WARNING: Aux fan failure can cause fire. Preventative fix inside. - PeachParts Mercedes-Benz Forum

300SD81 · #1 05-18-2015, 12:34 PM

Resealed my AC yesterday, aux fan fails while I'm charging the system today. Thankfully I caught this in time before there was any damage.

What happens is that is the fan shorts, and it's only on low speed, the low speed resistor limits the current enough to keep the fuse from blowing, but gets extremely hot. The resistor also happens to be in a spot where lots of leaves and twigs tend to gather. Bad combo. I caught a small fire down there because I was charging my AC system, it would have been far worse if I was driving.

There's a simple fix for this. Bypass the low speed circuit, so that the fan always comes on high speed if needed. This is pretty much desirable if you live in a hot climate anyway. If the fan shorts while on high speed, it will instantly blow the fuse.

Simply remove relay D, and connect it's coil input to relay C's coil input.

Aux fan resistor. FIRE HAZARD!!!

Low speed relay eliminated, bridged to high speed.

Wiring diagram...

300SD81 · #2 05-18-2015, 01:03 PM

Actually, upon further inspection of the circuit, there seems to have been a major reversion in 1991. For the 1991 model year, the aux fan circuit contains NO FUSE. WTF MB?

The above fix works if you have anything other than a 91. If you have a 91, do the above and then cut the red wire going to the high speed relay, and insert a fuse holder inline. I guess 15 or 20A would be about right.

1990 and older. Fan circuit is protected by fuse.

1991. No fuse in circuit, add one.

tyl604 · #3 05-18-2015, 03:33 PM

I have a more basic question. Which fitting to you use to add Freon? And do you hold the can upright or upside down? How do you identify the correct fitting?

Hit Man X · #4 05-18-2015, 09:11 PM

Quote:

Originally Posted by tyl604

I have a more basic question. Which fitting to you use to add Freon? And do you hold the can upright or upside down? How do you identify the correct fitting?

If you are asking questions like this, you really need to learn more about auto HVAC before jumping into it as you can very easily kill the compressor and injure yourself big time. At the very least, read everything on the MB CDs for the W126 and take the EPA test so you can buy R12 legally, you will learn quite a bit.

How do you know your compressor is good? If low, does the system hold vacuum... if so, how are you certain? Where did the previous refrigerant go? Are you planning to just add refrigerant and not check head pressures? How do you know what refrigerant is still in the system? Did you know 134A and R12 use different oils and are not compatible?

In any event, I agree the resistor is in a poor location. I vacuum out in there often. I like having the ability to use 'auto' and low speed so I keep that area clean!

Just finished a rewire of that area in my 300SD due to a small fire, all the HVAC did was high speed. Pretty annoying. The repair was a big time PITA... booster has to come out to really get in there.

300SD81 · #5 05-18-2015, 10:44 PM

Quote:

Originally Posted by Hit Man X

In any event, I agree the resistor is in a poor location. I vacuum out in there often. I like having the ability to use 'auto' and low speed so I keep that area clean!

Just finished a rewire of that area in my 300SD due to a small fire, all the HVAC did was high speed. Pretty annoying. The repair was a big time PITA... booster has to come out to really get in there.

This isn't the blower motor resistor, but the resistor for the electric fan infront of the radiator. I definitely wouldn't want to eliminate low speed on the blower. Is there another one there on the 300? My SD is parked at my mom's house until I finish an engine rebuild, probably in like 5 years at this rate. The 560 only has 1 resistor there.

I'm tempted to slap together a circuit to get rid of these resistors and use PWM. Less wasted power, no heat, and software-adjustable fan speeds.

Carson357 · #6 06-02-2015, 02:15 PM

Quote:

Originally Posted by 300SD81

Actually, upon further inspection of the circuit, there seems to have been a major reversion in 1991. For the 1991 model year, the aux fan circuit contains NO FUSE. WTF MB?

The above fix works if you have anything other than a 91. If you have a 91, do the above and then cut the red wire going to the high speed relay, and insert a fuse holder inline. I guess 15 or 20A would be about right.

1990 and older. Fan circuit is protected by fuse.

1991. No fuse in circuit, add one.

instead of adding the fuse holder i opted for a relay that had a fuse built in.
001-542-96-19 this relay is identical to original minus the color and fuse

Hit Man X · #7 05-18-2015, 11:08 PM

On the first gen W126, under the booster is where the blower fan resistor is located in addition to the aux fan resistor. Believe me, I spent YEARS trying to find out why the hell I had only high speed blower fan. PBUs and both relays swapped making myself nuts.

PO had a small melt down in there, the aux fan still worked on low (shockingly) but the wiring was very crispy. I spent quite some time in there with a soldering iron, heat shrink, and quality wrap.

This is the same reason my car had no ABS working, LF sending wire was bad. I have dirty exciter rings I believe, but hey... at least the system goes out of fault mode once the vehicle starts.

I believe MB moved the blower fan resistors beside the actual blower on the second gen W126. Seems to stick in my mind as there.

My 300SEL had a broken wire to the fan resistor and the fan was jumped to run high speed only for years. Worked fine just as your diagram shows. I fixed it and now use low again.

I missed the 'aux fan, high speed only' detail in your initial post. Sorry about that!

lsmalley · #8 05-18-2015, 11:50 PM

Instead of bypassing the low speed fan why not just write in a fuse before the resistor so if you get a short then the fuse is there to protect the other wiring.

300SD81 · #9 05-19-2015, 12:11 AM

Quote:

Originally Posted by lsmalley

Instead of bypassing the low speed fan why not just write in a fuse before the resistor so if you get a short then the fuse is there to protect the other wiring.

The problem is that the resistor limited current low enough not to blow the fuse, but got so hot that it started a fire.

I've already decided I'm designing a circuit board to fit in place of both relays, and use PWM to do low speed without a resistor. I'll probably have a couple extra to sell, should be cheap.

ps2cho · #10 05-18-2015, 11:51 PM

This is why its good to keep the engine bay clean

Glad you caught it though.

lsmalley · #11 05-19-2015, 11:48 AM

Excellent idea.