Alternator Charge at Idle

[JMD]

In the process of tuning my Sniper and reading datalogs, I noticed that the alternator is typically not charging when the car comes to idle. Well, sometimes it does, sometimes it doesn't. It does reliably hold 14.4v at 900 rpm and holds steady voltage from there, but below that reads anywhere between 12-14.5 volts. I haven't noticed any issues with the car, ignition or EFI because of this but wanted to throw it out there and see if this is something I need to address. If so, I believe I could simply swap the pulley to one 10% smaller, which should get the alternator to reliably excite at 800rpm. Any thoughts?

It's a one wire, internally regulated 160amp alternator, just fyi.

[JJK]

From my understanding, one wire alternators will not start charging until they reach a certain RPM which is typically 1000 or 1500. Are you sure you are not measuring the battery voltage instead of alternator voltage at low RPMs, hence dipping down to 12v.

[JMD]

It's almost a linear drop in voltage once rpm drops below 875. It's normally between 14.4-14.6v over 900rpm and drops in direct correlation to rpm. It drops to 14v around 850 rpm, 13v about 825 and tends to hover around 12.5v at 800rpm idle. The battery itself puts out 12.3v when the engine is not running so any reading above that would indicate the alternator is charging. Obviously the internal regulator is reducing voltage output below 900rpm, causing a voltage drop through the entire electrical system until it gets down to 12.3v which is just the battery supplying power. As far as I can tell everything is working properly and the alternator is charging at anything over idle speed, so I'm not too worried about the battery dying unless I'm at idle for an extended period (which is pretty unusual).

I'm measuring voltage through the Sniper sensor datalog. It doesn't indicate amperage, just voltage. I'm mostly wondering if this is typical or if I need to swap an alternator pulley to spin it faster so it maintains +14v at idle.

[BornWestUSA]

My one wire holds 14 volts at idle. That is roughly 750 rpm for me. I have read about them not charging at low rpm by design tho.

I would calculate pulley speed before changing anything, as in what will it be spinning when you redline the engine or miss a shift?

[JMD]

What is the max pulley speed supposed to be? I have a rev limiter set at 6300 in case of a missed shift, so it will never spin above that.

I'll have to double check, but I think the pulley on there now is a 2.375" (2 3/8") pulley. If I increase the pulley speed by ~10% so it charges at 800 rpm I would need a 2" or 2 1/8" pulley.

Can anyone else weigh in on whether I need it to output 14v at idle or not? It seems like a good idea for it to do so, but I'm not sure what typical voltage output is supposed to look like at idle.

[Redstang69]

Turn your lights on at idle as well as your fan and see what the voltage is. Looking to see if the VR kicks in and allows a charge. If no charge with those loads on then I would have a slight concern.

[Waterman]

One wire alternators were designed for race cars and custom cars with minimal wiring. They SHOULD charge at idle AFTER seeing an RPM high enough to initially excite them. IF you have issues then just swap out with a more std, more available, cheaper 3G alternator, which the FFR harness is designed for. You do not need 160 amps in a car with no power anything. The pulley should swap over easily. My thoughts.

[rich grsc]

One wire alternators are just fine on old farm tractors, and race cars, just not on street driven cars. A 100amp 3g alternator is cheaper and always works

[CraigS]

My 1-wire was the similar to yours. I never worried about it as I was carbed. If your efi doesn't seem to be affected you can probably ignore it. I am not clear on your alt operation though. Mine did not charge immediately after startup if the rpms stayed low. However after the rpms ran up to about 1200-1300 it started charging and would keep charging even when the engine dropped back to 800 idle. OTOH, a smaller pulley is an easy and inexpensive experiment.

[gbranham]

I tend to agree with Craig...if there are no symptoms, maybe leave it alone. Had you not noticed it in your Sniper data logging, would you have ever worried about it? FWIW, I have a 1-wire alternator as well, and couldn't tell you if it's charging below a certain RPM or not; it drives fine.

Greg

[phileas_fogg]

1-wire alternator here as well, with the Holley Terminator fuel injection. I can see on my volt meter when the alternator is charging, usually at RPM>1200. No problems at idle.


John

[JMD]

It may not be worth worrying about. My only concern would be that I'm running a smaller than average battery so don't have the amp hours accessible to most vehicles...of course there's hardly any significant draw on it other than the ignition system, radiator fan and (future) heated seats. I imagine the battery alone could operate the ignition system for quite a while all by itself, probably longer than I would ever need it to. I may put a smaller pulley on the 'to do' list this winter. Obviously it hasn't really been an issue so far, so not pressing in the least.

[Rdone585]

I agree with Craig, Greg, and John... if there aren't obvious problems, let it ride. If you put a smaller pulley on the alternator, you may overstress it because it will be running more cycles per RPM of the motor. This may reduce the lifetime of the alternator. I burned up two that way with my older engine setup, because I had swapped the standard V-belt pully for a serpentine pulley that was a little smaller, admittedly due to track use with a fair share of higher RPMs. If your worried about the voltmeter reading being near 12 volts (where a battery should be at full charge), just rev up the engine a little and drive. The alternator runs the voltage up to 14 volts in order to charge the battery, but the battery will only take a charge if it needs it. If the voltmeter gets down to 10 volts, then evaluate the circumstances leading to that and go from there.
Robert

[JMD]

I measured the pulleys today and what I’m seeing is starting to make a lot more sense. The alternator pulley is 2.3”, which is fairly standard, but my crank pulley is only 4.6”. From what I’ve read a 5.5” crank pulley is a pretty standard size and 5" and smaller is considered an underdrive crank pulley. With this crank pulley I have a ratio of 2:1 so the alternator is only spinning twice as fast as the crank…which is pretty slow. Most street applications have closer to 3:1. So even if I put on a 2” alternator pulley it will only be a 2.3:1 ratio, which is still on the slower side and won’t over rev the alternator. It probably isn’t absolutely necessary, but I’ll put it on the list as a winter project. Something to do while the body is off and getting painted. :-)

[VdubJoe]

Have one wire ones on 3 vehicle's. All work fine . Two are around 2 years old running fuel injection.
Puttting one on the mk4 with a fitech dual unit.