Tell me if I have this right about the Auxilary Fan and when it should kick in...
 

I've been doing some "searches," so let me know if I have this foundation of knowledge right on how to troubleshoot an auxillary fan not coming on (82 300SD):

If the fan is not working you could run a wire to the battery just to see if it works at all.

If it works, then the next step is to check the connections at the reciever / dryer.

Next is to look at a "relay" (also known as the "ice cube" relay).

See the pic - is that the right relay? (Originally Bosch 0 332 205 002. 5 pole terminal).

If so - Where the heck is that on a 82 126 300SD - under the cover with the rest of the fuses up by the firewall are somewhere under a cover near the fender????

From my research there looks to be two conditions when the fan is supposed to come on, and the fan has two speeds. There is a "high" speed triggered from a sensor near the thermostat when coolant temps exceeds 100* C, the low speed is triggered based on the air conditioner creating pressure in the system at the receiver / dryer (most common condition).

The auxilllary fan's main function is to pull air through the condensor to assist the A/C. So the auxillary fan helps your A/C condensor mostly, and kicks into "high" speed in severe overheating to help out your radiator.

The cooling system shouldn't need the auxillary fan to keep the car temp in the normal range - right???

There is an auxillary fan switch on the reciever / condensor, but I think if you remove it, you loose your freon. Is that correct???

So if it's not the fan, or the relay, I probably need to talk to an A/C specialist.

I'm trying to troubleshoot my friends 82 300SD... thanks all!

Just for comparison sake.....On my '87 SDL (different engine), the aux fan comes on at 105C. I could never get it to come on for the AC, although I believe that you have the operation correct. And if you take the pressure switch off the rec./dryer, you will lose freon.

bumb
 

Just giving this thread a "bump" to see if anyone else had input on my original post (my questions and assumptions).

Thanks all!

The primary function of the aux fan is to bring down high side pressure when it gets dangerously high. This is to save the a/c system from excessive and damaging high side pressure.

Some of the high pressure switches mounted to the r/d are in actuality temperature switches. Pressure directly correlates with pressure. If it is a temp instead of a pressure switch, you can change it without losing refrigerant.

Regardless of whether it is a temp or pressure switch I would not change it until I determined that it is bad. Get a wiring diagram for your car and it will probably indicate the pressure at which it trips. If it is temperature, you need to look at the scales on a high side gauge to correlate this with pressure. Then simply monitor the high side gauge with no ram air and the engine reved to about 1,500 RPM. The high side pressure will increase because there is little air flow. When the pressure reaches specification if the switch does not actuate then replace it.

Good luck,

the end of the story...
 

Just filling in the rest of the story. I was able to ground out the fan in the friend's 82 - fan worked. Then we got my 84 hot enough to get the aux fan to kick in, then we swapped ice cube relays and got my fan to get on again so his relay was fine as well... so he has bigger A/C issues.

thanks everyone for all the help - LOVE THIS FORUM!!!

aux fan went on despite no a/c
 

after a highway run yesterday in fairly warm conditions (about 85deg F ) my aux fan went on after the car was at idle for a short while (about 15 minutes). it only stayed on for about 30 seconds. the temp gauge showed nearly 100 deg C at the time the fan went on.
as a reference my a/c was not on at the time, the CC was in the off position (and the a/c doesnt work anyhow).

84300DT,
I believe that is the correct operation. After the hard run and pulling up an idling, the temp went up to where the sensor on the water pipe went to ground.

I can ground that wire on my 84 300D and the fan will not come on. The two wires going to the dryer temp switch when connected will cause the fan to operate. Not sure why it will not trigger from the engine temp circuit.

I am having a similar problem with my 1981 300SD w126. I just recharged (fixed) the AC system but I have no fan..
I was sitting in the parking lot the other day and it clumb right up there to 100+ with the AC on and no dice on the fan working. I wanted to see if it would do anything but it seems that it won't. Is the fan supposed to be on whenever the AC is on? If so...how? Heh...I don't have a wiring diagram for my car...and I think I need one..just trying to find one.
THanks!

Somewhere I have posted the exact temperatures and pressures at which the factory considers those items " working correctly".....

know where?

On the Diesel forum ?

"The cooling system shouldn't need the auxillary fan to keep the car temp in the normal range - right???"

Well, that depends.... there is more resistance to air flow with the condensor in front of the radiator.... so with that in place it may be necessary , particularly at low speed or stop and go traffic to have it available...

"The auxilllary fan's main function is to pull air through the condensor"

No, the pulling should be done by the engine driven fan... this fan is in front of the condensor and would provide PUSHING of air...

There is another component that I think you may have overlooked. Its the inline Fan resistor.

My fans did not work and it turned out that the resistor contacts where corroded. Cleaning them and the two connection points holding the resistor in place put my fans back in business.

This resitor is typically located either, close to the fan motor, or on the driver's side fender behind the headlight.

Phil