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How to test alternator only getting 12-13v max


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36 replies to this topic

#1 ericem

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Posted 03 July 2007 - 12:59 PM

Well, i got my battery replaced a few months ago with a OEM battery. But i replaced my alternator with a non-oem alternator. Now I get 14-14.1v with the lights off. but when i turn on the lights and the max ac, at level 3 fan, i am at 12-12.5v on idle, and at 1-4,000 rpm's max i get is 13.1v. Could this explain my hesistation issue, light dimming at idle, ABS light at times and inconsistent fan speed? What do you think I should do get a OEM alternator? or find another rebuilt, i think the one my dad got doesn't have enough amps.

#2 PAezb

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Posted 03 July 2007 - 01:22 PM

Well, i got my battery replaced a few months ago with a OEM battery. But i replaced my alternator with a non-oem alternator. Now I get 14-14.1v with the lights off. but when i turn on the lights and the max ac, at level 3 fan, i am at 12-12.5v on idle, and at 1-4,000 rpm's max i get is 13.1v. Could this explain my hesistation issue, light dimming at idle, ABS light at times and inconsistent fan speed? What do you think I should do get a OEM alternator? or find another rebuilt, i think the one my dad got doesn't have enough amps.


See heading numbers 6 and 7 at this link:

http://www.repairfaq...#CARBATTERY_006

As you increase the electrical load on the battery and charging circuit, voltage will drop somewhat, especially at idle. Your numbers are within norms. An ABS or Battery light along with dimming lights could be a sign of an issue with the charging system. Make sure your battery terminals are clean (a little steel wool cleaning followed with a thin film of dielectric grease or vasoline) and cables and their clamps are in good condition. Typical rating for most Subaru alternators is around 70 amps. Some rebuilts can be hit or miss for 100% power load delivery (all diodes working). Where did you buy the alternator?

Did your hesitation issue start with the alternator replacement?

#3 ericem

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Posted 03 July 2007 - 02:19 PM

Well this is the second non-oem replacement, and before it still had light dimming at idle. I just did a grounding mod with a wire going from strut tower to strut, to neg of battery. Another wire from the block to the neg terminal as well. Haven't driven at night yet after I did the mod, but the car seems to shift better, and idle unnoticeably, but i still got a ABS light, its weird see when I hit the brakes sometimes I hear the abs kick in on light braking keep in mind on drive hot asphalt, then the abs light just turns on. I know one CV boot is torn a little still need to get around to replacing it, and the front rotors are warped a bit, as well as the brake fluid still needs to be changed, brakes are very mushy, and can't lock the brakes, as well as worn pads.

#4 Cougar

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Posted 03 July 2007 - 02:22 PM

It sounds like one or two of the output diodes may be bad in the alternator. The best thing to do is have it load tested at a shop and see what the results are. The test will let you know if it is ok or not. They can do the test without removing the alternator.

#5 edrach

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Posted 03 July 2007 - 02:23 PM

Either a bad connection somewhere or a bad alternator. You should get well above the 13.1 volts at the battery under load (i.e. with the lights on) and with the rpm above 1500. Get the alternator tested at a local auto parts store. Most of them will test it for free for you.

#6 ericem

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Posted 03 July 2007 - 02:30 PM

Either a bad connection somewhere or a bad alternator. You should get well above the 13.1 volts at the battery under load (i.e. with the lights on) and with the rpm above 1500. Get the alternator tested at a local auto parts store. Most of them will test it for free for you.


Could it be the Battery? Terminals seem fine getting like a .1 v difference between the battery and alternator terminals.

#7 vic/se

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Posted 03 July 2007 - 02:37 PM

you should have 12,5 volts minimum! and no more then 14.1 volts .

#8 ericem

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Posted 03 July 2007 - 02:40 PM

I thought it was no more then 14.4v?

#9 PAezb

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Posted 03 July 2007 - 02:54 PM

Well this is the second non-oem replacement, and before it still had light dimming at idle. I just did a grounding mod with a wire going from strut tower to strut, to neg of battery. Another wire from the block to the neg terminal as well. Haven't driven at night yet after I did the mod, but the car seems to shift better, and idle unnoticeably, but i still got a ABS light, its weird see when I hit the brakes sometimes I hear the abs kick in on light braking keep in mind on drive hot asphalt, then the abs light just turns on. I know one CV boot is torn a little still need to get around to replacing it, and the front rotors are warped a bit, as well as the brake fluid still needs to be changed, brakes are very mushy, and can't lock the brakes, as well as worn pads.


What was the purpose of the grounding mod? Be sure you have no shorts.

If you have the alternator tested, have them test the specific gravity of the battery also - it's potency may not be what it should. Check for any small cracks on the battery casing that may be leaking acid, especially around the terminals.

Have you tried putting a smart charger on the battery to determine whether it is fully charged? Many chargers can also perform basic charging diagnostics with the engine running - invest in one of you don't have one already - around $50 for a decent one.

As for the ABS, it may or may not be a symptom of a problem with the electrical on the vehicle. Sounds like you have some separate maintenance issues to deal with on the brakes anyway. Might want to check the ABS and it's relay to be sure it's not maintaining a closed circuit (always drawing power)

#10 ericem

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Posted 03 July 2007 - 04:02 PM

Ya because my dad was thinking the ABS was drawing to much power. I remember the relay used to stick, but I just sprayed some contact cleaner on the relay and all connectors. Then it stopped sticking. Now I just hear the abs randomly come on then following a ABS light. But it doesn't keep the pump running.

#11 Manarius

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Posted 03 July 2007 - 04:13 PM

Get the alternator load tested.

If it fails, replace it and call it a day. If it doesn't, there's something else wrong. Maybe you need to replace stock wires running from the alternator to the battery with some heavy duty ones.

#12 Cougar

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Posted 03 July 2007 - 09:23 PM

Terminals seem fine getting like a .1 v difference between the battery and alternator terminals.


That should be good, especially if you had a good current load on it while testing.

#13 ericem

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Posted 03 July 2007 - 11:29 PM

Well the alternator is still under warrenty. So might as well find it. :rolleyes:

#14 edrach

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Posted 04 July 2007 - 05:34 PM

Could it be the Battery? Terminals seem fine getting like a .1 v difference between the battery and alternator terminals.

Are you testing that voltage with the lights on or off? Voltage drop is near zero if you're not drawing any current. If you want a true test, always test under load (as much current draw as you can manage); turn on the lights, blower, radio and anything else that pulls current.

#15 ericem

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Posted 04 July 2007 - 05:46 PM

Are you testing that voltage with the lights on or off? Voltage drop is near zero if you're not drawing any current. If you want a true test, always test under load (as much current draw as you can manage); turn on the lights, blower, radio and anything else that pulls current.


had lights on radio turned up, defrost on, wipers on, max ac on and max fan. Don't knwo what else sucks the battery?

#16 edrach

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Posted 04 July 2007 - 11:14 PM

had lights on radio turned up, defrost on, wipers on, max ac on and max fan. Don't knwo what else sucks the battery?

That's enough. 1/10 amp with that load indicates a reasonable connection where you measured it.

#17 Manarius

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Posted 05 July 2007 - 11:49 AM

How about we go and have the alternator tested and then report back instead of continuing to guess when the clear identifier will be when the alternator gets load tested.

#18 ericem

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Posted 05 July 2007 - 05:32 PM

Don't feel like it today. So your all going to have to suffer waiting. :lol:

#19 ericem

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Posted 10 July 2007 - 11:34 PM

Well replaced the alternator. Is 13v good with lights, high beams, and max ac on with highest level fan?

#20 Manarius

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Posted 11 July 2007 - 06:03 AM

Well replaced the alternator. Is 13v good with lights, high beams, and max ac on with highest level fan?

If you're sitting at idle, that should be plenty. Did you replace the alternator because it failed a load test or just because you felt like it? You may be asking for blood from a turnip here...

#21 Cougar

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Posted 11 July 2007 - 09:53 AM

Well replaced the alternator. Is 13v good with lights, high beams, and max ac on with highest level fan?


That is good, especially if it is at idle. At idle with a good load you could see around 12 volts but it should be between 13.0 to 14.8 volts with the engine running around 1200 RPM.

#22 ericem

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Posted 11 July 2007 - 10:40 AM

That is good, especially if it is at idle. At idle with a good load you could see around 12 volts but it should be between 13.0 to 14.8 volts with the engine running around 1200 RPM.


Ya i got 13v at 500rpm idle.

#23 ericem

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Posted 11 July 2007 - 08:04 PM

Hmm, did another test today. Got 11-11.7v with wipers on, radio, lights, high beams, max fan and mac ac. Nothing aftermarket is in the car. Going for a official test to find out if its the battery tommorow at subaru.

Does anyone know what the warrenty usually is for Subaru batteries?? The dealer installed it. Has been ALMOST a year.

#24 ericem

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Posted 11 July 2007 - 08:21 PM

Woot its 30 months. They better not charge me to check it :rolleyes:

#25 Cougar

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Posted 11 July 2007 - 08:54 PM

It sounds like the battery charge may be slightly low. The alternator needs to be running with the engine RPMs above 900 for it to put out a fair amount of charging current. You are not going to be able keep a good charge rate going at 500 RPM.




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