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Old 06-17-2001, 03:59 PM
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Join Date: Apr 2000
Posts: 71
Okay anyone have an answer for this one?

1978 W123 300D

Air conditioning blows cold air, but compressor never cycles off, nor does the aux. fan ever kick in with the A/C on. Well, actually, the aux, fan never kicks on at all at any temperature. When I first got the car, the aux. fan ran constantly, so I replaced the sensor at the thermostat. Fan stopped running constantly. If I jump the sensor at the thermostat, the fan will run.

Any ideas anyone??


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Old 06-17-2001, 06:46 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The compressor is ultimately cycled by the ETR (evaporator temp regulator) switch. It is behind and below the glovebox, accessible by removing the lower dash panel (relatively easy to expose -right in front). Unless the temp is getting cold it will never cycle. If the A/C is functioning properly the evaporator will freeze into a block of ice within a half hour in humid climates. If it can't make it below 50deg it might never cycle and it surely won't freeze up.

The same holds for the aux fan. It is activated by an engine temp switch of 212 degrees and a thermostatic switch mounted on the drier. The early thermostatic switches had to be pretty hot before they would set it off. If your A/C system is low or working poorly the thing will never creat the heat necessary to turn the fan on.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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