W123 Aux/Condenser fan specs? - PeachParts Mercedes-Benz Forum

gottarollwithit · #1 10-19-2020, 01:47 AM

Anyone know how many amps this thing is supposed to draw????

I've been chasing this nasty fuse popping problem with my aux fan.

Any experience with ACM Germany??? This is my SECOND brand new fan, still same problem persists

I'm using RollGuy's relay harness, so the Aux fan is triggered on by the factory AC compressor wiring, draws power from the glow plug fuse, grounds to the D fender, and is fed power with a 15A fuse. The factory wiring only serves to trigger the compressor and fan, not actually power it.

Ultimate question: Is the fan supposed to draw more than 15 amps?? Or, are these fans just crap? Perhaps just pop in a 20A fuse? The burned wiring would be isolated to RollGuy's harness, so it's not a huge deal.

hercules · #2 10-19-2020, 05:08 AM

Usually draw between 20-25 amps. When new the fan popped 16 amp fuses.
The fix was to install a 25 amp fuse, Mercedes update.
My 85 300sd still uses that.

Diseasel300 · #3 10-19-2020, 09:35 AM

The fix in Post 2 was for the BLOWER fan, not the Aux fan in front of the condenser.

Running amps for the condenser fan should be 8-10 amps. The fact you've swapped fans with a brand new one and are still blowing the fuse means you have a wiring fault somewhere. How is the fuse blowing, dark window and ragged edges of the fuse element? If so, that's a short-circuit. Typically overloads will have a smooth rounded edge of the fuse element where the element simply melted.

gottarollwithit · #4 10-19-2020, 01:53 PM

Well, there’s not much to Rich’s relay harness. Kinda find it hard to believe that he buggered it up. Couldn’t be the OE wiring because it’s basically not used. That said, different type of blow with each new fan.

Original OE fan, slow blow after a min or so of run time
1st new ACM fan, slow blow after a min or so
2nd new ACM fan, popped instantly.

These were all 15a fuses.

What do you guys think about wiring the fan directly to another car battery and putting a DC current clamp meter on? That way I could figure out what the fan actually is drawing, external to any car wiring.

hercules · #5 10-19-2020, 02:06 PM

Diseasel 300 you are correct,thinking of the larger 126 fan.

Diseasel300 · #6 10-19-2020, 02:12 PM

Quote:

Originally Posted by gottarollwithit

Well, there’s not much to Rich’s relay harness. Kinda find it hard to believe that he buggered it up. Couldn’t be the OE wiring because it’s basically not used. That said, different type of blow with each new fan.

Original OE fan, slow blow after a min or so of run time
1st new ACM fan, slow blow after a min or so
2nd new ACM fan, popped instantly.

These were all 15a fuses.

What do you guys think about wiring the fan directly to another car battery and putting a DC current clamp meter on? That way I could figure out what the fan actually is drawing, external to any car wiring.

Who was it that said the definition of insanity is repeating the same thing over and over and expecting a different outcome?

The fan isn't the problem. You have a wiring fault somewhere. Likely a rubbed or chaffed wire or a hot connection at the fuse itself. Even the big 16" fan used in the 2nd gen W126 only pulls ~10A on high speed.

gottarollwithit · #7 10-19-2020, 02:34 PM

This is why I seek advice here.

So, instead of trying to gauge how much the fan itself draws, I should pick apart Rich’s harness and see if I can clean it up

5cylinder · #8 10-19-2020, 08:13 PM

Quote:

Originally Posted by Diseasel300

You have a wiring fault somewhere.

A mere six words that cleanly cut through so much.

gottarollwithit · #9 10-20-2020, 12:42 AM

As the fuse instantly popped, I’d usually agree with the wiring fault theory.

This is a pretty simple harness. There’s the + and - trigger wires, they connect to the factory compressor clutch harness. All that does is supply signal voltage to close the relay. Then there’s a + wire that has an in line fuse that leads to the relay. This wire is tied into the glow plug fuse, but could go to the battery instead. Finally there’s a relay switched output wire that goes to the fan. All of these wires have less than 0.2 ohms resistance, probably less (DVOM sucks).

Relay has almost zero resistance when energized

Rollguy supplies a ground wire for the fan, so the fan is directly grounded to the inner fender

Maybe there’s crazy high resistance between one of the + wires and ground?

resago2000 · #10 10-20-2020, 08:19 PM

did you tape off the factory fan wires?

gottarollwithit · #11 10-20-2020, 10:36 PM

Nope, i probably should though. Unused factory harness for the fan is just dangling there, but i'll clean it up

Fuse that keeps popping is on Rollguy's harness, not the stuff in the factory fuse box. I jumped the in line fuse last night, unplugged the fan, and turned on the juice. No smoking or anything crazy. That indicates that Rollguy's harness isn't shorting out.

Maybe the pigtail on the fan is shorting out? It's new - kinda unlikely.

New question: This aux fan uses a relay switched +, and just grounds to the chassis. If the chassis ground connection isn't perfect (even though it is), will this cause high resistance and pop fuses?

Diseasel300 · #12 10-20-2020, 10:44 PM

High resistance does not pop fuses. Excessively low resistance does (short circuit means taking a short-cut around the load to ground instead of through it). Something in your setup isn't right to keep blowing fuses repeatedly. Even the twin 12" fans used on late W126 and W124 cars are powered from a 16A torpedo fuse.

gottarollwithit · #13 10-21-2020, 05:19 AM

That sad, if I disconnect the fan, at the fan, then turn on the power, wouldn’t that be a good way to see if the harness is shorting to ground?

resago2000 · #14 10-21-2020, 10:58 AM

yes.

What wattage is the fan and what amp fuse is blowing?
Inductive loads IE. fan motors can pull double at initial spinup
also, make sure the fan can spin freely..

I've had those inline blade fuse things be faulty (bad soldering).

test the resistance on either side of the inline fuse wire. if you see any resistance, it could be faulty.

Diseasel300 · #15 10-21-2020, 11:47 AM

Quote:

Originally Posted by gottarollwithit

That sad, if I disconnect the fan, at the fan, then turn on the power, wouldn’t that be a good way to see if the harness is shorting to ground?

That only proves the harness open circuit. You need to have the fan connected and measure resistance at your power wires leaving the relay.

Disconnect the fan from the harness and measure the resistance of the fan motor at its main terminals.

Your fan motor and the resistance you measured at the relay wires should be identical (and I do mean IDENTICAL).

Fan Amps = 14.4V / Fan Resistance. <- This is the most running amperage the fan will ever pull.

As an example, the spare 16" fan I have from a W126 measures 1.78Ω. 14.4V/1.78Ω = 8.08A Actual running amperage will be less due to the windings heating up and increasing in impedance and the natural current reduction of rotating electrical fields due to induction.

If you have a sensible resistance and amperage rating of your fan motor, you can rule it out. You've been through 3 of the things and the stock fans run fine on the stock wiring in the car.