Pretty sure I’m using the DVOM properly. Depending on the position of the fan, it ranges from 5 to about 94 ohms, measured directly at the fan pigtail.

This resistance is the same at the relay and ground wire bolted to the inner fender.

However, that resistance would indicate drawing less than 1A.

Why is my fan resistance so much higher than your example? By definition, fans are supposed to have resistance and more resistance means it draws less current. Ideas?

And... resistance through the spade type inline fuse is either 0 or 0.1 ohms

No ideas? Is there a spec on aux fan resistance?

Anybody? Or... do i just have crazy ideas here?

Is it better to have a high resistance fan, or low?

It is really odd that you have such high resistance on the fan. Diseasel's 1.78 Ohms sounds more reasonable.


You're saying resistance through the fuse is zero-ish. Is that the resistance from one side of the fuse to the other side, or is it resistance across the fuse to ground, through the fan? If it's the latter, you definitely have a short circuit.

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Resistance across the fuse, from one side of it to the other is zero. There's a + wire leading to one leg of the fuse, and wire leading to the relay on the other leg. Resistance is zeroish through all of this.


I understand that a fan should have resistance. It does afterall have windings, brushes, etc. The question is, should it always be in the 1 ohm neighborhood, or is 50 ohms+ normal??

I'm not prepared to be dogmatic about the right resistance for your fan. I just don't remember my rotating machinery class that well.

From the battery side of the fuse, through all the wiring and through the fan to ground, what is the resistance? If it's not the same as the fan resistance, there is some kind of problem.

__________________
'75 John Deere 450C (times 2)
'83 240D plus parts car
'87 F350, 6.9L IDI
'89 F350 crewcab, 7.3L IDI
'89 F-Superduty, 7.3L IDI
'91 F-250, 7.3L IDI
'91 F-350, 7.3L IDI
'94 F-350 Crewcab, 7.3L IDI
'94 E-350 Ambulance, 7.3L IDI

Collectively, it "should" be the same as the fan resistance, plus a super tiny bit more. I've measured the resistance through all of the individual parts, and the only thing with resistance is the fan.

Now that you mention it, I suppose it would be safe to unplug the fan, pop in a new fuse, and energize the circuit (to flip the relay on), then measure resistance between + input and ground

OK, new question:
Are w123 aux/condenser fans polarity sensitive?? What happens if you mix up the + and -?

Rich’s harness simply uses female bullet connectors instead of the special MBZ connector - in other words you can easily mix up the 2 wires.

The fan is a simple brushed DC motor.

I reconnected the fan and installed a 20A fuse. Ran for a min without probs, same goes for using a 15A fuse.

Perhaps, I wired the fan wrong and it caused the initial 15A fuse to instantly pop?

I wouldn't try to measure resistance with the power on - measure either current or voltage drop instead. Measure resistance through the fuse and fan motor to ground with the power disconnected.

If you swap the positive and negative leads, I would expect the motor to turn backwards.

If you hooked it up wrong, you would have had to make a short to ground to pop fuses, right?

__________________
'75 John Deere 450C (times 2)
'83 240D plus parts car
'87 F350, 6.9L IDI
'89 F350 crewcab, 7.3L IDI
'89 F-Superduty, 7.3L IDI
'91 F-250, 7.3L IDI
'91 F-350, 7.3L IDI
'94 F-350 Crewcab, 7.3L IDI
'94 E-350 Ambulance, 7.3L IDI

I dont know about that fan, but many can run backwards.
however, if the frame of the fan is metal and tied to the negative wire, then wiring it backward would be a short to chassis

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That’s a really good point. Glad I posted up. Wouldn’t be surprised if the fan motor is mounted on something metal, which is then directly mounted on the chassis. I bet I caused a short circuit by wiring it backward!

I think i might have cracked this one.
I put a 20A fuse in the dedicated aux fan circuit and it hasn't popped or smoked out yet.

With a 15A fuse and the motor off, the fan could turn on/off and run great. With the engine on, fuse pops shortly afterward, but with an inconsistent duration of time until popping.

Theory: As Diseasel mentioned:
Fan Amps = 14.4V / Fan Resistance

If my voltage w/ engine off is 12V, then it's drawing less current, thus not popping a 15A fuse. Engine on, fuse pops shortly afterward, inconsistently likely due to slight variation in fuses and V output of my alt. I difference between 12V and 14.4V is just enough to pop a 15A fuse

While a short is possible, I think these ACM W123 fans need about 15A. Plus, there's no soft start, so the initial spike in juice needed to start spinning also helps pop a 15A fuse.