w124 Aux fans - PeachParts Mercedes-Benz Forum

rrgrassi · #1 05-07-2021, 02:12 AM

Guys,

When I run my a/c, that aux fans do not come on. Are the switched by pressure or freon temp?

Thanks!

Usaguy · #2 05-07-2021, 02:30 AM

The a/c aux fans on your car work by means of a pressure switch that is on the "high" side of the system next to the drier.

It should close at 16 bar (240 psi) and then open at 12 bar (180 psi).

The pressure switch activates a relay in the fuse box which then sends power through a resistor (to give slow speed) and then to the fans.

I would start by rotating the fuses in their holders (to remove possible corrosion) and if you still have aluminum fuses then I would recommend upgrading them to the newer style that don't corrode.

Fuse 7 does the switch and Fuse D is power to the fans

Of course your system may not be getting to a high enough pressure in the first place.

rrgrassi · #3 05-07-2021, 04:05 AM

Ok, so it is pressure. Thanks for the info. I am using Duracool. Someone did a poor 134 conversion. Added the the suction port fitting, and not the high port. They left the R12 fitting unchanged. Found that out the hard way when trying to attach gauges.

I am going to change the fan operation to when the compressor is engaged, and depending on ambient temp, so that they do not come on during the winter.

Usaguy · #4 05-07-2021, 04:59 AM

Quote:

Originally Posted by rrgrassi

Ok, so it is pressure. Thanks for the info. I am using Duracool. Someone did a poor 134 conversion. Added the the suction port fitting, and not the high port. They left the R12 fitting unchanged. Found that out the hard way when trying to attach gauges.

I am going to change the fan operation to when the compressor is engaged, and depending on ambient temp, so that they do not come on during the winter.

On a properly working a/c system and a good fan clutch there's no need to have them on all the time

rrgrassi · #5 05-07-2021, 06:01 AM

True, but when stopped at a long traffic light, or is stop and go traffic, I think they are needed.

Diseasel300 · #6 05-07-2021, 09:38 AM

Quote:

Originally Posted by rrgrassi

True, but when stopped at a long traffic light, or is stop and go traffic, I think they are needed.

So let the pressure switch turn them on when they're needed. As Chris said, if your fan clutch is doing anything, they're only in use on hot days or extended standing. The refrigerant needs some heat in it to do work, if you overcool the condenser, you will have very poor A/C performance on moderate or cooler days. There is no better way to control the fans than the refrigerant pressure.

rrgrassi · #7 05-11-2021, 05:25 AM

At what pressure are they supposed to activate?

speednjay · #8 05-11-2021, 07:50 AM

Quote:

Originally Posted by christuna

On a properly working a/c system and a good fan clutch there's no need to have them on all the time

Sure if you live somewhere where the climate is cooler. I find that the clutch fan no matter what brand is flat out useless. The electric fans do almost 100 percent of the cooling for me and all the Mercedes I’ve owned

87tdwagen · #9 05-11-2021, 08:56 AM

Quote:

Originally Posted by rrgrassi

At what pressure are they supposed to activate?

Chris provided the answer in an earlier post:
"It should close at 16 bar (240 psi) and then open at 12 bar (180 psi)."

Note that the pressure switch switch could be bad, likely if original, and if you change it, you will need to evacuate/refill the refrigerant. Since you are opening it up and if the dryer is old, may as well replace that too, they are not expensive.

FWIW, after redoing my AC system, the aux fan come on in Low speed whenever the AC is activated, goes to high speed based on the pressure/temp increases as needed.

Diseasel300 · #10 05-11-2021, 09:30 AM

Quote:

Originally Posted by speednjay

Sure if you live somewhere where the climate is cooler. I find that the clutch fan no matter what brand is flat out useless. The electric fans do almost 100 percent of the cooling for me and all the Mercedes I’ve owned

The electric fans move a tiny fraction of the air that the clutch fan does. They are there solely to assist the A/C condenser and act as an emergency backup when your clutched fan packs up. When working correctly, the clutched fan moves a hurricane's worth of air through the radiator and condenser. Stop buying cheap fan clutches or pulling from junkyard cars.

Diseasel300 · #11 05-11-2021, 09:36 AM

Quote:

Originally Posted by 87tdwagen

FWIW, after redoing my AC system, the aux fan come on in Low speed whenever the AC is activated, goes to high speed based on the pressure/temp increases as needed.

If you charged the system correctly, the pressure should be high enough to trigger the aux fans to remain on only when it's 85˚ or higher outside. Below that they should be cycling on and off, or remain off depending on ambient temperature and load on the system. If they're always on, you may be overcharged which is really hard on the compressor.

Additionally, high speed is not triggered by the refrigerant circuit. Earlier cars have a temperature switch somewhere in the cooling system (location varies with engine) that triggers the high speed relay. Later cars have a temperature sensor in the cooling system that feeds the climate control unit to accomplish the same task. High speed fan is always a function of coolant temperature, not refrigerant temperature/pressure. This isn't a great way to trigger high speed fan operation since your engine is running hot and you have a 5-10˚ window before the compressor is disabled due to high temp.

tjts1 · #12 05-11-2021, 09:56 AM

Quote:

Originally Posted by Diseasel300

The electric fans move a tiny fraction of the air that the clutch fan does. They are there solely to assist the A/C condenser and act as an emergency backup when your clutched fan packs up. When working correctly, the clutched fan moves a hurricane's worth of air through the radiator and condenser. Stop buying cheap fan clutches or pulling from junkyard cars.

I'm sorry but that's simply not true. In a hot climate and low speed the AUX fans do all the work while the viscous clutch fan is a glorified paper weight. Until I learned about the cool harness mod and later complete electric cooling conversion I would sit there and watch the temp gauge creep upward of 100c until the aux fan kicked in. The clutch would lock up and pass the rolled up news paper test, the problem is the fan is too dinky to do any good at idle when the air temp is 100f+.

The clutch fan is a garbage solution for the om603/m103/m104. There's not enough room between the engine and radiator to fit a vicous clutch fan large enough to do the job. The less powerful om602 was fitters with a much larger fan (wider blades) than the om603.

Diseasel300 · #13 05-11-2021, 10:09 AM

Quote:

Originally Posted by tjts1

I'm sorry but that's simply not true. In a hot climate and low speed the AUX fans do all the work while the viscous clutch fan is a glorified paper weight. Until I learned about the cool harness mod and later complete electric cooling conversion I would sit there and watch the temp gauge creep upward of 100c until the aux fan kicked in. The clutch would lock up and pass the rolled up news paper test, the problem is the fan is too dinky to do any good at idle when the air temp is 100f+.

The clutch fan is a garbage solution for the om603/m103/m104. There's not enough room between the engine and radiator to fit a vicous clutch fan large enough to do the job. The less powerful om602 was fitters with a much larger fan (wider blades) than the om603.

Living in a climate that routinely hits 110 in the summer time, I've NEVER had an issue with my 603's overheating. Use a fan clutch that isn't crap and they lock up quite well at low speeds. The air they move is surprising, even at low engine RPM's.

Fun fact: The metal alloy fan on the 603's is the exact same part number as the V8. Funny how they don't overheat either.

tjts1 · #14 05-11-2021, 11:49 AM

Quote:

Originally Posted by Diseasel300

Living in a climate that routinely hits 110 in the summer time, I've NEVER had an issue with my 603's overheating. Use a fan clutch that isn't crap and they lock up quite well at low speeds. The air they move is surprising, even at low engine RPM's.

Fun fact: The metal alloy fan on the 603's is the exact same part number as the V8. Funny how they don't overheat either.

Try the Cajon pass on a Friday afternoon in August.