Mercedes-Benz Forum - View Single Post - chasing electrical gremlin with AC
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post #5 of (permalink) Old 06-26-2019, 10:53 AM
Mike D
BenzWorld Extremist
Date registered: Aug 2009
Vehicle: '72 250C, '74 280C, '85 500SEL, '81 300CD.
Location: Tucson, Az.
Posts: 957
Mentioned: 3 Post(s)
Quoted: 137 Post(s)
First things first. Don't jump to suppositions when dealing with electrical systems. Start at the beginning of the circuit and follow it to the end.

Check the fuse which supplies power to the blower systems. I believe it is the #8 fuse in the fuse box. It is probably fine but check it anyway. With the key in the "run" position check for power on both sides of the fuse. Rotate it and check for corrosion. If it is one of the aluminum strip fuses now is a good time to change to a copper one. Power to both sides?

As per your diagram 83.2, locate the relay labeled #9. This is/was a metal canister and is usually mounted on the fender well. I don't have my /8 here at the moment so I can't post a picture. This is an "old school" type of relay and doesn't look like the newer style. If you feel up to it, now is a good time to replace that old can with the newer type.

This is the relay which is activated by the temperature control knob. With the key on to run position, temperature switch to off,check for voltage to pin/terminal #30. The numbers are stamped/marked on the relay. Power?

With the key still on, turn the temperature switch to "colder", check terminal #30 again. Power? Now check terminals #86 and 87.

Here's a link to help you understand relay circuits.

That's enough to get you started. I'll monitor your progress and try to guide you in the proper direction.
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