Aux fan ideas... - PeachParts Mercedes-Benz Forum

Shern · #1 04-19-2022, 11:43 PM

Still trouble shooting a fuse that keeps blowing.
Original post

So here's the deal: When I fire up my AC, the Aux fan fuse blows. I have one of Rich's relays installed. We troubled shot a bit and it's definitely not an issue with his relay. This leaves the aux fan, any wiring, and any unknown unknowns.
The Aux fan is on a circuit of it's own, meaning not sharing with anything else-I believe it's fuse C.

When I unplug the fan, and jump the AC compressor, no fuse blows.

Seems like an obvious solution, right? But when I apply 12v directly to the Aux fan, it runs fine. It makes no noise (I've re-lubricated the bearing), doesn't spark, etc. So here's the question:

Is it possible for the aux fan to be drawing some crazy amount of amperage and still visibly run fine? If it isn't the Aux fan, what else could it be?

I've got a fresh Sanden compressor installed and a few cans of charge standing by and I am being completely shutdown by an electrical issue that is seemingly beyond me...

JHZR2 · #2 04-20-2022, 02:01 PM

The last situation was fuse 12 I believe.

So now you’ve removed the aux fan from that circuit and fuse 12 is ok but the aux fan circuit is not?

Are we sure the aux fan itself is OK (i.e. no insulation failures)?

Shern · #3 04-20-2022, 02:07 PM

Correct. Fuse 12 is fine now, the issue is now entirely on fuse C.

It’s possible the two were connected, but now it’s just C.

If the fan is faulty, shouldn’t it give some indication while running 12v directly?
If the fan is fine, I suppose it could be the harness? I have no idea how to test that without cutting the wires and connecting them directly

ykobayashi · #4 04-21-2022, 11:39 AM

Hey Shern,

When you apply 12v to the fan, are you applying it at the connector under the car? Or are you applying it at the output of the relay? The reason I ask is a lot can be going on between those points. I.e. shorts to the body.

So let’s say you’ve powered the fan directly at its two pin connector under the car. It can be drawing more than 16A. You can measure this. Put your meter (on amps mode with the leads in the A socket and negative) and put this inline with either the ground side or 12v side. Inline NOT across. If you put it across the terminals it’ll pop the fuse in the meter which is rated usually at 10A. This may also pop if you do it correctly simply because your fan draws more than 10A. In this case you’ll know your fan is out of spec, otherwise you should be able to determine your fan current. So maybe it’s better not to try to measure current this way.

Another thing to check is if the hot side of your fan circuit is shorted to ground. Take your meter in resistance mode (ohms) and measure the resistance between the fuse C (either terminal) and the body of the car. The resistance should be very high. If it is low like 1 ohm that means you are connecting your power through a short likely in the harness to the body. Perhaps a cable got pinched while doing cooler lines?

You can measure resistance of the fan by disconnecting it and measuring across its terminals. It could be drawing too much current. Take 12V divide by the resistance in ohms and that will be the theoretical DC current in amps. I assume since it is fused at 16A that the current is in the neighborhood of 10A and the resistance about 1 ohm. Mine measures 1.3 ohms between the two terminals on the plug leading to the fan. (Located under the bumper). That means it draws around 10A DC when running at steady state.

All resistance measurements done with car off.

ETA - had kind of senior moment when we were talking IRL last weekend. I gave you a crash course in using a Freon charging manifold. It went kind of fast and I don’t recall telling you to only charge on the low side by closing the red valve and opening the blue valve. I think I got distracted when I got to that part. That’s important. Watch a YouTube video on the procedure. I like the one by Eric The Car Guy because it’s simple.

Shern · #5 04-21-2022, 12:34 PM

This is very helpful!

I’ve been testing the fan directly at the plug, ie unplugging it and applying 12v with alligator clips to the battery. Resistance is measuring about an ohm. I’m convinced I have a short somewhere. This all began after I installed a bigger fuel tank. Possibly a total coincidence, possibly I pinched something in the trunk. This used to be an issue on fuse 12 which controls dash instruments, fuel sender and aux fan sensors in the AC drier. After Rich installed his custom relay, fuse 12 is no longer blowing. Now it’s just fuse C (aux fan relay alone).

This also all began after a 9 hour drive north and probably lots of vibration. Last night I did a deep dive into the archives and found a few posts by user Jay_bob (electrical engineer by the sounds of it) about removing the fuse cluster and installing a small incandescent bulb in place of the fuse, removing each wire to said fuse and testing one at a time. I have the wiring diagram but I’m struggling to make heads or tails of it.

When you say “the hot side of the fuse” -which is that?

Also, copy re: charging AC!

Edit: beginning to make sense of the wiring diagram…

ykobayashi · #6 04-21-2022, 01:09 PM

The fan could be good. Meters aren’t that accurate at those low numbers. Say it is one ohm. Then when the car is running your alternator is regulating at 14V a 1ohm fan load would draw 14A. 14V = 14A x 1ohm. That is dangerously close to the 16A fuse c. When the engine is off the voltage is closer to 12V, so it’s 12V = 12A x 1ohm. Not so bad. Shouldn’t be blowing if your ohmmeter measurement is accurate.

The wire harnesses out back by the tank shouldn’t have much to do with the aux fan. All that stuff is right on the drivers side fender, the fuse box and rounded under the oil cooler. It looks really easy for the wires in that wonky routing to rub on the frame and short out. Especially if you’ve been working down there by the cooler.

So the easiest test is check the resistance between the fuse c and the body. I mistakenly said high side of the fuse. Both sides should be hot and “high”.

My gut feel is that circuit is finding an easy path to ground through a cut section of insulation on a pinch.

Jay bob’s light bulb trick sounds really good. The bulb filament is a dead short cold but as it starts to glow the resistance goes up thus saving your wiring harness from catching on fire. Pretty smart way to chase shorts. In your case you don’t have to remove all the fuses, just clip the light across the fuse c. It’ll glow till you break the offending short to ground. It’s cheap and you won’t be blowing fuses one after another while you search.

ETA - oh yeah and when you do the glowing light test do it with the fan plugged in and with the fan not plugged in. The coil in the fan should (at 1 ohm) be enough to turn on the light. So disconnecting the fan may be necessary to find a shorted harness.

Shern · #7 04-21-2022, 09:15 PM

Fixed it (massive sigh of relief).

Couldn’t find a test light or a bulb that would fit in the fuse slot. In the interest of saving fuses, I blitzkrieged a few things at once. Not totally sure which was the culprit.

1.) Removed the fuse box and cleaned up the fuse C terminal and connecting wires in the back.

2.) cleaned up RH ground point.

3.)Took fan apart. Again. Re-measured resistance at fan plug, determined my multimeter was kaput as display showed 91 ohms! Borrowed one from a friend, again, 91 ohms! Took fan apart, blew out and polished commutator (is that the name of the copper contact point for the brushes?) polished brushes (I was desperate), reassembled and took a new reading of about 3.6 ohms. A dramatic improvement. Applied 12v directly to fan. Figured running it for 5-10 might help the brushes settle. New reading: 1 ohm. Nice.

4.) took harness apart for both fan and connector and polished the metal.

5.) drained some water(wtf?!!) from the wire harness sheath. Yeah that one was weird… there was about 10ml of water contained within the harness sheath. Could this have been causing a short? Not sure…

Finally tested continuity between all points on the aux fan circuit, hot and ground. Found nominal resistance.

Popped a new fuse in slot C and jumped the compressor. Fan spun up immediately and fuse held. Time to charge the system

Thank you for the guidance!

ykobayashi · #8 04-21-2022, 10:26 PM

Yay!

JHZR2 · #9 04-21-2022, 11:06 PM

Not sure I truly understand the finding. You had a high resistance fan, but the high resistance would limit current thereby preventing the fuse from wanting to open.

Cleaning it and taking it from 91 ohm to 3.5 ohm to 1 ohm just means more current will flow.

A seized fan with good windings can cause a fuse to blow. That didn’t sound like your issue.

I have to wonder if there is still a short somewhere.

Shern · #10 04-22-2022, 12:00 AM

You’ve got me… perhaps cleaning the Aux ground did the trick?

As Ykobayashi mentioned, a 1 ohm fan is still drawing dangerously close to the limit of the 16amp c fuse.

May have been something in the fuse box wiring, though I inspected each sheath as carefully as an untrained eye could and nothing looked out of sorts.

In any event, the fuse is no longer blowing instantly, so something happened…

ROLLGUY · #11 04-22-2022, 10:08 PM

I am glad you found the problem, as I was all out of ideas or potential solutions. Although it would have been nice to know exactly what caused both electrical problems, I wouldn't complain if I were you! Sometimes things just fix themselves, or we fix several things at one time, and never really find out what was the thing that finally fixed the problem.

Shern · #12 04-22-2022, 11:19 PM

Tell me about it.
I was really hoping for an ah-ha moment: a visible short to frame, anything...

It was interesting, however, how much water came out of that harness sheath.
I've been thinking about that today. I removed about 5 inches of sheath, basically up until the first bend but couldnt' find any wire damage, nor could I find any point of ingress further up the chain. I do religiously wash my engine bay, however, and wouldn't be shocked to find that I did get water somewhere I shouldn't have. That was another thing I did after installing that new fuel tank which is when the problem first arose.

BillGrissom · #13 04-24-2022, 03:30 PM

Thanks for posting. I hadn't even noticed that the radiator fan relay is sourced from a fuse in the firewall fusebox. That sounds like plastic waiting to melt since that fan is high-current. I had fuse #8 melt plastic in my 1985 300D, which supplies the cabin blower (also high current), which is a similar problem in 1960's U.S. cars on "blower high" position. In my 300D's, I rewired the blower speed box (above glovebox) to be supplied straight off BATT+ (wire pushed thru right firewall grommet) via 30A relay actuated by the former 12 V supply wire. Perhaps I should look at rewiring fuse C to use an external aftermarket fuse to spare the factory fusebox.

JHZR2 · #14 04-25-2022, 09:52 AM

Quote:

Originally Posted by BillGrissom

Thanks for posting. I hadn't even noticed that the radiator fan relay is sourced from a fuse in the firewall fusebox. That sounds like plastic waiting to melt since that fan is high-current. I had fuse #8 melt plastic in my 1985 300D, which supplies the cabin blower (also high current), which is a similar problem in 1960's U.S. cars on "blower high" position. In my 300D's, I rewired the blower speed box (above glovebox) to be supplied straight off BATT+ (wire pushed thru right firewall grommet) via 30A relay actuated by the former 12 V supply wire. Perhaps I should look at rewiring fuse C to use an external aftermarket fuse to spare the factory fusebox.

On my w140, they use a maxi-fuse for the aux fan, probably for that reason.

I’d suspect that a similar strip fuse retrofit like is used for the blowers on the w123 and 124 (and maybe others?) would be prudent for the aux fan too??