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The Dude abides...
 
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Join Date: Oct 2011
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Call to all alternator experts & voltage gremlin hunters

Iíve been working through a low system voltage problem since getting my 87 Targa back on the road. I first noticed the issue as a draining battery and was not sure if it was an issue with a lack of driving/old tired battery, etc.

The battery has been replaced. The battery drain issue continued to be a problem unless it was kept on the trickle charger and resulted in additional confusion on my part as to the true nature of the problem.

Upon searching our great forum, I found various threads on assessing the system voltage while driving and purchased a cigarette lighter volt meter. System voltage at driving load was found to be 11.8 - 12.3, considerably lower than the optimal voltage range expected from a healthy charging system.

Since learning of the low voltage I promptly purchased and replaced the voltage regulator, which includes built in brushes. I was very confident that this would fix my issue and was anxious to test drive the car and observe the new volt reading on my meter.

And it did work, at least initially. I fired up the engine and the levels were 13.1, the highest level by far since I started monitoring system voltage. This was good, and I assumed would continue to increase to the 13.5-14.5 V levels expected of a health system. And then, almost as quickly, it stopped functioning as I would have hoped/expected. In that same initial test drive, the happiness turned to a WTF moment when the voltage readings dropped to what they had been and then lower, bottoming out at 10.7 V.

Can anyone tell me what the heck is going on?!

Iím looking for some insight from those of you who are much more accomplished than I. Also looking for advice for further troubleshooting, rather than simply ď...send the alternator out for diagnostics and repair.Ē

Donít get me wrong, I am out opposed to giving the unit to someone to fix. Iím just interested in the why and the how to fix information, if it exists out there.

Thank you for your help.
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'87 Targa Granite Green
Old 10-11-2017, 09:18 AM
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Does the battery drain when just parked without a charger on it? If yes, you have a load that is not turning off and slowly draining the battery. You'll need to find the draining source by putting a multimeter in line between the battery terminal and cable (lots of youtube video and how to do this). Once you have the multimeter connected and set to current, take note of the current draw. Anything over 1 amp will be too much and drain your battery. Start pulling fuses (taking note on what fuse you are pulling) to find out which fuse make the current draw drop. Something might be ON, have a short, or bad ground.
I wouldn't trust tool much on the volt meter at the cigarette lighter. You'll need to test at the battery terminal to get a true reading.
Old 10-11-2017, 09:33 AM
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your local flaps should be able to run a quick, free diagnosis on your car to check your alternator's health (without having to remove the alternator). That should give you a clear picture of what's up.

That said, sounds like you may have a bad diode/failed windings on the alternator itself that's preventing it from providing a sufficient charge. A failed voltage regulator will generally allow the alternator to pass too high a voltage (generally shown as 15+ volt spikes) rather than unduly restrict the alternator's output.
Old 10-11-2017, 09:34 AM
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ROW '78 911 Targa
 
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Did you look at the commutator for glazing where the brushes ride and clean it? Needs to be bright and shiny.
Is the commutator worn, did you seat the brushes, did you test the diodes?
Is the belt at proper tension?
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Old 10-11-2017, 09:35 AM
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Samp1800 -- Agree that battery drain could be a factor, but that would need to be an incredibly strong drain to bring voltage down to 10.7 volts during a drive (while the alternator was charging) -- one wouldn't see that large of a drop if running high beams AND ac together with a (in any way) healthy alternator.
Old 10-11-2017, 09:37 AM
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Quote:
Originally Posted by darrin View Post
Samp1800 -- Agree that battery drain could be a factor, but that would need to be an incredibly strong drain to bring voltage down to 10.7 volts during a drive (while the alternator was charging) -- one wouldn't see that large of a drop if running high beams AND ac together with a (in any way) healthy alternator.
I agree with you there, but I don't trust getting voltage reading from the lighter socket.
Old 10-11-2017, 09:39 AM
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I think the volts at idle without lights/on should be 14.2. Test at the battery but the cig. socket should work as well. If you want the alt. rebuilt THE guy is here in San Jose. Now does it out of his house.
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Old 10-11-2017, 10:29 AM
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I'll go with bad grounds.
Battery, tranny, alt, VR, etc...
Clean up with wire brush, sand paper, and use needle nose to tighten all female crimp connectors.
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Old 10-11-2017, 11:03 AM
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Btw, there are testing procedures for the alt and VR.
Simple one grounds the input from the VR to the alt so the alt goes to full charge.
Measure V at alt, if not good, it's the alt.
If V good at alt, VR or wiring is the problem.
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Old 10-11-2017, 11:06 AM
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Even if they look serviceable, replacing the transmission and front battery earth straps can get you an extra volt at the battery.

The battery voltage when the car is cold will be higher than after it has warmed up, again by about a volt. Hot climates never see as high a voltage as cold climates.

You will lose at least 0.2 of a volt between battery and cigarette lighter, even if everything is as new.

I have a cigarette lighter voltage meter and it occasionally loses its contact with the metal and reads low until it is wiggled again.

Personally, I would check the battery voltage with a meter at the battery terminals before you draw conclusions.

It is quite normal for a 911 battery voltage to drop to 12.5 volts at idle if lights, wipers, defogger are on but 10.7 volts is way too low. To give you some idea, it would take a load of around 60A to pull it that low at idle.
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Old 10-11-2017, 11:07 AM
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yup, bad ground at the transmission ground strap I'd bet. Mine did the same thing. Would be ~13.5 volts but then drop off after a few minutes of driving it.

Try this test:
Get it warmed up and use your volt meter to determine when its low on voltage. Leave it running in this low voltage state and pop the engine lid open.
Grab some jumper cables and connect one end to the fan shroud or somewhere else on the motor that is metal. Connect the other end of the jumper cable to the chassis where you suspect it can ground out. This could be where the lid latch is or maybe even the metal bracket where the rear fuses are mounted.
Now check your volt meter and see if the additional path to ground has solved your charging issue. That's how I found the issue with mine. Very simple to test.

years of oil and grime on my ground strap had caused it to have way too much resisitance. Probably could have cleaned it well and wire brushed the connection points but i replaced it instead.
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Old 10-11-2017, 11:12 AM
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Don't bother cleaning ground straps, degradation is internal. Buy new.
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Old 10-11-2017, 11:19 AM
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Many seem to forget that there's a large cable that runs first from the alternator to the starter,
and another cable from there to the battery. Either the terminal at the alternator (usually just
crimped & not soldered) and/or loose, or the connections at the starter are loose or need soldering.

Hopefully not using a junk Hugo regulator, but the OEM Paris-Rhone regulator. Also, if a flaky
rebuilder worked on the alternator, there maybe intermittent internal solder connections.
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Old 10-11-2017, 08:33 PM
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I have had a low voltage problem twice in my car over the past 25 years both times the problem was isolated to a failed exciter diode in the alternator. There are three exciter diodes, one for each phase that feed a signal to the voltage regulator. Think of it as a feedback loop, lower signal to VR= lower output from alt. A loss of one exciter diode will cause your symptoms. Since you have an 87 and you replaced the VR and new brushes I will assume it is the alt with internal VR. The three exciter diodes are small and not the six power diodes that rectify the three phases. I have previously posted the repair I have done using generic 1N4001 diodes that you can buy at Radio Shack. Naturally, you will need to have some soldering skills to do the repair. Estimated repair time with just the alt is 2 hours and cost for parts under $5.00. For most car owners the easiest solution is to have the alternator load tested and repaired by a pro and the only downside is cost. In my own case, I have the skills and enjoy doing the work.
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Old 10-12-2017, 12:36 AM
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Lots of good ideas.

In my experience, don’t rule out the possibility that there is more than one issue.

A free load test at the local FLAPS is a good idea. However, I had my alternator/VR load tested once and it passed but problems persisted until it was replaced. Suspect that it was an intermittent failure.

Good suggestions to replace the transmission and battery ground straps. Good preventative maintenance even if not the problem. Good idea to check tension on the belt and make sure it is shimmed properly.

Get some Deoxit D5 and emory paper to clean contacts and grounds. I also cut about 1/2 inch to clean copper on the positive wires at the battery and installed a new terminal.

It does sound like you have a parasitic drain. My only concern is Jonny’s note regarding amperage. If you have a drain that big you will blow the fuse in a meter in a hurry looking for a parasitic drain. I would check the voltage at the battery off, idle and at 2k rpm to see if the voltage drop at the battery is still as big as measured at the cig lighter. If it is, there is a big load somewhere.

Do you have anything aftermarket installed? Oil cooler fan? Stereo amp? Alarm system? Any place someone may have hacked into the factory wiring would be a place to investigate. Could someone have installed something on an unfused circuit?
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Last edited by tirwin; 10-12-2017 at 05:22 AM..
Old 10-12-2017, 05:20 AM
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On my Targa, I had a defrost relay that was stuck, drained the battery overnight. I found it by taking advice from a good friend, simply disconnecting the negative lead to the battery and putting the amp meter between the negative terminal of the battery and ground and pulled/reinserted fuses one by one until I hit the problem area. Finally going to the rear power panel and pulling the big relay, I nailed it. I then took the relay apart and sure enough, one of the contacts was closed.
Old 10-12-2017, 06:10 AM
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Well, thanks to all for your many suggestions.

I have been out of town until early this morning, but I can answer a few of the questions.

1. Alternator belt is shimmed properly and tension is good
2. OEM voltage regular purchased and installed
3. Aftermarket stereo amp installed, but has been for 20 years
4. Grounds are connected and solid
5. Alternator has not been rebuilt (faulty rebuilder suggestion)

I will start moving through the list of suggested troubleshooting items.In the meantime, I have the battery charging (yet again) just to be sure the low voltage observed wasn’t due to low charge, overwhelming the new voltage regulator.

I will get back to the list as I make progress.
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Last edited by bigel; 10-12-2017 at 05:42 PM..
Old 10-12-2017, 05:29 PM
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Quote:
Originally Posted by bigel View Post
Well, thanks to all for your many suggestions.

I have been out of town until early this morning, but I can answer a few of the questions.

1. Alternator belt is shimmed properly and tension is good
2. OEM voltage regular purchased and installed
3. Aftermarket stereo amp installed, but has been for 20 years
4. Grounds are connected and solid
5. Alternator has not been rebuilt (faulty rebuilder suggestion)

I will start moving through the list of suggested troubleshooting items.In the meantime, I have the battery charging (yet again) just to be sure the low voltage observed wasnít due to low charge, overwhelming the new voltage regulator.

I will get back to the list as I make progress.
As mentioned up-thread, marginal cable connections, e.g. from the alternator, can result in
intermittent voltage drops. One can always in effect directly measure the alt voltage by measuring
the charging light voltage (blue wire at the bulb) when the battery voltage is low. If it's greater
than the battery voltage by more than 1-2 volts, there's a voltage drop from the alt to the battery.
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Old 10-12-2017, 07:11 PM
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Update

So, I have been been utilizing the advice provided by many and have been eliminating the laundry list of potential problems from the list.

Hereís the latest on my situation.

As mentioned in a previous post, I put the battery on the charger and tested the voltage. The battery is holding right around 13 V (13.1 - 12.9 depending how the wind was blowing).

I performed a test for parasitic draw as suggested by samp1800 and others; result was 50 mA with multimeter in line with negative terminal side of the battery. So, no parasitic draw. Thatís good...

Next I tested the voltage at the battery (b/c of concern with trusting the cigarette lighter socket voltage thingy, which by the way read exactly the same as the multimeter at the battery terminals). As mentioned above, battery read 13 V prior to starting. (Pic below)



Upon testing the battery with the engine idling, the voltage dropped, albeit nominally, to 12.76 V...a far cry from 13.5 - 14.5 V. (Pic below)



Taking Ďpampadoriísí advice, I tried augmenting the system ground by connecting one side of a jumper cable to the fan shroud and other to the chassis, no increase in voltage. This I tried from various locations; fan shroud, lid latch, engine support, intake ground connection and still no increase in voltage at idle.

So, does this mean my problem is likely the alternator? Keeping in mind that I have already changed the VR. Or could this condition still be caused by a bad ground? I have cleaned and reattached the following grounds.

- Ground adjacent to fuse panel in frunk
- Connections from VR to alternator
- Ground connection on engine intake
- Ground strap from transmission to chassis (with engine out 1 yr ago)
- Aftermarket ground from amp to chassis (frunk)
- Ground from alternator to engine block (during rebuild 1 yr ago)

Another question, can a FLAPS location identify an issue with the alternator vs. ground issue vs. something else?

I appreciate everyoneís input. Iíve just got to figure this thing out.

Onward!
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Old 10-14-2017, 08:23 AM
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Why do you continue to focus on just the ground lead? Doesn't the alternator connect to the
battery with another cable too?
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Old 10-14-2017, 09:40 AM
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