High amp draw when radiator fans are on

[gsable]

I am running two 37 amp radiator fans. When the fans kick in, my ammeter goes from -0- to 45 amps and stays there at all speeds. Fortunately, I have a kill switch on the fans so I don't drain the battery when the car is turned off.

The alternator is a single wire unit, so there is no regulator.

Here's the question: Is the problem the alternator or the battery, or something else? At this time, I cannot tell you the rating on the alternator or battery. However, what should they be to handle the draw from the fans?

gsable

[Rick Parker]

gsable:

What fans are requiring that much current? 74 amps! That seems quite high. Most of the Spals and similar are about 18-24 amps.

Rick

What is the potential indicated swing on the gauge (from - amps to +amps?)

[Yetiman]

Maybe the 37 amp rating is the max. draw when they first fire up (inrush current)

Regardless, your charging system should have no problem at all keeping up; with them as long as they don't run for an hour straight after you shut the engine down.

Cobra's typically don't have much in the way of current consuming accessories (power windows, electronic everything, air conditioning etc.) so even the most basic charging system should be adaquate.

[Excaliber]

Even with the fans running the Amp gauge should NOT be showing negative current drain. If it is there IS something wrong. Amp guages are not "that accurate" and to measure actual amp draw (37-74 or whatever) you would need a more proffesional gauge.

Bottom line: Assuming the fans draw 75 amp and the alternator is rated at 60 amp. THEN you will see a 15 negative drain. In that case you need to "up size" the alternator or limit fan run time.

But 45 amp drain? Nope, something is way wrong with this picture. It MAY be as simple as the Amp gauge cannot be (and should not) be "fully trusted" for accurate readings.

[wtm442]

I agree with Excaliber (Slick). Small amp gauges are very inaccurate. You also might want to try looking at the battery voltage when the engine is running at idle (#1) and compare it to when the fans are on at idle (#2). If the voltage at #2 test is less than 13.2 Vdc (I think thats the recommended minimum with fully charged battery), you might want to considered a higher amp alternator to make sure the battery is still charging when the fans are on at idle speed. My Smith's gauges in SPF look original, they are repetitive, but the accuracy is questionable.

[trularin]

May I suggest:

The alternator will only deliver it's max rated current of say 65 amps ( standard 1960s alternator ).

The car, other than the fans, draws anywhere between 5 and 40 amps, depending on anything and everything "ON".

So, say the car draws 20 amps. The fans may have a max rating of 74 amps ( most probably inrush ), but for discussion purposes, let's just say 74 Amps.

20 plus 74 is 94 Amps. Subtract the best the alternator can do, 65 Amps, and you get 29 Amps.

This 29 Amps has to come from somewhere, hence the battery and a negative reading on the Amps meter.

If you can find one, a high current lab meter will give you a really indication of what your car draws.

You are probably beyond the changing system with the 74 Amp draw and the rest is coming from the battery.

On top of all of this, heat robs the changing system by as much as 20%.

Hope this helps.

[vettestr]

I checked your photos and it is blank but I was trying to see if you are carb'd or injected. If carb'd and no computer ECM to harm then a cople tests will get you on the correct path.
When running you can disconnect NEG. battery cable and see if engine runs or dies. If it runs your charging system is supplying demand, id dies it is in fact overcoming supply and pulling from the battery. Be careful of sparks etc.. Turn engine off and hook up cable. You can now place a meter in series with the battery cable to fans to verify actual current flow but watch polarity taking reading. Be careful about this reading and meter quality and lead size. There are many inductive type meters out there that are much easier.

[lineslinger]

75 amps to run two fans??
There are small houses that run on a hundred amps.
Those have to be some very serious electric motors to demand that kind of juice. Even at initial startup they require 75 amps to get turning?
We operate a 20,000 watt HMI or Zenon par motion picture lamp that usually requires 32 - 50 amps depending on the lamp.
There should be a lable somewhere on the fan motor listing the max amperage consumed.
Are you sure it is not 18.5 amps per fan motor (which still sounds high) but more reasonable. Even with the high rpm's those fans turn, 75 amps??
Maybe I am just being naive but that really does sound like way too much muscle required for a couple of cooling fans.

[Excaliber]

Lineslinger, I hear that! 75 amps??? DAM!

[Yetiman]

Originally posted by lineslinger


75 amps to run two fans??
There are small houses that run on a hundred amps.

Yes, but houses do it on 10 to 20 times the voltage (120 volts, most panels are 240 volts) thus 10 to 20 times the wattage which is what really counts.

Thats why 6 volt electrical systems sucked so bad... Everything requires twice as much current to do the same work as in a 12 volt system.

That said, the current rating for the fan motors seems pretty excessive which is why I wondered if it wasn't the inrush current and not the running current.

[flipper35]

Is there a decimal missing perhaps? 3.7 instead of 37? Though that would seem low.

[vettestr]

Well the old formula is E= I/R .
A bad motor brush or bad ground would cause current to increase but 75 amps is about what a starter cranking can draw. Something not right here and a 18 gauge wire (on most fans) would be smokin!!
I bet ya it is more like about 17 to maybe 20 amps for 2 fans and we need to verify all readings and start over here.

Edited to go back and re-do math ...... return from lunch at ????

[gsable]

Thank you all for the logical, understandable responses.
I guess it's time to get some serious measuring done. I'll let you know what I learn, but it may take some time.
gsable