Outer Limits 4x4 Board

Currently with the engine running nicely at 750 rpm my alternator outputs 14.2V.

If I switch on the headlights it will drop to 14V, if I then switch on the heater fan it drops to under 14V and the engine loses about 50 rpm and runs rough.

I didn't really think that a drop of about .25 V was a problem, but when tied in with the engine losing revs I am starting to suspect the alternator is faulty.

What are your thoughts on this please?

What's the vehicle? Engine? Did you do the test hot or cold? Silly question, but was it standard low beam (say 2 x 50W) or retina-burning roo-baking high beam with 1kW of aftermarket spots?

If your alternator isn't turning fast enough to maintain voltage output then the regulator will push field current to maximum, which makes the alternator as hard to turn as it ever will be. If your engine doesn't have a lot of power at 750rpm then the power required to keep the alternator turning could represent a significant percentage of the engine's output. Depending on your engine's control system (carby & distributor, or full electronic control of everything?) what you describe isn't necessarily a sign of a problem.

Or it could be - depending on what you've got.

EFI Holden 5.0 in a 4 Runner. Standard Hi beams and heater fan at full speed. Hot test.

I also discovered while testing things last night that the voltage supplied to the coil/ignition module and injectors via the adaptor loom drops as the revs increase. A hot wire from the battery to the coil positive saw the volts remain constant as the revs increased and the motor run stronger on the road.

So time to ditch the Marks relay box and put in some decent relays and wires I think.

I have still got to test the volt and rev drop with lights and heater on now that the ignition power problem has been foind.

Cheers

I agree with Scott's explanation. The extra load on the alternator is simply making the engine work harder. Headlights and heater blower are pretty much the biggest current draws in the whole electrical system.