michaelwoosley

Hello everyone. I've been working on a 1990 Mercedes 420SEL... with multiple issues. It had a problem not reaching normal operating temperature so I replaced the thermostat which fixed that problem (by the way, the thermostat is NOT where some would expect it. It is located in the lower housing connected to the lower radiator hose). The second problem that is rather perplexing is the heating and cooling system, particularly the heating as I know the expansion valve is bad and I'm not willing to exert the effort to change it.

Right now, the power button does turn the fans on or off. However, any other button push does not change the fact that heated air ONLY exits the dash vents.

I can hear nothing when I change the circuit to defroster to floor for example. I know where 3 of the door actuators are located (seen these on a YouTube video).

Regardless of which button is pressed, I can hear nothing actuating or moving. I would've thought at least the actuators would move, even with the diaphragms busted.

Has anyone had any experience with this type of malfunction? If so, please, what did you observe or fix to make it right?

Thanks.

Mike

Any help will be highly appreciated.

Thanks,
Mike

knownworldwide

If the diaphragms are busted (most likely) then the actuators will not work at all. They are extremely common failure items. The one behind the center vents is particularly tricky to replace but it can be done. There is one behind the glove box as well as one on the left side up under the steering wheel area.

As for heat, the expansion valve has nothing to do with HEAT. The expansion valve has everything to do with the AC system and it isn’t that hard to replace. The mono valve controls the heat.