W108 A/C Knob Malfunction - Mercedes-Benz Forum

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post #1 of 4 (permalink) Old 08-22-2016, 10:46 AM Thread Starter
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W108 A/C Knob Malfunction

Hello All,

I'm working on getting my A/C in top notch again. I have another thread addressing a compressor noise, but from the cabin, I need some help with the switches too.

I have the under-dash unit with two knobs as seen here:

Trouble is, with BOTH of my switches, it's either full on, or full off. There's no adjustment for fan speed or temperature. If I spin the left switch past "HIGH," the fan just turns off. If I turn the right switch past "CITY" i believe (or coldest), it goes warm.

I'm assuming the coil in the switches are broken. But I can't seem to find a part number for replacements. I found them in the old Mercedes Part Catalog as seen below, but that doesn't have the real part number. Can yall offer any help here?

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post #2 of 4 (permalink) Old 08-22-2016, 02:38 PM
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Vehicle: 1972 280SEL daily driver. 904 Dark Blue with cognac interior. euro 4.5 engine
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So I had a similar issue with my fan switch. I discovered there were no new ones to be had and I hesitated to put in another 45 year old switch from a scrapped car (not to mention people wanted a fortune for them). I chose to replace it with a modern 3 speed switch. I purchased mine from oldairproducts.com. I no longer have the infinitely adjustable fan speed, but I have reliable high, medium, low. Existing knob fits on the switch.

I believe the other switch is a simple thermostat with a capillary tube. They also sell those with either an 18" or 48" capillary. 48" is probably longer than you need, but pretty sure 18" is too short.

BTW, I also replaced my AC blower motor with one for a ford mustang (I forget which 60's vintage) and it works great. Same connections and similar mounts.

If you are looking to stay "original", I can't help.....sorry
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post #3 of 4 (permalink) Old 08-24-2016, 08:58 PM
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If you can tell me how to test the knobs on the unit I removed from a 1969 280s parts car with AC, I would be glad to tag them when I get around to it.


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post #4 of 4 (permalink) Old 08-25-2016, 08:58 AM
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Fonzi...I believe the following is correct for the W108/W109 system. The fan speed switch has three connections on the back. One is voltage in, one is power out for the clutch (it actually goes to the thermostat switch) and one is fan power out. When the switch is off, there should be an open circuit between the power in and all the outputs. When you turn the switch, the resistance between voltage in and power out to the clutch should go from infinite to 0 ohm and stay there as the switch turns. The resistance between voltage in and power out to the fan should sweep smoothly from 0 ohms to fairly high resistance...0 ohms being high speed and higher resistance being slower speed.

The capillary switch, I believe, should be 0 ohms or open circuit depending on how far the knob is turned and the temperature applied to the end of the capillary tube. Theory being, for a given knob position (desired air temp) the capillary tube will open the contact when temp on the tube is below the set point (turn off the compressor) and then close the contact when temp rises (turn on the compressor)
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