Thread: How can I tell if my electric fan is drawing too much current.

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Today I was out for the shake down cruise in the newly finished roadster pickup, and when I got home the 30 Amp fuse that supplies power to my "wrecking yard" Pontiac Fiero fan was blown. I replaced the fuse, and the fan seemed to work all right, and the fuse didn't blow after a couple of minutes of running. My question is, how can I check the current draw, to see if there is an internal "bind" in the fan that makes it draw too much current, thus blowing the fuse.----Also, how do I know if a 30 Amp fuse is sufficient. I used a Ron Francis "basic" fuse panel that has a built in grounding relay for the fan, and it recomends a 30 Amp fuse, but is that really big enough. I am sure that there has to be some logical, scientific method, using an Ohm Meter or something to get an answer. I don't want to blindly go to a bigger fuse and burn up the car. The only 2 glitches that I actually experienced were the fuse blowing, and for some reason my electric speedometer seems to not be working. Other than that, everything seems to work as it should.

This is kind of a mad scientist idea, but------I have an ammeter left over from another project which will read +/- 70 Amps. The fan has its own dedicated 30 Amp fuse. Why couldn't I pull the fuse out, and run a lead from each side of the fuse holder through the ammeter, then turn the fan on, and see if it draws more than 30 Amps? The roadster pickup has a new voltmeter that reads +14 V when the engine is running, and +12 when the key is on and engine is not running. I know that when the fan kicks on that it is drawing a lot of current, because the alternator belt will start to squeal a bit---this indicates that the alternator load is increased quite a bit when the fan kicks on.----anybody have a comment on this????

Brian,

Most of the fan kits come with 30 amp fuses. MY CCI fan reccomends a 30. Is there a chance of a short somewhere?

Putting an ammeter in line with the fan lead would be the perfect way to see if you have a heavy draw. Not a mad scientist method at all.

When the key is off, you're reading the battery 12V directly. When the engine is running, the alternator joins the mix. The output of the alternator is higher than 12v.

The increased alternator load when the fan kicks on is an indication that the fan may be a problem - although, as I remember, electric motors draw the most current when they first start up, then the load levels out when the motor comes up to operating RPM.

Also, for the two wires sizes that you may have, the following is the current capacity:

14 ga maximum load is 32 amps
12 ga maximum load is 41 amps

both are wires in free air, not in a wire bundle

Is it possible that the air flow at speed is causing an increased load on the fan motor, thus drawing more current?

OR, is it possible that, under acceleration, there is a binding of the fan causing an increased amp draw? I'm not sure what kind of motor is used in an automotive fan....some can take stall without damage (like a shaded-pole motor) and some cant.

The ammeter idea is good EXCEPT that you have no fuse in the line....try to run your ammeter test with the fuse included. Use at least 12 ga wire.

mike in tucson

Henry Rifle---I know its not a short---if it was a short, the fuse would die right away. I may try the inline ammeter trick tomorrow. Damn, I knew that being a design engineer had to be good for sumthin!!!!

Robot---all the wires involved with that fan circuit are #10 gauge. It doesn't cost that much more, and I didn't want to impede the current flow, or have overheated wires.

10 ga is great!

Not to be too elementary, but did you check to see if the fan was moving air the correct direction? Sometimes, with the shroud, the exiting air can fool you. Take some paper (like a $100 bill) and put it in front of the radiator to see if it is pulled against the radiator, indicating the air flow is front-to-rear. If the thing were reversed, it would cool at idle but stall the fan at speed.

Such a problem is common with belt-drive fans where someone uses a v-belt fan on a serpentine system or vice-versa.

You should run relays on a fan....
Here's a circuit that you can put in that will do some nice things...
It turns the fan on automatically at the temp the sender is set at.
It turns the fan on as soon as the a/c gets turned on.
It gets turned on manually with a switch (cruise parades, etc)
You can't forget this one, and it won't pop fuses or breakers.
Uses standard automotive relays.
I set my truck up this way and it works great.
http://community.webshots.com/album/79580450MvBZqX
Jeff

Yes, the fan is blowing the right way. Yes it is wired thru a grounding relay. No there isn't a short. I performed the amazing ammeter stunt and the results are (drumroll)-----the fan spikes to 60 Amps when it comes on, then drops back to a constant 28 Amps while running. I don't really want to replace this fan, because of a ton of custom made bracketry. I think on Monday I will do some investigating to see if there is a self resetting circuit breaker that I can wire into the circuit, which will allow a temporary surge without going kaput. If I am really lucky, there may be one that will plug in where the 30 Amp fuse should go. What I really would like to know, is how many amps should a 15" fan draw.

The 28 amps sounds high. Did you get a spec sheet with the motor? How about a slow-blow fuse to take the start-up load?

Henry---the local junkyard doesn't give spec sheets. They tell you what the fan came off of and charge you $25. If it don't run, they might give your money back, on a good day. I know that all over the world there are hotrodders with 15" electric fans setting around their garages, collecting dust.---I was hoping someone could do what I just did---hook one up thru an ammeter and a 12 volt battery and tell me what their results were.

[ I think on Monday I will do some investigating to see if there is a self resetting circuit breaker that I can wire into the circuit, which will allow a temporary surge without going kaput.

I'm not sure about blade design, but I bought a 30 amp one at O'Reillys for the heater fan in my '73 F100

brianrupnow you got enough people telling you how to do it, so all I want to add is id use a 30 amp. Circuit breaker in the place of that fuse if I was you. Just a little add protection.

Brianrupnow, I run 2 electric fans, a 12" and an 10" on my Model A. I run them through a contoller by Centeck and it is fused at 20 amps at the main fuse panel with no problems. I have it wired with a single 14g wire. Hope this helps.

Well, I got the speedometer working and calibrated for kilometers O.K.----one wire had come off its terminal under the dash. I am still not sure with the fan issue---I jumpered out the fuse with a peice of #14 ga wire and let it run for half an hour. The wire got warm but not hot, and remember the rest of the wire in that circuit is #10 gauge. All the parts stores are closed here for Canada day, today, but tomorrow I will definately find a blade type circuit breaker with a delayed response time and try it. When I am going down the road this engine runs at a constant 180 degrees, and the fan doesn't try to come on. It's only after setting for 7 or 8 minutes that the temp creeps up to 190, then the fan comes on. The fan only has to run for about 5 minutes, and the temp cools back down to 180 and the fan shuts off. I watched this fan cycle on and off in my garage for about 20 minutes, and everything seems to work perfect, with the fuse jumpered. This fan appears to be a 2 speed fan, and right now I have it wired for the high speed setting. I may try wiring it for the lower speed setting, and see how this affects the fuse.