Electric fans and revs

[jumper]

When the electric fans come on the engine revs go down
What would the cause be ?

[skid]

they require power to make them spin and its gotta come from somewhere.

the same as when you turn on the air con in your car

[TJ]

Aircon normally has a mechanism of sorts attached to the throtle system whereby the rpms are automatically adjusted as soon as the aircon clutch is engaged by the compressor. My previous vehicles used to go from 850 rpms to 1000 rpms as soon as the aircon was turned on (i.e. clutch engaged). On the TJ, aircon load is not enough to bother the engine!!!

But Skid is completely correct in that the draw of the fans might be causing it (more like when you turn all your head beam and auxiliary lights on and the rpms drop a little).

[jumper]

The 40 was at idle with no draw other then the hei running the engine. i have a 110amp bosch alternator that is working, the fans pop a 20 amp fuse but not a 25 amp fuse.

[skid]

The 40 was at idle with no draw other then the hei running the engine. i have a 110amp bosch alternator that is working, the fans pop a 20 amp fuse but not a 25 amp fuse.

might have to look at your wiring.

I used to pop 50 amp fuses with my electric fan until we put a relay in place.

[darinz]

My revs stay constant ut the volts drop from 14 to just below 13 at idle. It needs about 200rpm more to go back to 14 volts on the gauge. That is with twin electric fans and a 110amp 12v alternator.

[KiwiBacon]

Are you petrol or diesel?
If petrol are you EFI or carby?

Diesels and EFI have a much more steady idle speed and are less likely to be bothered by alternator loads. Carbys could fluctuate the most.

[jumper]

Petrol Chev 350 with Holley carb. i was thinking that if the idle is lower than it should be, and i think it is, then the alt might not put out full power so the load from the fans causes a problem. does this sound likely

[KiwiBacon]

20amps on 12v is around 250watts, while it's not a huge amount unless your engines induction system has a way of compensating then it will drop the revs a little.

Stick a multimeter on it and check you've got enough voltage at idle. A smaller alternator pulley will make it turn faster and charge more at lower revs.

[DieselBoy]

Like the other guys said, when the fans come on, they draw power. They draw their power from the alternator. By drawing power from the alternator, it places load on the alternator, i.e it takes more effort to drive the alternator. The extra effort required to drive the alternator caused by the draw on the electrical system created by the fans working, places load on the engine. Hence the engine rev's drop as it labours slightly withe extra load on it.

Maybe increase the idle by a 50 or 100rpm if its a big issue??