W124 Auxiliary Fan
 

Should it run when the AC is switched on?

The car was cold and I turned on the AC at idle to see how the AC felt (its getting a checkup soon) and a friend suggested I check the fan as well since its weak around town but okay on the highway. AC felt cool at idle but the auxiliary fan didn't kick on.

I put 12V to the connector and the fan does work.

So if the fan is supposed to kick on when the AC compressor comes on, where should I look now? Relay? If so, where is that guy.

It's temp controlled, comes on only when it needs to.

The fan has two speeds. Low speed is triggered by the AC high pressure switch, the switch with pigtails on the drier. Jump the pigtail ends and the fan should come on. High speed is triggered by the coolant temp switch, the one on the upper radiator hose turret on the cylinder head. Jump the contacts on the 2-pin connector and the fan should come on.

If the fan doesn't come on on low speed, it's either a bad relay or more likely a bad step down resistor behind the left headlight and ABS pump. A bit of a PITA to get to.

The fan doesn't come on just because the compressor is engaged. There's also the high pressure factor.

Sixto
87 300D

In addition to what Sixto has provided -- There are two relays in the relay box (behind the fuses) that turn on the fan in high or low speed, depending on which switch, as described by Sixto, is activated. The relays are usually reliable. I would check the high-pressure switch in the refrigerant line first. Unfortunately, replacing this switch requires evacuating and refilling the refrigerant.

OTOH, the high-speed switch in the water jacket can be replaced without draining the coolant, which is nice. Sixto and I have both replaced our stock switches with one that turns on the aux fan at a lower coolant temp (100C) to additionally protect the engine. The stock switch turns on at 105C.

Plus the OE (gray top?) cuts the compressor at 128*C vs the red top at 105*C. Suffer the ride home or replace the head :)

Sixto
87 300D

Any part#'s?

I put them all in another thread. Let me try to find it.

Jeremy

Part numbers are:
006 545 64 24 (original equipment in OM603 only) 105ºC/128ºC with blue top
006 545 42 24 (used in M102 and OM601 engines) 105ºC/120ºC with gray top
006 545 61 24 (used in M102-3-4 and OM602 engines) 100ºC/110ºC with red top

Don't forget to install the crush washer if you change switches.

Thanks! I think I'll get the red top for my wagon.

Oddly, the 86/87 SDL use the gray top while the 87 300D/TD use the blue top. Go figure. There's also a green top 105ºC/115ºC switch in some M103s. Don't you love pick-n-pulls?

Sixto
87 300D

Color blind?
 

If you get the switch at a dealer, don't tell them what your car is, it will just confuse them. Just give them the part number. On-line suppliers, same thing. Wrecking yards, go by the color.

For the switch in the head you can also adjust the turn on temp by placing a resistor in parralel. Can't remember the exact value but different ones make the temp different. I adjusted the one on my 103 to kick on at 95 and got away with a bad fan clutch for several years (was poor college student).

Revised list
 

Thanks, Sixto, I'll revise my list.

Comment: The OE switch in the 1987 300D/TD does not turn off the a/c compressor until the coolant gets to 128°C. In my opinion and Sixto's, this is waiting much too long and may be one reason why this engine is known to crack its head. Replacement of this switch with one of the others, mechanically and electrically identical except for the temperature settings, is advised. About $30-40 new from your dealer or $5 from a wrecking yard.

Edit (6-28-2011): I had the red and gray part numbers reversed. Chris caught my error, thanks. Here is the corrected list. BTW, I've noticed that the factory original (blue top) switch in my '87 has the Mercedes part number stamped into the brass base. The replacements have the temperatures but not the part number, you have to go by the color of the insulator and sometimes the part number is on the box.

Mercedes-Benz auxiliary fan (high speed) and compressor cut-out switch part numbers are:
• 006 545 64 24 (used in OM603 of '87 300D/TD only) 105ºC/128ºC with blue top
• 006 545 61 24 (used in M102 and OM601 engines plus OM603 of '86/'87 300SDL) 105ºC/120ºC with gray top
• 006 545 45 24 (used in some M103s) 105ºC/115ºC with green top
• 006 545 42 24 (used in M102-3-4 and OM602 engines) 100ºC/110ºC with red top

Don't forget to install a new crush washer if you change switches.

Jeremy

Drove the car a half hour in 85* weather with the AC on. Got home and the auxiliary fan was not on at all.

Is that indicative of a low speed relay?

The aux fan's low speed circuit doesn't respond to ambient or system temperature, it responds to refrigerant system pressure. The FSM suggests the high pressure switch triggers the aux fan at 20bar/290psi. Bridge the pigtails on the high pressure switch with the engine running and compressor engaged. If the aux fan doesn't come on, it could be a bad relay, bad step-down resistor, bad fan or bad wiring. If the aux fan comes on, the relay is fine but it tells you nothing about the switch. I suppose you could put a gauge on the high side but I don't know how you force 20bar at the receiver/drier without the risk of overheating the engine.

Sixto
87 300D

Jeremy, you have the red and grey numbers reversed.

Chris

I had the same problem....
 

When my auxiliary fan wouldn't operate, it was the step down resistor...it was cracked and broken from age.

As I have the #14 head, I was always afraid I would overheat the engine.

I removed the resistor from the circuit and used a small bolt and nut to connect the wires, then used shrink wrap tube to seal them.

Now when the pressure goes up on the AC, the fan runs at high speed...no low speed anymore, but who cares...I'm inside the car in the cool!

The Tenor Man

Correction
 

The list in post #13 is now corrected and I made a couple of additional comments. Chris, thanks for catching the error.

Jeremy

Where is the step down resistor???

IIRC, it's screwed to the left inner fender, low down, just behind the left headlight. I have a picture somewhere, let me confirm & edit this post.

Edit: Yes, it's next to the ABS pump (between it and the fender). For pictures, see my post #18 in this thread.

Jeremy

If anyone wants to check the relay, it is relay C.

Relay C is the second relay back behind the read cover of the fuse box. In order to get to the rear fuse box, you need to take the six Phillips head screws out of the fuse box.

So unless, I refill more R12 into the system about 290psi on the high side, the auxiliary fan will turn on. Is this right?. Right now I have it about 190, the AC seems to work fine but no Aux. Thanks.
-Lui

I'm resurrecting the AC in my 87 300TD. I have it to the point where it is charged and compressor is coming on, however the aux fan is not coming on when the compressor is engaged.

As a troubleshooting measure is temporarily bypassing the step down resistor as described by "the tenor man" a bad idea? Or are there potential electrical problems introduced by doing this?