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Q: airconditioner malfunction on 1997 Mercedes-Benz E420

when my air conditioner is on full cool it blows 55 degrees on the passenger side and 71 degrees on the drivers side. why?
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This has electric motors that control the air flow, and if one of those motors are not working this will happen. You'll have to have it checked by a shop that has the appropriate Mercedes scanner to check it out.
Your car has a valve that moderates temperatures so that driver and passenger climate control functions properly. The valve is under the hood typically (depending on year and version of your E420) on the passenger side somewhere near the right shock tower. It's called the Duo-valve. The seals in the valve typically wear out and cause one or both sides of the climate control to behave as you describe. It is a engine coolant valve; not an AC valve. I suspect that hot engine coolant is flowing into the driver side causing the temp to be warmer than the passenger side (in simple terms: the heater is stuck in the "on" position ;-)

It should be noted that your car has an electronic diagnostic that you can perform by operating the climate control buttons in a certain sequence. This diagnostic can confirm for you what the core temp of both of your inside-the-car heater cores are. Obviously, when running on full AC on both sides, they should both read the same temp and both be quite cold. Search online for more info on mercedes ac diagnostics and you will learn the method I described above.
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