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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.
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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?
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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.
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Quote:
Sixto 87 300D |
Color blind?
Quote:
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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).
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Revised list
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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 |
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