I have a 1985 Euro 300D, model W124, with air-conditioning. The control unit has a driver-side and pass-side temp wheel. Problem: running in EC-mode, at irregular times, the heater blows like an inferno for a few minutes and then will resume normal operation. No pattern has been detected, this has been going on now for months.

I have removed and looked at the control unit, and it looks ok. I did resolder one loose wire. I have replaced the twin solenoids near the battery controlling water-flow. I believe the interior temp sensor is ok; but even if it was not, the controller seems to ignore that signal, since the heater can come on despite the whole car being hot due to being in the sun.

Is it likely the controller even though nothing on it looks burned out ? Are there some more tests I can do to help determine the problem ? I'd hate to shell out $500 for a new controller, only to find out that's not it either.

Any help is appreciated.

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Michael H
1986 300D W124 5-spd manual 250k miles

Not at all sure, but I *think* your car has a duovalve with two units similar to the monovalve on US cars. If so, could you be looking at an early failure of the rubber diaphragm that's intermittently allowing full coolant flow into the core on one side or the other?
My car behaved somewhat similarly until it finally failed completely, when a simple (and cheap!) monovalve core replacement fixed it.

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Craig Bethune

'97 SL500, 40th anniversary edition

'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)


SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)

Thanks Craig. But, when I replaced the solenoids, I actually replaced the whole "mono-valve" unit; not cheap ! So, I'm thinking I will remove the controller again and closely examine it - maybe I'll get lucky.