I noticed today that my auxillary fan does not appear to be coming on. On my three other diesels (123's), the auxillary fans were rewired so the fan came on when you turned the car on. I would like to do the same thing to this car. Has anyone done this, and if so, can you please enlighten me on the best way to tackle this project.

Thanks,
Mike

__________________
Mike

'80 300D
'84 300D
'85 300D
'87 300D

If the temp gauge isn't going over 100C, that's not a problem.

It should come on at low speed when the AC compressor is engaged and at high speed when the engine coolant temp is over ~105C. There's a 3 pin temp sensor where the upper radiator hose connects to the engine. Grounding one of them will get the aux fan going full tilt. It's safe to ground the pins one at a time for testing.

Why do you want to leave the fan running?

Sixto
95 S420
87 300SDL

From what I have heard, and read over the years, the fan does not come on until the temp has already gotten too high. My father's mechinic in Florida rigs these fans so they come on as soon as you turn on the car. The temps here in Houston are quite hot and I want to keep this car as cool as I can. Is there any harm to having the fan on all the time?

Mike

__________________
Mike

'80 300D
'84 300D
'85 300D
'87 300D

Keeping the Auxilliary fan(s) on is NOT a good idea. They will wear out much faster then if they were to come on only ocassionaly.

__________________
'95 E300 Diesel, 264,000 Miles. [Sold it]

My fan only comes on when the car is idling for a while and the a/c is on.
I think the low speed is not working.
Is there a resistor in the line somewere?

I'm not really concerned about the fan wearing out too fast. I think I would much rather replace a fan, than an AC system or engine which got a little too hot.

Mike

__________________
Mike

'80 300D
'84 300D
'85 300D
'87 300D

If your cooling system is up to snuff you shouldn't need the fan. My car never got hot enough in I-95 traffic with the AC blasting in 95 degree weather to need it, it was running around 90-95C. How is 105c to hot? The MB enginers thought it was the right temp for the fan to come on.

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

The resistor is beside the receiver/dryer on the W124, somewhere in that vicinity (left front, behind headlight) on the W123, I believe.

It's fairly common for the connections to corrode or the resistor to fail so that the fan won't run in slow mode. Test by jumpering the two long pigtail wires at the pressure switch on the receiver/dryer (the switch with no leads, just lugs, is the low pressure switch). If the fan comes on, you are probably fine.

Running the aux fans all the time is rough on the fans and alternator, they are designed to provide proper cooling in intermittant use. You are wasting fuel, too, by running them all the time.

The cooling system will tolerate temps under the red zone indefinitely, and the system it DESIGNED to run at higher temps under extreme condtions -- teh higher the temperature difference between the air and the coolant inside the radiator, the more heat is tranferred to the air.

If you aren't seeing anything higher than 90 C on the gauge and the aux fan runs when "triggered" by grounding the correct wires, leave it alone, it's working properly.

If you see 100C regularly in slow traffic, you need a new visco clutch, it's not working.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Do what I did! I installed a by-pass switch so if I want to over-ride the thermo switch to turn the fan on, I just flip a toggle swich on under the dash and the fan comes on. When not on, the thermo switch will kick-in at or slightly above 100C.

I liked the idea of keeping temps down while stopped in traffic because the 603 heads are tender and margins for maintaining in the safe temps are closer than almost all cars.