Voltage Drop from Alternator to Battery

putrio · 10-31-2022, 02:52 PM

Good Afternoon NASIOC! Been a while since I posted. Summary - installed new clutch, flywheel, starter motor (it was grinding), alternator, and battery. Car sat for a long time, and wouldn't start. I charged up the battery and it started right up. Drove it to a movie, and on the way went through some water. I hadn't put the undermount shield back on. About 5 minutes later car started acting like the alternator had died, unable to keep things powered, and eventually it died and had to be towed. Let it sit for a few days, charged the battery, good to go! For about 5 miles. Then the rear-diff light and battery light came on. Shortly after, it died again. In my ex wife's driveway. That was awesome. Her new husband gave me a jump, and I got home. Because this isn't my daily, I haven't buckled down and worked on it until fixed, but after that, I did have the battery and alternator tested, and they both tested ok.

Fast forward to now. Using a multimeter, I've tested the voltage on the battery on a fresh charge, 12.67v. Starting the car, I get 14.6v, which slowly drops down over about 5 minutes until it gets below 13, at which point I turn the car off, charge it back up, and keep testing.

Tests:
Positive Battery Terminal --> Alternator Post. Multimeter at 2v DC: 1.3 (which appears to be a giant voltage drop)
Positive Battery Terminal --> Alternator post nut: 1.3
Positive Battery Terminal --> Wiring bracket that goes between nut and post: 1.3
Positive Battery Terminal --> Cable going from Alternator to fuse box, punctured near one of the splice points: 0.05.

All tests from negative battery terminal to ground points are under .1.

Based on this info, I am considering unsheathing the main wire from alternator to the fuse box along the entire length to see if something has happened. I don't see anything externally that can point me to a specific location.

Looking for thoughts on this plan of action, and/or suggestions on other tests to perform before cutting into the sheath/electrical tape protecting the wire.

Thanks!

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rtv900 · 10-31-2022, 03:24 PM

is there a reason you don't want to just put a new alternator on this thing and see if that completely solves the problem?

putrio · 10-31-2022, 04:07 PM

Other than the current one being 8 months old, testing ok, and not wanting to spend another $350 on a new one, not really. Coming up on Christmas and I'd like to avoid spending money I could put towards the kids :-)

What I am basically doing is testing voltage along a single line (the wire going from A - B) and seeing voltage across it means there is resistance along the path which forces the voltage to go through the multimeter instead of the wire. That resistance would in theory cause less voltage to get to the battery, causing it to lose charge like I'm seeing during driving. Or idling, or whatever.

I need to figure out why there's a voltage drop and the best way to fix it, and if it did go bad in 8 months, there could be an underlying problem that's going to cause alternator #3 to go bad also.

rtv900 · 10-31-2022, 04:29 PM

why not rig up a cheapo voltage gauge inside your cab so you can watch it
regular analog voltage guages are cheaper than picking up a single beer on your way home.
You could just do a hoky wire job and run wires out of the hood and just close the door to the cabin over them as a temporary guage to watch

Also, in theory it should read your system voltage no matter where you hook up to, even a cigarette lighter or something. Obviously this is in theory and I realize you are trying to zero in on a particular section of wire, but still, it will at least give you some useful information.

putrio · 10-31-2022, 05:08 PM

The Accessport does a battery voltage gauge, but I can't really drive the car anywhere without risking getting stranded, even the grocery store 3 miles away. The AP gauge does show it slowly dropping from 14.X down to 12 while it idles though...I wonder how hard that part of the wiring harness is to replace.

benflynn · 10-31-2022, 05:40 PM

Throw a jumper cable from alt to batt positive and see what it does.

benflynn · 10-31-2022, 05:41 PM

I have also had to clean the ground going from batt to starter to get good power from
The alt

putrio · 10-31-2022, 06:10 PM

By jumper cable do you mean just connect the two directly, see if that resolves the issue, bypassing the wiring harness path?

I will try cleaning things from battery to starter, since I did replace that (it was touched) and it could be dirty now. Thanks!

putrio · 10-31-2022, 06:10 PM

By jumper cable do you mean just connect the two directly, see if that resolves the issue, bypassing the wiring harness path?

I will try cleaning things from battery to starter, since I did replace that (it was touched) and it could be dirty now. Thanks!

benflynn · 10-31-2022, 06:33 PM

Yeah, if it helps you’ll know it’s one of those 2

I chased an occasional batt light flashing kind of occasionally, batt always charged but it’s annoying. New alt didn’t help, cleaning the ground did, adding more grounds didn’t and then getting the green connector on the alt to make better contact seems to have been the trick. Diode trick was next and waiting to go.

putrio · 10-31-2022, 11:23 PM

It’s being really weird honestly. After running for a few minutes I wasn’t reading any voltage drop, but the battery had dropped down to 12v and the light was on on the dash. I think I read something about the diodes going bad and it showing AC at the battery which I did see so maybe the alternator did get fried when I went through some water without the under guard on….

Blackfin · 11-01-2022, 12:12 AM

Quote:

Originally Posted by putrio

Fast forward to now. Using a multimeter, I've tested the voltage on the battery on a fresh charge, 12.67v. Starting the car, I get 14.6v, which slowly drops down over about 5 minutes until it gets below 13, at which point I turn the car off, charge it back up, and keep testing.

Tests:
Positive Battery Terminal --> Alternator Post. Multimeter at 2v DC: 1.3 (which appears to be a giant voltage drop)
Positive Battery Terminal --> Alternator post nut: 1.3
Positive Battery Terminal --> Wiring bracket that goes between nut and post: 1.3
Positive Battery Terminal --> Cable going from Alternator to fuse box, punctured near one of the splice points: 0.05.

All tests from negative battery terminal to ground points are under .1

You measure a positive voltage between the battery +ve and the alternator post which means a current is flowing from the battery to ground through the alternator. If we assume for the the moment than the cable itself is sound (no corrosion in the crimps, clean connections etc) then the current flow is pretty significant to cause such a sizeable drop across a comparatively short run of thick (low AWG) wire.

One of the only things that comes to mind that could cause that is one or more shorted diodes in the rectifier assembly.

You said the alternator was replaced 8 months ago and "testing ok": What test(s) were actually run? Were output vs RPM, load, charging voltage, ripple voltage, rectifier and regulator functions all verified?

putrio · 11-01-2022, 10:42 AM

Quote:

Originally Posted by Blackfin

What test(s) were actually run? Were output vs RPM, load, charging voltage, ripple voltage, rectifier and regulator functions all verified?

That was a thorough answer/set of questions, thank you! I took the car in to an autozone and they tested the alternator there, I don't know what tests they performed. I'm going to remove it this morning and take it in again after I verify there's AC voltage at the battery while it's running, since that indicates the diodes aren't converting to DC properly.

putrio · 11-01-2022, 11:10 AM

There was definitely AC voltage at the battery. Alternator is removed, will try to get it warranty replaced and post an update this evening!

putrio · 11-01-2022, 03:29 PM

Warranty replacement successful, but not in stock so I have to pick it up tomorrow. I'll update after I get it installed, whenever that might be. Hopefully tomorrow. RTV900 - Maybe I should have just replaced it from the getgo like you questioned at the beginning :-) I needed to be able to say it was faulty though or they wouldn't have swapped it for me under warranty.

rtv900 · 11-01-2022, 04:37 PM

Quote:

Originally Posted by putrio

RTV900 - Maybe I should have just replaced it from the getgo like you questioned at the beginning

totally, but yeah I get they are a pain to remove

but at those auto parts stores they can hook it up to that device in the store and test them in 2 minutes
I got a dud once too. The guy gave me all kinds of attitude when I said it was bad and was clearly lazy and didn't want to set it up on the tester. Then he was surprised when it came back bad, didn't give me crap after that and just handed me a new one.
Finding a bad part is best case scenario. Nothing worse than testing one thing after the next and they keep testing good and you make no progress.

Max Capacity · 11-01-2022, 04:55 PM

Sounds like just one more story about junk replacement parts out there now days.

Good luck with the replacement.

Max Capacity · 11-01-2022, 04:55 PM

yet another double post...

AliBenn · 11-01-2022, 06:52 PM

shot in dark here(from experience)

have u traced the altn harness to the bottom of the fuse box mounted behind L headlamp?

i had 'same' issue
i found that the wire from altn to fuse box had come loose
i clicked in it and fixed issue
must of come loose with the work being done on my car(short block)
GL

AliBenn · 11-01-2022, 06:53 PM

shot in dark here(from experience)

have u traced the altn harness to the bottom of the fuse box mounted behind L headlamp?

i had 'same' issue
i found that the wire from altn to fuse box had come loose
i clicked in it and fixed issue
must of come loose with the work being done on my car(short block)
GL

10-31-2022, 02:52 PM	#1
putrio Scooby Newbie Member#: 494757 Join Date: Dec 2018 Chapter/Region: MWSOC Location: Cincinnati, OH Vehicle: 2011 STI Hatch Black	Voltage Drop from Alternator to Battery Good Afternoon NASIOC! Been a while since I posted. Summary - installed new clutch, flywheel, starter motor (it was grinding), alternator, and battery. Car sat for a long time, and wouldn't start. I charged up the battery and it started right up. Drove it to a movie, and on the way went through some water. I hadn't put the undermount shield back on. About 5 minutes later car started acting like the alternator had died, unable to keep things powered, and eventually it died and had to be towed. Let it sit for a few days, charged the battery, good to go! For about 5 miles. Then the rear-diff light and battery light came on. Shortly after, it died again. In my ex wife's driveway. That was awesome. Her new husband gave me a jump, and I got home. Because this isn't my daily, I haven't buckled down and worked on it until fixed, but after that, I did have the battery and alternator tested, and they both tested ok. Fast forward to now. Using a multimeter, I've tested the voltage on the battery on a fresh charge, 12.67v. Starting the car, I get 14.6v, which slowly drops down over about 5 minutes until it gets below 13, at which point I turn the car off, charge it back up, and keep testing. Tests: Positive Battery Terminal --> Alternator Post. Multimeter at 2v DC: 1.3 (which appears to be a giant voltage drop) Positive Battery Terminal --> Alternator post nut: 1.3 Positive Battery Terminal --> Wiring bracket that goes between nut and post: 1.3 Positive Battery Terminal --> Cable going from Alternator to fuse box, punctured near one of the splice points: 0.05. All tests from negative battery terminal to ground points are under .1. Based on this info, I am considering unsheathing the main wire from alternator to the fuse box along the entire length to see if something has happened. I don't see anything externally that can point me to a specific location. Looking for thoughts on this plan of action, and/or suggestions on other tests to perform before cutting into the sheath/electrical tape protecting the wire. Thanks! * Registered users of the site do not see these ads.

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10-31-2022, 03:24 PM	#2
rtv900 Scooby Guru Member#: 428511 Join Date: Aug 2015 Location: philadelphia Vehicle: 2016 STI	is there a reason you don't want to just put a new alternator on this thing and see if that completely solves the problem?

10-31-2022, 04:07 PM	#3
putrio Scooby Newbie Member#: 494757 Join Date: Dec 2018 Chapter/Region: MWSOC Location: Cincinnati, OH Vehicle: 2011 STI Hatch Black	Other than the current one being 8 months old, testing ok, and not wanting to spend another $350 on a new one, not really. Coming up on Christmas and I'd like to avoid spending money I could put towards the kids :-) What I am basically doing is testing voltage along a single line (the wire going from A - B) and seeing voltage across it means there is resistance along the path which forces the voltage to go through the multimeter instead of the wire. That resistance would in theory cause less voltage to get to the battery, causing it to lose charge like I'm seeing during driving. Or idling, or whatever. I need to figure out why there's a voltage drop and the best way to fix it, and if it did go bad in 8 months, there could be an underlying problem that's going to cause alternator #3 to go bad also.

10-31-2022, 04:29 PM	#4
rtv900 Scooby Guru Member#: 428511 Join Date: Aug 2015 Location: philadelphia Vehicle: 2016 STI	why not rig up a cheapo voltage gauge inside your cab so you can watch it regular analog voltage guages are cheaper than picking up a single beer on your way home. You could just do a hoky wire job and run wires out of the hood and just close the door to the cabin over them as a temporary guage to watch Also, in theory it should read your system voltage no matter where you hook up to, even a cigarette lighter or something. Obviously this is in theory and I realize you are trying to zero in on a particular section of wire, but still, it will at least give you some useful information.

10-31-2022, 05:08 PM	#5
putrio Scooby Newbie Member#: 494757 Join Date: Dec 2018 Chapter/Region: MWSOC Location: Cincinnati, OH Vehicle: 2011 STI Hatch Black	The Accessport does a battery voltage gauge, but I can't really drive the car anywhere without risking getting stranded, even the grocery store 3 miles away. The AP gauge does show it slowly dropping from 14.X down to 12 while it idles though...I wonder how hard that part of the wiring harness is to replace.

10-31-2022, 05:40 PM	#6
benflynn Scooby Specialist Member#: 515308 Join Date: Jun 2020	Throw a jumper cable from alt to batt positive and see what it does.

10-31-2022, 05:41 PM	#7
benflynn Scooby Specialist Member#: 515308 Join Date: Jun 2020	I have also had to clean the ground going from batt to starter to get good power from The alt

10-31-2022, 06:10 PM	#8
putrio Scooby Newbie Member#: 494757 Join Date: Dec 2018 Chapter/Region: MWSOC Location: Cincinnati, OH Vehicle: 2011 STI Hatch Black	By jumper cable do you mean just connect the two directly, see if that resolves the issue, bypassing the wiring harness path? I will try cleaning things from battery to starter, since I did replace that (it was touched) and it could be dirty now. Thanks!

10-31-2022, 06:33 PM	#10
benflynn Scooby Specialist Member#: 515308 Join Date: Jun 2020	Yeah, if it helps you’ll know it’s one of those 2 I chased an occasional batt light flashing kind of occasionally, batt always charged but it’s annoying. New alt didn’t help, cleaning the ground did, adding more grounds didn’t and then getting the green connector on the alt to make better contact seems to have been the trick. Diode trick was next and waiting to go.

10-31-2022, 11:23 PM	#11
putrio Scooby Newbie Member#: 494757 Join Date: Dec 2018 Chapter/Region: MWSOC Location: Cincinnati, OH Vehicle: 2011 STI Hatch Black	It’s being really weird honestly. After running for a few minutes I wasn’t reading any voltage drop, but the battery had dropped down to 12v and the light was on on the dash. I think I read something about the diodes going bad and it showing AC at the battery which I did see so maybe the alternator did get fried when I went through some water without the under guard on….

11-01-2022, 11:10 AM	#14
putrio Scooby Newbie Member#: 494757 Join Date: Dec 2018 Chapter/Region: MWSOC Location: Cincinnati, OH Vehicle: 2011 STI Hatch Black	There was definitely AC voltage at the battery. Alternator is removed, will try to get it warranty replaced and post an update this evening!

11-01-2022, 03:29 PM	#15
putrio Scooby Newbie Member#: 494757 Join Date: Dec 2018 Chapter/Region: MWSOC Location: Cincinnati, OH Vehicle: 2011 STI Hatch Black	Warranty replacement successful, but not in stock so I have to pick it up tomorrow. I'll update after I get it installed, whenever that might be. Hopefully tomorrow. RTV900 - Maybe I should have just replaced it from the getgo like you questioned at the beginning :-) I needed to be able to say it was faulty though or they wouldn't have swapped it for me under warranty.

11-01-2022, 04:55 PM	#17
Max Capacity Scooby Specialist Member#: 149347 Join Date: May 2007 Chapter/Region: NESIC Location: Tolland, CT Vehicle: 05 LGT ,09 SpecB Black/silver	Sounds like just one more story about junk replacement parts out there now days. Good luck with the replacement.

11-01-2022, 06:52 PM	#19
AliBenn Scooby Guru Member#: 487 Join Date: Nov 1999 Chapter/Region: TXIC Location: Greatwood/Sgrlnd Vehicle: 1998 /2005 STunIcorn Acadia Green RS-T	shot in dark here(from experience) have u traced the altn harness to the bottom of the fuse box mounted behind L headlamp? i had 'same' issue i found that the wire from altn to fuse box had come loose i clicked in it and fixed issue must of come loose with the work being done on my car(short block) GL

11-01-2022, 06:53 PM	#20
AliBenn Scooby Guru Member#: 487 Join Date: Nov 1999 Chapter/Region: TXIC Location: Greatwood/Sgrlnd Vehicle: 1998 /2005 STunIcorn Acadia Green RS-T	shot in dark here(from experience) have u traced the altn harness to the bottom of the fuse box mounted behind L headlamp? i had 'same' issue i found that the wire from altn to fuse box had come loose i clicked in it and fixed issue must of come loose with the work being done on my car(short block) GL