alternator voltage question

[EstoMax]

Hey Guys,

I have been diagnosing my alternator on suspicion that it may be going south, and i measured some voltages with the car running and off. What i got was:

with engine running:Across battery terminals: 13.9VEngine Bay Battery terminal to engine bay ground: 13.95V (My battery is in the trunk, so a .05V drop for 8 feet of wire is ok in my opinion)Alternator bolt (the rubber shielded one on alt) to engine bay ground: 14.15V

engine/acc off:Across Battery terminals: 12.7V

My question is, should there be a 0.2V drop from the alternator to the battery cable? I'm pretty sure they are just one cable.. that's a lot of resistance in that case.. And are these voltages in range? I know the FSM says alternator voltage should be 14.1-14.7, but is this at the alternator terminals or battery terminals..

cheers for inputMarko

[Cone Junky]

.2 Voltage drop is borderline OK. But the best way to measure true voltage drop is to place one lead on the alternator and the other on the battery while the system is charging. Then the voltage displayed on the multimeter is true voltage drop.

[del82]

Well, your numbers pass, .5v is a bit on the high side for a trunk battery kit, so you might consider a new set of cables. At that length they get expensive, don't they? Heh heh...

You can take it to most auto parts stores and have em run a test on it, too, to have a look at your diode assembly. It's usually free. If that passes, then your alternator is fine, if a little worn.

[Vegascorbin]

+1 for all OK.

Remember that your meter can account for .1 variation (depending on the quality or the meter).

I would verify all connections are clean and tight and re-test in a few months.

[snake240]

i would also test all the grounds.

[EstoMax]

cool, they seemed ok. Just sometimes when i switch on my fan or headlights will stumble the idle and the lights will dim.. seems like the alternator is slow to respond to the increase in current demand.

del82 i said .05 not .5. I would be worried with a .5V drop too, thats excessive. I guess I'll check voltage drop on the alternator/battery line to confirm that. Do they sell this cable seperately or is this something i buy the parts and make? Haven't replaced this yet .. haha

cheersMarko

[snake240]

also, check voltage regulator.

[del82]

del82 i said .05 not .5. I would be worried with a .5V drop too, thats excessive. I guess I'll check voltage drop on the alternator/battery line to confirm that. Do they sell this cable seperately or is this something i buy the parts and make? Haven't replaced this yet .. haha

cheersMarko

Haha...well, that's me once again proving why I wear glasses. Anyway, yeah, .05 is actually excellent, and don't worry about the headlights dimming a bit at idle, that's also very common. It's got more to do with the amount of power from the alternator that's actually available at idle, which isn't anywhere near the max output. The motor bogs momentarily because, well, imagine someone stepping on the back of your shoe while your walking. Since you don't know its coming, you'd stumble just a bit before turning around to backhand em so long as the idle corrects itself within a second or two, nothing's gone wrong.

If it's still bothering you, and/or you just feel the need to tinker with it, you can pop off the IAC motor and clean it and the port it sits over with some fuel injector/throttle body cleaner. That might help get some of your idle response back.