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.

__________________
68 280SL - 70 280SL - 70 300SEL 3.5 - 72 350SL - 72 280SEL 4.5 - 72 220 - 72 220D - 73 450SL - 84 230GE - 87 200TD - 90 190E 2.0 - 03 G500

Nissan GTR - Nissan Skyline GTS25T - Toyota GTFour - Rover Mini - Toyota Land Cruiser HJ60 - Cadillac Eldorado - BMW E30 - BMW 135i

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

__________________
Cheers!
Scott McPhee

1987 300D

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.

__________________

"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

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?

__________________
www.jerseyplanning.com
1987 300TD 440K - My car
2005 Jeep Grand Cherokee Laredo 107K - Tim's car
2000 - CLK320 100K - Deb's car
1994 C230 150K - Josh's Car

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

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

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.

__________________

"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

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

__________________
www.jerseyplanning.com
1987 300TD 440K - My car
2005 Jeep Grand Cherokee Laredo 107K - Tim's car
2000 - CLK320 100K - Deb's car
1994 C230 150K - Josh's Car

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

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.

__________________

"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

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).

__________________
1990 300e 230k (old reliable) sold
1987 300D 230K sold
1987 300D 232K 5 speed
1998 E300 140K

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

__________________

"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

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?

__________________
68 280SL - 70 280SL - 70 300SEL 3.5 - 72 350SL - 72 280SEL 4.5 - 72 220 - 72 220D - 73 450SL - 84 230GE - 87 200TD - 90 190E 2.0 - 03 G500

Nissan GTR - Nissan Skyline GTS25T - Toyota GTFour - Rover Mini - Toyota Land Cruiser HJ60 - Cadillac Eldorado - BMW E30 - BMW 135i

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