H/O Alt, volt drop in reverse and at idle.

[Lowkey 10+ year member]

I have a 2003 Hyundai Santa Fe with the 2.7L V6. I installed a high output alternator from AutoTech engineering, 143 amps at 650 RPM and 286 amps at 1800 RPM. 1/0 ground directly from alt and a separate ground from battery. Using the factory power wire (from the old alt) in addition to a new 1/0 power run. Alt came with a 2 wire clip (original harness not used) 1 wire goes to alt top post and the other to a 12v ignition source (I used a fuse tap).

Voltage fluctuates a bit but in park, neutral or while driving it sits around 14.7-14.9. When I'm in drive but stopped, like at a traffic light, it will drop to low 14's. In Reverse it drops to mid 13's. Is this normal activity?

I'm using a volt meter that plugs into the cigarette lighter which I know isn't great (I've ordered a legit meter already) could this be the issue entirely?

It's also possible that the auto parts store gave me the factory sized belt when I asked for one .5" shorter. I googled this after the fact and realized that the 79" belt he gave me is the stock size. Would this somehow cause lower voltage in reverse and at idle?

I'm wayyy out of my depth here any advice/insight would be really helpful. TIA

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[metalheadjoe 10+ year member]

What kind of equipment are you running? If your equipment is pulling a lot of current, your voltage will drop when your alternator is supplying 143 amps verses 286 amps. Dropping lower in reverse than stopped in drive doesn't make much sense. Is there a difference in engine speed between reverse and stopped in drive?

 

[Buck 5,000+ posts]

What's the max redline on your motor in RPM's?

 

[Buck 5,000+ posts]

If you have a high revving motor, the pulley can only be so small. High revving motors can over-spin an alt, so they have to put a larger pulley on say a 6500 RPM DOHC v6 vs a 5000 RPM OHV V8. So the lower revving engines will make more alt power at idle, because the alt can spin faster at idle.

 

[Buck 5,000+ posts]

If your motor doesn't rev that high, then there very well could be a separate issue. I've been around a lot of 4 cylinder honda motors, and they tend to have this problem. Some of them won't charge at all at idle, like the alt is spinning so slowly that it's not even charging until they hit like 800 or 1000 rpm. Seen that quite a lot, actually.

 

[metalheadjoe 10+ year member]

If there is too much slack in your belt, you will hear it squeal when the belt slips.

 

[Popwarhomie 5,000+ posts]

Lower engine RPM = lower alternator RPM.

 

[Buck 5,000+ posts]

If there is too much slack in your belt, you will hear it squeal when the belt slips.

This is a real thing too. Like we always changed out the V belts on the old school chevy's because no V belt has enough surface area to grab onto a 300 amp alt lol

 

[Lowkey 10+ year member]

What kind of equipment are you running? If your equipment is pulling a lot of current, your voltage will drop when your alternator is supplying 143 amps verses 286 amps. Dropping lower in reverse than stopped in drive doesn't make much sense. Is there a difference in engine speed between reverse and stopped in drive?

Just have a single CAB-22, wired at 2 ohm so 1500 RMS. This oddity happens regardless of having the radio up though. I can listen more carefully to the engine next time and see if I hear an audible change when I shift into reverse.

*edit* I went out just now to see listen, it definitely has a shift in engine noise in reverse, I'd say it sounds softer/slower. Oddly the voltage didn't drop to 13s this time but seemed to settle around 14.4/5

 

[Lowkey 10+ year member]

If there is too much slack in your belt, you will hear it squeal when the belt slips.

I heard some "chirping" in the drive-thru earlier but can't be sure if it was my car or someone elses. Haven't heard it since though.

 

[Lowkey 10+ year member]

What's the max redline on your motor in RPM's?

RPM turns red at 6500.

 

[Buck 5,000+ posts]

RPM turns red at 6500.

So your alt starts charging when you get a little over idle RPM? How big is the pulley on the alt?

 

[Lowkey 10+ year member]

So your alt starts charging when you get a little over idle RPM? How big is the pulley on the alt?

I'm not sure because if I'm in park and idling its in the high 14s but when I put it in drive it drops until I let off the brake. I suppose its idling higher in park then in drive with the brake applied? I'm not a car guy really so I'm not sure if that makes sense.

Not sure on pulley size, I could get the measurement I'm sure. They (Alt maker) did say that it would be smaller than stock and it typically needs a half inch shorter belt. *unless your tensioner is able to pick up the slack*

 

[Buck 5,000+ posts]

I'm not sure because if I'm in park and idling its in the high 14s but when I put it in drive it drops until I let off the brake. I suppose its idling higher in park then in drive with the brake applied? I'm not a car guy really so I'm not sure if that makes sense.

Not sure on pulley size, I could get the measurement I'm sure. They (Alt maker) did say that it would be smaller than stock and it typically needs a half inch shorter belt. *unless your tensioner is able to pick up the slack*

Yes, going into reverse drops engine RPM's by 100-200 rpm usually, so your voltage is going to drop even more when you put it in reverse or drive.

This is a common problem with smaller motors that rev high, like I say. An alt can only spin so fast before it breaks. So if an alt can only spin at X rpm, then X alt RPM on high revving motors is reached at a overall higher RPM. They can't let you spin that alt so fast at idle that it overspins itself if you ever rev up to 6500 RPM.

 

[Buck 5,000+ posts]

So say my explorer which revved out at about 5000 RPM, my alternator is spinning faster at idle because they can put a smaller pulley on my motor, since my motor spins 1500 RPM's less than yours. It's just gearing, essentially.