Troubleshooting Aux Fan - PeachParts Mercedes-Benz Forum

phinsup · #1 10-06-2009, 09:52 AM

I keep blowing fuses, I replaced the aux fan because the original one was pretty noisy, the new one is much quieter, however as I suspected it still blows the fuse. It even melted the fuse box a little behind the contact. It will work fine for a day or two then blow, which leads me to believe it's not a short somewhere, could a bad relay cause this?

I'm kind of leaning towards rewiring everything, new relay, new fuse and just bypass the fusebox, however after browsing the forum I am a little confused, some people say the w123 fan has a cooling sensor and a freon sensor, looking at the wire diagrams I only see the freon sensor, so I'm pretty sure that the aux fan is only a/c based, but want to verify.

Like I say I'm really leaning towards replacing the relay and all the wiring and putting in an inline fuse. Unless someone thinks a bad relay could cause fuses to blow?

Thanks again guys!

Brian Carlton · #2 10-06-2009, 10:20 AM

You are correct..........only the a/c system runs the auxiliary fan on that vehicle.

How did you determine that the auxiliary fan blows the fuse? The fan only runs when the a/c system is on and the system temperature exceeds a specified point. This only occurs when the vehicle is operated at low speeds. The auxiliary fan is generally not engaged for sufficient time to blow the fuse unless the fan is drawing significantly more current than specified. I have my doubts about this.

phinsup · #3 10-06-2009, 10:25 AM

I'm in south florida so it runs quite a bit unfortunately. I'll be sitting in traffic and I can hear the fan, then it quits, pull over and the fuse is blown. I can also tell when it's blown because my a/c goes from cold to hot air when sitting at a stop light.

Brian Carlton · #4 10-06-2009, 10:33 AM

Quote:

Originally Posted by phinsup

I'm in south florida so it runs quite a bit unfortunately. I'll be sitting in traffic and I can hear the fan, then it quits, pull over and the fuse is blown. I can also tell when it's blown because my a/c goes from cold to hot air when sitting at a stop light.

Which fuse is blown and what size fuse are you using?

phinsup · #5 10-06-2009, 10:35 AM

Fuse C, top all the way to the right and I replace it with a red fuse. Last time it even melted the fuse box a bit.

blackestate · #6 10-06-2009, 10:42 AM

If you are pulling enough power to melt the fuse box a little, there is a big issue. I would check the wires for shorting to the body, or as you seem to want to do, run new wires.
Do not bypass the fuse.
Search for what happens when you do. Some cars have burned to the ground over a failed aux. water pump. Some very nice cars. The fuse is doing it's job. Find the cause. Most likely a wire where it crosses a sharp point on the body.
(this could cause random issues as the wire moves when hitting bumps etc.)

lutzTD · #7 10-06-2009, 10:44 AM

did you for some reason disassemble the plug and get the wires wrong? could the fan be running backwards? this would cause high load while moving as it would be fighting the air moving through the grill

phinsup · #8 10-06-2009, 11:12 AM

Im not talking about bypassing the fuse I'm planning on residing a new relay in with new wires and a new online fuse it's not going to be dangerous in fact I'll be upgrading to larger wire guage so if anything it will be safer I would never bypass a fuse that was blowing. No wiring was changed it was working fine one day then all of a sudden it start blowing fuses my best guess is somewhere a wire chafed or the relay has an issue of some sort like internaly grounded my plan would fix either

Brian Carlton · #9 10-06-2009, 11:27 AM

Quote:

Originally Posted by phinsup

Im not talking about bypassing the fuse I'm planning on residing a new relay in with new wires and a new online fuse it's not going to be dangerous in fact I'll be upgrading to larger wire guage so if anything it will be safer I would never bypass a fuse that was blowing. No wiring was changed it was working fine one day then all of a sudden it start blowing fuses my best guess is somewhere a wire chafed or the relay has an issue of some sort like internaly grounded my plan would fix either

You've got the correct fuse number (c) and the proper sized fuse.

If your problem is the relay or a wire that is chafing, the fuse will melt whether the fan is running or not. You can test this by disconnecting the fan at the plug and see if the problem occurs.

My bet is on too much current draw from the fan.............which you can check if you have a suitable ammeter.

Doing all that work to bypass the wiring and the relay doesn't make a whole lot of sense if the problem is within the fan itself.

phinsup · #10 10-06-2009, 11:34 AM

First thing I did was replace the fan it blew the fuse before and after the new fan

Brian Carlton · #11 10-06-2009, 11:45 AM

Quote:

Originally Posted by phinsup

First thing I did was replace the fan it blew the fuse before and after the new fan

OK, if we rule the fan out..........now's the time to take an ohmmeter and look for the issue.

Pull the fuse and disconnect the fan.

Put one lead of the ohmmeter on the positive side of the circuit (at the plug) and one lead on the negative side. You should get infinite resistance. If there is a reading on the ohmmeter (resistance not infinite), you can now see your problem.

It becomes a task to find it............but, the relay is the first possibility. Pull the relay and test again. You're trying to disconnect various sections of the circuit and find one that allows the ohmmeter to return to infinite resistance.

phinsup · #12 10-06-2009, 12:01 PM

Ok I did a run down and found no chafed wires I pulled the relay and am going to run to napa and see if they have one

phinsup · #13 10-06-2009, 02:32 PM

The odd thing is it doesn't actually blow the fuse it's melting the red plastic of the fuse rather then melting the fuse wire

ah-kay · #14 10-06-2009, 02:40 PM

Quote:

Originally Posted by phinsup

The odd thing is it doesn't actually blow the fuse it's melting the red plastic of the fuse rather then melting the fuse wire

If that is the case. I would check the contact of the fuse holder. The fan draws a lot of current and a bad contact would generate heat and melt the plastic. Sand the fuse contacts and the fuse holder coppers at both end.

Brian Carlton · #15 10-06-2009, 03:03 PM

Quote:

Originally Posted by phinsup

red plastic

That's part of your problem. The plastic can't take the heat. The proper fuses are ceramic and the melting problem is eliminated.