Mercedes Benz E-Class W124 - Merc diesel heater stuck on HOT

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Sun 28 Mar 2010 13:34
Mercedes Benz E-Class W124 - Merc diesel heater stuck on HOT - robertp

1987 W124 300D Saloon. 250,000 miles. Auto transmission. Has Aircon (but not functional for many years due to leaking pump)

Problem - the heating controls don't allow the temperature to be set. When engine is at operating temperature, heater puts out maximum heat regardless of settings. Problem evident once last year and disappeared before diagnosed. Now returned and looking permanent.

Temp setting wheels move as normal. No fuses blown. Heater blower works normally. Air supply controls work normally.

Possibilities - failed internal temperature sensor? Heater valves - are these motor driven?

Thanks in advance for any advice.

Rob

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Sun 28 Mar 2010 14:51
Mercedes Benz E-Class W124 - Merc diesel heater stuck on HOT - elekie&a/c doctor

The heating system on these is controlied by a "duo water valve".It is a twin solenoid operated valve that restricts the flow of water into the heater matrix.(left and right).The rotary wheels on the heater panel adjust the return voltage from the valves thus changing the flow of water and consequently the heat output.At this sort of mileage it is very likely that the internal diaphragms have perished,so a new valve is reqd.It is situated just in front of the brake servo.There are 3 coolant pipes and a 3 -pin electrical connector.Might be an idea to check for voltage to centre pin with ignition on.This should be present at all times.hth

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