1994 C280 auto climate control problem

[Fred Mopar]

unit will not work in auto mode. will put out hot air only on HI setting. otherwise puts out cool air on other pre set temps such as 74 degrees. inside senser replaced by local expert(sic) after he claims he diagnosed the problem, said it could take many hundreds of dollars to diagnose further. local cooling shop traced the problem to either the control valve or a pump but they said they could go no further.

[benzfan]

The pump they refer to has no bearing on the heater function. It will continue to circulate warm coolant after the car has shut down, but only if you tell it to. Mine hasn't worked in years, but it doesn't affect the heater. You really need to see a dealer to diagnose this accurately without spending extra money. Search this forum for a thread regarding c280 A/C woes and you will gain some insight into the self-diagnostic functions which might help you pinpoint bad sensors, if any.

[stevebfl]

Actually the pump does play a big part in keeping the heater core hot when the engine speed drops like in traffic. A common problem we have seen on some models is after sitting at a light and the heater on mid setting, the valve goes to a higher duty cycle as the heater core cools due to low flow at idle. Upon accelerating the heat bursts upon the car as the vale is still wide open and the increased engine speed floods the heater core.

I would suspect a sensor problem or a controller problem, but you probably need a technician that understands the system and has the information to test it properly.