I haven't had to work on that problem on a 500E but I have seen it on other 124 chassis cars.

Aside from electronic problems there is one other common failure that gets over looked. That is the auxilary water pump. On other chassis the pump is just used when it gets real cold (like below 59 deg - we hardly ever see them work {bg}). On the 124 chassis the aux water pump is necessary to keep a uniform temp in the heater core.

The Climate control system uses mixing of heat and cold to get the desired temp. If the heater core is changing temperatures because of different engine speeds the controls can't keep pace.

This problem doesn't occur if you are on the road at a constant speed. It occurs in traffic and after leaving a light. While idling the heater core drops in temp as the flow is reduced (without the aux pump). In order to compensate the heater valve is opened more of the time. As one starts from the light and the engine speed increases the valve is open too much and the temp gets hot fast; and of course the controls try to stop it and often go to far the other way.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician