A friend's 1995 E320 is getting a bit hot during summer when A/C is on and idling. I read this post: '93 w124 M104 Aux fan ground location.

I am thinking it may be the resistor because it does now seem to have a low speed. If I recall correctly (from my 1987 300TDT), when A/C is engaged the Axillary fans turn on low. When Temps reach 105, it goes to medium, and then at 120 is run at full.

I have been reading that it is only 2 speed, low and high. Either way, it appears to turn onto High at 105 and does nothing when A/C is turned on.

Should the fan be engaging on low when the A/C is on?

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000

The low speed fan is activated by the pressure in the high side of the a/c system reaching a set level. That doesn't happen immediately, but generally does occur after letting the car idle for a few minutes (depending upon cabin and ambient temperatures).

High speed fan is activated when engine coolant temp reaches 105. There are only two fans speeds. Well, I suppose if you count "off", there could be three. The a/c is disengaged when engine coolant reaches 115ish.

Most likely the low speed resistor or wiring leads to it are burned out. The first step is a visual inspection - the resistor is located near the a/c receiver/dryer. Make sure it's present, not crispy, and all wiriing is attached. Step 2 is to probe it using an inexpensive DVM. Test resistance across the resistor. Then let the a/c run for a bit and test for voltage between the resistor and ground.

Later 124 chassis cars had a wiring change that reduced the wire diameter running from the fusebox to the resistor. As a result the wiring sometimes overheats and breaks away from the resistor. If this is the case, it should be obvious.

Power for the resistor comes from a relay in the fuse box. The relay is activated by a switch on the a/c dryer. If you open up the fuse box - completely remove the cover - you *may* be able to hear the relay click when low speed fan is activated.

It's a whole lot easier to troubleshoot this when you have the wiring diagram in front of you...

So I replaced the fan resistor and still no dice. The axillary fan does not engage until 110-115. My low fan function is not working. The old resistor looked a little crispy but not too bad. What next?

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000

you have low AC pressure......

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1993 300E, 2.8 M104 .....

We had the system on a gauge and it was fine...I do not have the numbers but my friend who is a great mechanic said it was perfect. He also added a BG additive/lubricant to the A/C system and it helped a TON. A/C is blowing ice cold here mid summer in Central Florida. Only when I am in stop and go or idleing with A/C on does the engine get too hot. Fan clutch passes the friction test when at operating temperature.

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000

It could be failed pressure switch, failed relay, failed relay fuse, failed wiring.

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CENSORED due to not family friendly words

Jump the wires on the pressure switch. If the fans work then the circuit is ok....

If the fan works and the circuit is ok, also if your ac is pressure is Ok then you need a new pressure switch as Tjts indicated. A new pressure switch is cheap, but you will have to evacuate and recharge the AC system...!!!

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1993 300E, 2.8 M104 .....

Arthur Dalton PMed me with just the right info. He had me jump the fan at the low pressure switch. The fans did not turn on but I did hear a click from Fuse box. From here he guided me to the relay which is behind (towards firewall) the regular fuses. All I needed was the hood straight up and a phillips head screwdrivers to access it. Six screws is all it took.

Once the panel comes off there are two relays, each with blade style fuses. Just as Arthur had supposed, the original fuse was 15 amp and was blown. The relay is blue. According to him Mercedes changed it to 25 amp, but he uses 20 amp with success. I got a multi fuse pack and put in a 20 and voile! Cranked the car up with A/C on and fans turned on low immediately. Car now stays between 80 and 100. Even when idling!

Thanks so much Arthur, Hopefully this helps someone else!

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000