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Originally Posted by 400E
A very common problem with MB aux fans is the low-speed resistor fails. You can find this resistor behind the left headlight.
Low-speed fans are designed to run only when refrigerant pressure exceeds a certain limit, so in contrast to many other makes, your aux fans will NOT necessarily come on every time the AC is on. If you do wire them to run whenever AC is on, there is some concern (from other lists) that they won't last as long, and they're not cheap to replace.
High-speed fans come on when coolant temp exceeds a certain limit (don't recall exactly how high).
FYI, if your 400E is like mine (and I think it is), you don't have a Klima relay or over-voltage protection relay -- these functions have been incorporated into the 4 circuit boards in the protective metal case on the right side of the firewall (i.e., where the battery would normally be in most other W124s).
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I agree with everything you have stated. Inside the module box, one of the controls units is responsible for controlling the AC compressor as well as several other devices. This control unit is known as the Base Module (BM), it is the one fitted with fuses. At this time I would not worry about it being a problem because it sounds like it is doing it's job because the compressor is cycling. You might want to begin by checking the refrigerant level, if it is low it could explain the unpleasant temperature you are getting at the vents while idling.
Now the aux fans are another story, it sounds like you need to check the ballast resistor as 400E suggested. As well, you may want to remove the fuse box cover to expose the relay section, two of these relays control the aux fans. One is for hi speed and one is for low speed, some of the fan relays have built in fuses 15A (blue) 30A (green). If you discover your relays are fitted with fuses than I might suspect a new 15A fuse may solve the low speed fan problem.
FYI-the blue 2 pole switch on the intake manifold controls the aux fan high speed circuit. Fan cut in voltage is roughly 1.5 VDC and the cut out voltage is roughly 1.75 VDC. A 2k mini potentiometer (15 turn is best) adjusted to roughly 1450 ohms and then soldered to each leg of the sensor lead will help provide for better fan function. The low speed fans will cut in a little bit sooner and they will stay engaged a little longer................the advantage to using the mini pot is that you can make subtle adjustments whenever your driving conditions change. NICE modification.