Hey everyone,

It's pretty much "california spring" and I want to get an idea of people's experience with the aux fan before we get hot weather. I used my a/c a lot last summer but I don't think the aux fans are functioning as well as they should or at all even, the temp will start creeping up in traffic with the a/c on. I also feel the A/C might work better if the fans are functioning well.

last summer I did replace a fuse on the green relay behind the fusebox and I did hear the fan kick on once, but I never really hear the aux fan kick on when I have the A/C on Should it be coming on as soon as the A/C in engaged?

Any tips would be helpful

No. The aux fan is pressure controlled, so it will only come on when the pressure just after the receiver drier is above a limit. Probably around 300psi.

If the fan isn't coming on at all, you also want to check the low speed resistor.

quick and dirty function check with car running/AC on - pull off the two wires coming of the pressure switch on the drier and connect them to each other. This bypasses the pressure switch and should see the aux fan kick on. it will NOT be full speed.

if you have dual fans, a single fan will run. if the car truly starts running hot, it will kick both aux fans on MAX, to bring temps down to just under 100* C.

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Allen Kroliczek
Oak Grove Autosport | Oak Grove Autosport
01 G500, 82 300TD, quite a few more.....

thank you for the short simple test, will try that asap!

AFAIK, both fans run in high or low speed. That may depend on the year of your 124. In my '95 E300D, the fans come on in low speed if it's at all warm outside and the a/c has been running for awhile. They come on especially when the car is stationary or moving at low speed but if it's warm enough they come on even at highway speed.

The fans run in high speed when switched on by a switch in the water jacket (early 124s) or by the climate control (later models). Since you mention a "green relay," you have one of the later models. I don't know when they changed from relays with external fuses. The engine has to get really hot for the fans to come on in high speed. If they do, it's probably best to turn off the a/c until the engine cools a little.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

This is a 124, both fans run in either high or low speed.

OP we need the year of your 124.

They changed the aux fan control between 86-87 and all others.

86-87 uses a binary temperature switch in the water neck to directly trigger the fan relay on high speed.

88-up the water neck sensor is a thermistor. This goes to the CCU, and the CCU has an output pin that drives the high speed fan relay.

This relay applies direct voltage to the motor (no resistor).

Both versions use the dryer pressure switch to trigger the low speed fan (through the resistor).

For this reason you also have to be careful with the CCU controller, the 88-up has the extra feature for the aux fan control. You can put one of these in an 86-87 since the pins aren't wired in these model wiring harness and your fans will still work directly the original way. Of course you can also swap the water neck sensor and rewire the system to use the new style controller.

You can't use an 86-87 CCU in a later model as it does not support the CCU based fan control and your fans will not respond to temperature.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Before you throw any parts at it, ensure you are getting good flow through the condenser. Many have unbolted the condenser from the radiator and were surprised at the dirt accumulated there between them.

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Sam

84 300SD 350K+ miles ( Blue Belle )

excellent info, thanks! It's been raining here all week, and I've been so busy so I haven't had the opportunity to check a few things, but I will definitely do so very soon! the A/C works good, but i know it could be colder, it tends to warm up in traffic on hot days

If anyone is still looking for a guide on replacing their Aux Fan, check out the link below for a DIY guide. Hope this helps you all.

Auxiliary Fan Replacement


-Dmitry

thank you Dmitry!

Thanks for the responses everyone. Just checked it today, the aux fans are not coming on when the two pressure wires from the drier are connected. I have to check the fuse on the relay (green relay and fuse) I changed that fuse before and they worked at least once that I noticed.


as for the question concerning the year and model it's a 1995 e300d

so I hooked the aux fans directly to a battery and they fired right up. I am guessing i have to track down a failure in one of the control components.