aux fan 300SL

New to the forum...lots of good information here.

I have what seems to be a common problem with the aux fan not coming on at all. I've done a search on all the forums and have read the posts. Here are my symptoms. Fans do function with a remote 12vdc power source. #7 fuse intact and functioning. Jumpering HP switch at receiver-dryer causes relay to click but no power to fans. Removing connector at CTS has no result at relay or on fan operation. I removed the aux fan relay, removed the cover and inspected the relay. No apparent visible signs of damage. Contact points for both pressure side and temp side appear to be normal. I reinstalled the relay with the cap off and using a small piece of wood manually "made" each side of the relay with no results. I don't care to replace the relay if I'm not sure that it's no good.

Is there something else that I can look at? I would appreciate any and all suggestions.

Thanks...Bob

[Benzmac]

When the relay clicks, is there power getting to the resistor? IF so, the resistor could be the problem.

No, there is no power to the resistor. I even changed the relay but still no good. I'm stumped. Is there another fuse somewhere that I am missing?

Thanks for you help

Bob

[Arthur Dalton]

The #7 fuse is for the relay feed. So, if the relay is clicking, it is getting feed.
Now the relay has the LOAD side . That is fan/resistor feed.
Check outside the fuse box for a 30 amp fuse. It will be somewhere around the fuse box. This is the load protection fuse for the fan motor/motors.
The later ones had this fuse on top of the relay. It is there somewhere between the resistor and the relay.
A year of man. will help for the schematic....

Thanks, Arthur. It's a 1991 300SL. I thought there must be another fuse somewhere. I just can't seem to find it. but I'll keep looking.

OK, found the aux fuse box (just behind power brake assy). It has 3 30amp fuse strips. Guess which one was missing? Yep, the aux fan fuse, screws and all. Apparently someone has monkeyed with this system before (my wife bought the car a few years ago). I replaced it with the one from the rear window defroster. Here's what I get now when I troubleshoot.

With everything connected properly and the key ON, I removed the leads to the CTS and nothing happens. I jumpered the HiPress sw at the rec/drier and both come on, but they sound like they are at hi-spd. I jumpered with the CTS leads connected and disconnected and still sounds like hi-speed. The resistor shows 0 resistance when cold.

What now?

[Arthur Dalton]

Zero resistance is direct short in the resistor or someone has jumpered the resistor in order to get high fan on the a/c circuit.
Pull the resistor and see what you have down there.

Hopefully, no one has done some rewire of the sensor circuits....

Also, be sure you meter is on low resistance reading scale......

Thanks, again, Arthur. Unfortunately, I have to go out of town this afternoon for several days, so will have to do some more troubleshooting when I get back. I just got back from a test drive around town with the A/C on to try and drive the temps up. It's a cool day here today (upper 70's) and the temp didn't go above 80C. I checked the fans at engine idle with A/C on and they were both on at low speed...yea! Would this indicate that the resistor is functioning properly? However, still unable to get fans to high speed by disconnecting CTS.

[Arthur Dalton]

OK .. You may have misread the resistor..
[ when trying to explain some of these wiring problems, I always assume one knows how to set up a meter or knows to disconnect one side , etc. cuz we do this on "auto-pilot" , so to speak... but then I reread my post after it goes out and I say , "Damn, I should have mentioned XYZ. Is this guy giving me backfeed values????].

Anyway, the best test for the high speed is to get a 150-180 ,
ohm , 1/4 watt resistor and bridge the CTS harness plug.
Sometimes the removal works and sometimes it does not... I think it has to do with how close to cut-in ohm spec it is when trying this method .[ maybe SB or Doc knows that one]

But I have found the resistor bridge circuit to work , as the ecu now reads beyound the cut-in temp/ohms
Another is to get the temp/resistance chart and read the CTS with your meter using the temp gauge as reference.

Be aware that there are 2 relays . one for low fan/resistor, and one for the high coolant temp sender circuit....

[mbdoc]

The aux fans work a bit different on the 1990 to 1995 SL's than other MB's. The A/C pushbutton signals the low speed relay any time the compressor is on. The GREEN switch in the drier turns the aux fans to the HIGH speed when the A/C pressure reaches 295 lbs. This is done thru the pushbutton to the relays.
If the A/C isn't on the 107C switch will turn the fans on to the high speed at that temp, also thru the pushbutton.
The low speed resistor is under the left headlamp.

Thanks for all the good info. I'm really happy to have found this site. Of course, I would have saved myself some work if I had found that 30 amp fuse strip sooner, but I figure that I learned something in the process.

[Arthur Dalton]

Thanks for that Good Info..MB

Guess they are looking for more air with the constricted engine compartment on the SL/s..

That explains the high fan on a/c operation...