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Old 06-07-2012, 11:18 AM   #1
wrx_02
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Default Alternator Voltage

2002 WRX 90k miles Optima Red Top Battery
Stereo installed since 2004 with no issues

Voltage started to drop into the 11's and even 10 volts when I am at lights with the AC on. So I go down to the auto parts store and have the Alternator checked. They say it's bad so I get one from NAPA. Replace it with a 2006 STI Reman 90AMP.

Voltage still drops into the 11's so I suspect the battery may be bad from the stock alternator going out. Get my Optima replaced with a new one. Still getting the same issue.

Take the Car into Cobb for a timing belt and have them check for a draw in the system. They couldn't find any wires with a high resistance or draw on them. Suggest it is the Alternator.

I go back to NAPA and have them test the new alternator, they tell me it is not charging the system. So they order me another one. I put it on and still get 11's sometimes but they told me it tested as good. So I charge my battery last night thinking it was low from driving it around. They told me the battery tested good also.

Car finally hit 14's this morning but it was really cool outside, as it gets hot the Volts drop. Anyone have experience with the NAPA remand alternators of the Bosch? I can upgrade to a Bosch 110 but am not sure the issue could be somewhere else. Could the discharged battery cause the alternator to kick out less volts?
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Old 06-07-2012, 12:02 PM   #2
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wrx_02, your alternator should be charging the system at between 13.8 and 14.2 volts, or somewhere in that general area. If the output of the alternator is dropping to 11 volts at the output wire terminal on the alternator, it is not charging the system properly.

You've basically checked everything in the charging system and it is doubtful you'd get two bad alternators in a row, but I have heard of it happening. All that aside, I'd start checking for bad grounds. The main problem points would be:

1. Where the battery ground cable connects to the chassis.
2. Any grounding straps between the engine block and the car "frame" or body.
3. Any smaller ground wires on your car which tie back into the charging circuit.

Once you've gone through and cleaned the ground connections, I'd check the charging circuit again with a voltmeter with the engine at idle - - turn on any accessories you can think of; all lights, A/C (fan on high), stereo, etc. Alternator output should be around 12 volts with the full load, possibly just lightly under. 11 volts is too low. Then run the engine up to about 1500 - 2000 RPM; alternator output should be around 14 volts.
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Old 06-07-2012, 12:07 PM   #3
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^ good advice.
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Old 06-07-2012, 12:25 PM   #4
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I had some 8awg wire I grabbed from work and ran a few grounds.

Negative terminal to chasis
Negative to intake mani
Intake mani to alternator
Intake mani to chasis

I noticed the voltage was more stable and my car runs a lot smoother now. Just a thought.
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Old 06-07-2012, 12:28 PM   #5
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Quote:
Originally Posted by Halls06wrx View Post
I had some 8awg wire I grabbed from work and ran a few grounds.

Negative terminal to chasis
Negative to intake mani
Intake mani to alternator
Intake mani to chasis

I noticed the voltage was more stable and my car runs a lot smoother now. Just a thought.
Excellent!
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Old 06-07-2012, 12:45 PM   #6
wrx_02
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Quote:
Originally Posted by bassman12350 View Post
wrx_02, your alternator should be charging the system at between 13.8 and 14.2 volts, or somewhere in that general area. If the output of the alternator is dropping to 11 volts at the output wire terminal on the alternator, it is not charging the system properly.

You've basically checked everything in the charging system and it is doubtful you'd get two bad alternators in a row, but I have heard of it happening. All that aside, I'd start checking for bad grounds. The main problem points would be:

1. Where the battery ground cable connects to the chassis.
2. Any grounding straps between the engine block and the car "frame" or body.
3. Any smaller ground wires on your car which tie back into the charging circuit.

Once you've gone through and cleaned the ground connections, I'd check the charging circuit again with a voltmeter with the engine at idle - - turn on any accessories you can think of; all lights, A/C (fan on high), stereo, etc. Alternator output should be around 12 volts with the full load, possibly just lightly under. 11 volts is too low. Then run the engine up to about 1500 - 2000 RPM; alternator output should be around 14 volts.
Thanks for the info.

I did find this yesterday and was going to do some testing.

http://www.aa1car.com/library/charging_checks.htm

I did get some 8 gauge wire and strap some clamps to it. Tested it to make sure it worked with my volt meter.

Then I clamped it on the alternator power wire then to the positive end of the battery. Just to see if there was an issue between the connection, the fused box, then the wire to the battery. It did nothing. I haven't tired it with the new alternator yet. Just with the first one I got. May try that again tonight just to see. I was going to start checking ground last night but it was raining really bad here so I got the motor back together on my bike in the garage instead.

I trust the tech at Cobb though. He said he could not find an issue with any wires. I do need to look for myself though.
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Old 06-08-2012, 09:18 AM   #7
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Checked for loose grounds yesterday.

Also I checked some volts. Alternator housing to the negative battery terminal came out with 0 volts. Alternator output terminal to the positive end of the battery turned up 0 volts. So I could not find a drop in current.
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Old 06-08-2012, 01:19 PM   #8
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It sounds like you're on the right track, but you could still have a grounding issue. Using a voltmeter involves very little current draw. I you have a "low ampacity connection" (one that will not allow the full flow of high current) somewhere, you could still see a zero voltage drop until the car's electrical system has a high demand on the charging circuit. Try checking for the voltage drop with the engine running at about 1500 RPM, all accessories on, and then perform your measurements again.
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Old 06-08-2012, 02:04 PM   #9
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I checked the voltage drops when at idle with the AC on High, window defroster on, Lights on, stereo on etc etc....

I will check again and get someone to rev the engine for me.
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Old 06-08-2012, 03:33 PM   #10
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Check the alternator output voltage from the alternator output to the alternator case. Then check the voltage from the alternator output to the block and finally to the battery. If the voltage changes when you measure to the block, you have a bad alternator ground. If the voltage to the block is good but the voltage changes when you measure to the battery you have a bad battery ground.
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Old 06-08-2012, 04:15 PM   #11
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Good call chuck, will get it looked at Saturday.
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Old 06-11-2012, 02:08 PM   #12
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Quote:
Originally Posted by LIChuck View Post
Check the alternator output voltage from the alternator output to the alternator case. Then check the voltage from the alternator output to the block and finally to the battery. If the voltage changes when you measure to the block, you have a bad alternator ground. If the voltage to the block is good but the voltage changes when you measure to the battery you have a bad battery ground.

Couldn't get someone to rev the engine for me this weekend while I checked but I did check the idle volts.

Alternator output --> Alternator Case 11volts
Alternator output --> Engine Block 11 volts
Alternator output --> Battery 11 volts

This was loaded with the AC on high/radio/lights/deforster

I did notice that while driving and sitting at lights with just the radio on, even when it is hot, if I never turn the AC on it will stay at 14 volts an above. The second I turn the AC on the voltage will drop to 13.8 while on the freeway and will get to 12 flat or 11.8 while sitting still. On high or on low.

Thinking there may be a draw from the AC when it is on?
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Old 06-11-2012, 02:45 PM   #13
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Yes of course there's a draw from the AC. The engine fans are running and the compressor clutch is engaged, so that's a substantial amount of current. With additional loads at the same time, it will be enough to exceed what the alternator can put out at idle and the system voltage will begin to drop. This is certainly true with the OEM 75A alternator in the '02, and even though you said you've installed a 90A from an STi it may still not produce enough current at idle. You also don't know for sure whether that alt is really producing its rated output.
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Old 06-11-2012, 03:15 PM   #14
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Quote:
Originally Posted by Mulder View Post
Yes of course there's a draw from the AC. The engine fans are running and the compressor clutch is engaged, so that's a substantial amount of current. With additional loads at the same time, it will be enough to exceed what the alternator can put out at idle and the system voltage will begin to drop. This is certainly true with the OEM 75A alternator in the '02, and even though you said you've installed a 90A from an STi it may still not produce enough current at idle. You also don't know for sure whether that alt is really producing its rated output.

Yeah I guess your right but 2 whole volts seems like a huge draw, didn't know it that was normal for the wrx or not?

I am going to try to take this to a friend and have him tell me what it is putting out. This is all over my head, I am more mechanical not electrical.
NAPA did test it with their machine and said it was putting out enough volts but I take that with a grain of salt. I would think a 90amp would be enough, next option would be 100$ and get the Bosch 110amp. Which I could do but I feel like I would be just throwing money at it. When my old 75amp seemed to be fine, maybe NAPA brands just doesn't do what it's rated to do?
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Old 06-11-2012, 03:28 PM   #15
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Draw is measured in current not voltage.
What happens is that once the draw on the system exceeds what the alternator can put out at that RPM, the load will be taken up by the battery and the voltage will drop. With enough load it will drop down to battery voltage.
Regardless of what an alternator may be rated for in peak current output, it will only be able to produce a portion of that output at idle. Since the RPM-vs-output curves may be different, it's not a given that the 90A will do better at idle than the 75A.
Without actually seeing the output on a tester with your own eyes, you don't know whether it's putting out its rated output or what the curve looks like. If your friend has the right test equipment, have him check it.
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Old 06-11-2012, 03:32 PM   #16
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Thanks Mulder, feel like I am chasing my tail.
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Old 06-12-2012, 02:02 AM   #17
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Quote:
Originally Posted by Mulder
Draw is measured in current not voltage.
What happens is that once the draw on the system exceeds what the alternator can put out at that RPM, the load will be taken up by the battery and the voltage will drop. With enough load it will drop down to battery voltage.
Regardless of what an alternator may be rated for in peak current output, it will only be able to produce a portion of that output at idle. Since the RPM-vs-output curves may be different, it's not a given that the 90A will do better at idle than the 75A.
Without actually seeing the output on a tester with your own eyes, you don't know whether it's putting out its rated output or what the curve looks like. If your friend has the right test equipment, have him check it.
So would this be a reason why I sometimes get a voltage warning on my meth system? Let's say rolling 3rd gear and then wot pull to mid 4th. You think cause of the load of my engine/ meth pump draw, the meth pump isn't getting enough juice and in return showing me a voltage warning. It doesn't happen all the time, but seems like halfway through rpms and the wot driving does it more often.
How about a combination of lightened crank pulley and lightened alternator pulley? Too light of rotating mass? Maybe put stock alt pulley back on. I know I should probably just put a volt gauge in, but thought I would ask.
Sorry for hijacking and getting off topic.
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Old 06-12-2012, 03:46 AM   #18
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The alternator will stop working if power to the field exciter is interrupted. This is the lead that ties to the battery warning light. Power from the ignition runs through the lamp that is in series and on too the connection on the back side of the alternator.
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Old 06-12-2012, 12:44 PM   #19
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OK, you confirmed that your wiring is good. It is either the battery or the alternator. First make sure that the belt isn't slipping. Then you need an ammeter to measure the alternator output. If the alternator is putting out more current as the load increases and yet the voltage still drops, you have a bad battery.
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Old 06-12-2012, 12:54 PM   #20
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Quote:
Originally Posted by LIChuck
OK, you confirmed that your wiring is good. It is either the battery or the alternator. First make sure that the belt isn't slipping. Then you need an ammeter to measure the alternator output. If the alternator is putting out more current as the load increases and yet the voltage still drops, you have a bad battery.
Excellent advice, thank you and I will report back my findings. Yes all wiring is good. My initial thought was my belt slipping, but without a gauge its very hard to hear anything while pulling that hard lol.
I will put a newer belt on and see what happens and try to duplicate it again.
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Old 06-12-2012, 05:28 PM   #21
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A bad battery will present problems alright and it will depend on what kind of trouble the battery has. If the charging system can't provide the proper voltage I would suspect the internal voltage regulator before a bad battery. Having a load test done on the charging system is the proper way to find out if there is a problem with either the alternator or the battery.

If you haven't checked wiring on the back side of the alternator for proper operation and a possible problem then you haven't tested everything.
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Old 06-13-2012, 04:29 PM   #22
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From what he said the system does sometimes provide the proper voltage so I don't think it's the regulator. It could be a bad connection in the regulator but for two different alternators?

Wait a minute, check the alternator pulley size. If it's too big you won't get full power out of the alternator. It won't be the first time that a replacement alternator has the wrong pulley installed.

Last edited by LIChuck; 06-18-2012 at 09:07 AM.
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Old 06-14-2012, 01:00 PM   #23
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Quote:
Originally Posted by LIChuck View Post
From what he said the system does sometimes provide the proper voltage so I don't think it's the regulator. It could be a bad connection in the regulator but for two different alternators?

Wait a minute, check the alternator pulley size. If it's too small you won't get full power out of the alternator. It won't be the first time that a replacement alternator has the wrong pulley installed.

Can anyone let me know the diameter of a stock alternator pulley?
Quick Google search just shows aftermarket ones and on the parts website they don't list the size.
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Old 06-17-2012, 02:28 PM   #24
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If your alternator pulley is smaller, you should be getting higher voltage since your shaft would spin faster. Btw I have 120 amps alternator. I'll try to measure my alternator pulley tom.
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Old 06-18-2012, 05:02 PM   #25
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I can have the car idling, with the AC on and I can unplug the compressor and the voltage will jump up to 14 volts.

I need to test with just the blower on but I think with the AC off and the blower on High it will drop the volts to 11.9 also.
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