The heater in my 380sl seems to only work in town at low speeds. Once the engine warms up, and the speed increases such as highway speeds, the heater blows cold no matter what the setting is. Once you slow down as in stop and go traffic, or if you turn the car off, let it set a few minutes then restart it, the heater blows hot, until you reach high speeds again. I have checked the heater control valve, and it opens and closes. ( I have even disconnected it and the heat still doesn't work right.) I have also checked and replaced the aux. water pump. Any suggestions and help would be appreciated.
Thanks in advance.

Have you replaced the mono-heat valve insert?? IF not do it first. They are all bad if more than a year old. The rubber leaks and rips causing liquid lock under pressure.

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Benzmac:
1981 280GE SWB
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Hi, thanks for your quick response. Would this be the same as the heater control valve. (Located on the passanger side firewall?)