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Old 10-14-2002, 08:50 AM
Rick S
Posts: n/a
A/C compressor on when heat needed!

I have the opposite problem than most of the heating & A/C situations that are listed in the archives. The A/C works fine in our 1987 300e.

However, when the pushbutton unit is set to the automatic mode and the thumbwheel is moved to any heat settings, the center vents close but we get no obvious heat and the A/C compressor remains engaged.

Any guidance re: diagonostic direction would be appreciated.

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Old 10-14-2002, 10:12 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Unless you pick the EC position the A/C is always engaged. This is to allow the removal of humidity and dust from the air that will then be heated appropriately.

The fact that the center duct closes indicates to me that the controller probably is controlling. The next step is to see if the heater valve is being controlled. The valve is shut with current with the controller supplying the ground circuit. The power comes from fuse #7.

Check both sides of the circuit, during heat both terminals of the valve will be "hot". If confused try pulling the connection. You should get heat with the valve disconnected.

The 124 body is quite dependent on the aux water pump for water flow at low RPMs. To be sure of valve activity drive the car around the block in low gear to maintain enough RPMs to flow water even if the aux pump isn't working.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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