Low Voltage At Idle

MafiaMustang

New Member
Nov 4, 2016
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I have a 1990 fox with a 3g conversion. For some reason I'm getting low voltage at idle. I can See my headlights dim. When I bring up the RPMS the lights brighten. I have a fresh battery and an upgraded 3G alternator with upgraded wiring. I do have dual electric fans on the car but can't imagine they are taking that much juice. Another issue which may or may not be related my turn signals and reverse lights are not working. I checked fuses and all looks good. My hazards are working though.
 
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Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
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crank, alternator pulley sizes, and idle rpm?

Pretty much. You need to be sure you are spinning the 3G alt fast enough at idle to make full charge. Usually they come with a large 3" pulley on them. This slows them down on a fox.

What is your crank and alt pulley size?
 

MafiaMustang

New Member
Nov 4, 2016
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I have a underdrive pulleys on the car and the 3g alternator is whatever came with it. That could really cause such a fluctuation?
 

FastDriver

My dad had a bra
SN Certified Technician
Sep 5, 2001
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FYI, Mike, that part is misidentified as a 1 7/8" pulley. It is actually 1.69". So, less than 1 3/4". I bumped the rev-limiter down to 6k to keep the alt at less than max. My normal 7k rev limit would've spun the alt to almost 19k.
 

Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
33,092
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I think they are measuring the outer lip to make that claim. I assume you measured where the belt rides?
 

FastDriver

My dad had a bra
SN Certified Technician
Sep 5, 2001
4,543
1,247
214
Fort Knox, KY
I actually mic'ed all of it and the outer lip measures 1.94", which is how it's advertised on Summit Racing (<- click for link). I saw that pulley before I ordered it on another website, but I didn't realize that they were the same part number until afterwards.

I spoke with ASP who verified that I'd received the correct part, said they'd been advertised wrong like that for 20+ years and he didn't know why. He told me that he'd custom make me on for $75! Go figure. You order a part as advertised, pay for it, get it, it's wrong, and they want 3x more money to make the part you originally expected.
 
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