Low 11V charge at idle but normal just off idle

[jp455]

Hello all...bit of an odd issue here. Got low voltage at idle (11.5 or so), but the second it goes off idle its up to normal. Now the ground strap is brand new, as is the battery. The pulleys are a March serpentine setup, 75mm and 172mm for the Alt pulley and crank pulley so 2.3 ratio. Alternator is 3 wire and has plenty of power to run electric water pump, fans etc, but the issue is I have EFI now. So when I come to a stop and the ECU sees less than 12 volts it shuts down.

Any thoughts?

[a67]

Get a better EFI system? No reason it should shut down at just under 12 volts. Cranking voltage is usually in the 10 volt range, maybe it is something in the tune? Bob.

[GoodysGotaCuda]

No EFI system should shut down at 12v, i think there is a different issue. I've run a car off of 9 volts before..

Sounds like you need a different alt, or perhaps a smaller pulley. AND you have some tuning to do.

[dhutton]

Did you put a voltmeter right at the EFI controller? Maybe it is seeing much less than 12 volts due to a wiring issue. Check your grounds too.

Don

[rocketrod]

Was the voltage ever good, or did it just start. If it was good at one time, what have you changed since then? Also make sure you have multiple grounds connecting the engine/frame/body and they are all good clean solid connections.

[jp455]

Get a better EFI system? No reason it should shut down at just under 12 volts. Cranking voltage is usually in the 10 volt range, maybe it is something in the tune? Bob.

Its a brand new Holley Sniper...a better system would be a much more upgraded one. Im not an EFI expert, but I think cranking amps dont correlate to how many volts the system needs to run reliably. Ill look into that with Holley anyway.

[jp455]

Was the voltage ever good, or did it just start. If it was good at one time, what have you changed since then? Also make sure you have multiple grounds connecting the engine/frame/body and they are all good clean solid connections.

Actually the idle voltage has never been excellent, but it does have two brand new ground straps now. Maybe the 2.3 pulley ratio is too low for proper idle voltage?

[130fe]

What is your voltage reading off of the battery and alternator stud? I would look at those in addition to what RocketRod and Don said to do.

[minendrews68]

A lot of alternators don't "excite" (or start charging) until after a certain rpm. I'm thinking changing the pulley might create more rpm at the alternator without revving the motor up.

[jp455]

What is your voltage reading off of the battery and alternator stud? I would look at those in addition to what RocketRod and Don said to do.

Ill check this and come back...

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A lot of alternators don't "excite" (or start charging) until after a certain rpm. I'm thinking changing the pulley might create more rpm at the alternator without revving the motor up.

Thats what has me a bit confused...the second the RPM is off idle the voltage jumps to normal.

[blitzer454]

What is the voltage at the point where the sense wire is connected? It should maintain 14V at this point at all times.

[minendrews68]

At what rpm does your car idle?

[jp455]

What is the voltage at the point where the sense wire is connected? It should maintain 14V at this point at all times.

I’ll check...if it isn’t?

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At what rpm does your car idle?

Idle is steady at 800 or so

[jp455]

So had a look at the ground straps, cleaned them up and idle voltage is now up to 12. Its odd though, the straps are brand new and the contacts werent particularly rusty. It seems to me like just 12 volts at idle is still low.

[dontlifttoshift]

You want 3:1 on the pulley ratio. So get a bigger crank pulley or smaller alternator pulley.....or raise the idle speed.

Then check voltage at the EFI controller, if it doesn't match what the alternator is putting out, correct that.

[TheJDMan]

Yep, your alternator is spinning too slow at idle. Like others have said you need a bigger crank pulley or smaller alternator pulley. I started with a stock March serpentine front drive kit and initially I had the same problem. At idle the voltage would drop off even with a 140 amp alt. It was pretty obvious to me that the alt was simply spinning too slow at idle even with a 1000rpm engine idle speed. I installed a smaller alternator pulley and the problem disappeared immediately. You could test this by simply raising the engine idle speed.

[ULTM8Z]

Yep, your alternator is spinning too slow at idle. Like others have said you need a bigger crank pulley or smaller alternator pulley. I started with a stock March serpentine front drive kit and initially I had the same problem. At idle the voltage would drop off even with a 140 amp alt. It was pretty obvious to me that the alt was simply spinning too slow at idle even with a 1000rpm engine idle speed. I installed a smaller alternator pulley and the problem disappeared immediately. You could test this by simply raising the engine idle speed.

Yes, the alternator pulley does help.




Then check voltage at the EFI controller, if it doesn't match what the alternator is putting out, correct that.

Absolutely. Check ALL voltage taps on the ECM. I had mine where the battery tap was seeing full alternator voltage, but the switched igntion tap was 1.5V lower. Caused significant driveability problems until I discovered that discrepancy.

[rickpaw]

Also, where does the positive cable for the EFI connect to? Holley is very adamant that the + wire be connected directly to the battery.