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