Heater stops working when car is moving

MAXFLYJOHN

MEMBER
1989 MERCEDES BENZ 300SE
3.0L
6 CYL
2WD
AUTOMATIC
191,000 MILES

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

Sunday, June 10th, 2018 AT 10:22 AM

3 Replies

KEN L

MASTER CERTIFIED MECHANIC
51,841 POSTS

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

Images (Click to make bigger)

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Monday, June 11th, 2018 AT 12:40 PM

MAXFLYJOHN

MEMBER
2 POSTS

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

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Thursday, June 21st, 2018 AT 7:26 PM

KEN L