L • 6 CYL • 2WD • AUTOMATIC
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MAXFLYJOHN
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I replaced the mono-valve and the climate control unit. The heat and AC. work great when you first start the car after it has been run and has already been warmed up. As soon as you drive the car it becomes all AC, no heat at all. Stop the car and shut off restart and the climate control unit has full AC and heat adjustment.Start to drive and heat disappears. Help
Jun 10, 2018 at 10:22 AM
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STRAILER
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Hello,

This sounds like you have a temperature sensor that is not working right or the control unit itself is bad. Here are the locations of the possible troubles.

Check out the diagrams (below). Please let us know what happens.

Cheers, Ken
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Jun 11, 2018 at 12:40 PM
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MAXFLYJOHN
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Thanks for the information. Upon further research I have found how complicated this Mercedes heating and cooling systems are. It goes cold after 2,000 rpm's. The CCU was replaced but it appears by unplugging the auxiliary water pump it solves the problem. I have found a lot of people who have this problem with there similar Mercedes Benz's. Also, the monovalve must be in good working order and original Mercedes Benz or the springs are too weak and close prematurely. I have read so much about this lately. It is great when it works...But. Thanks for the help. John
Jun 21, 2018 at 7:26 PM
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STRAILER
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Glad you could get it fixed, that kind of problem can be tough. Please use 2CarPros anytime we are here to help

Cheers, Ken
Jun 22, 2018 at 10:07 AM