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Nigel Tufnel Wannabe
 
tchudy's Avatar
 
Join Date: Sep 2011
Location: Channahon, Il
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Alternator charging issues & blue exciter wire

Wondering if one of you can help with my alternator issue. The last couple years my alternator had no voltage at start-up, but if I revved the engine a few times the voltage came up to about 13.9. A few weeks ago I had no voltage even if I revved the engine. The alternator tested okay at autozone but I decided to replace the alternator wiring and add a Nissan alternator. I modified and wired the new alternator same as the diagram and instructions from Rennlist forums. I have 12.5 volts at the battery, the starter and the alternator with the key off. I have just under 12 volts with the car running at the battery. Revving the engine makes no difference.

After reviewing many forum posts I pulled the cluster and removed the "battery light" bulb. Bench tested and it is okay. I added a ground to the alternator body and it made no difference. My alarm system has not had the module attached for many years and it is not jumpered. The old alternator had worked okay the last few years so I doubt that the alarm is the issue. I went ahead and jumpered it anyway and no charge. Assuming the new alternator is good (I'll take it out and test if needed), what is the next step? I was thinking of sending a few volts down the exciter wire where it connects at the power brake booster. Thanks
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1986 Porsche 944 Turbo Project.
2005 VW Jetta GLS
2000 Toyota Solara...boring to drive but good mileage.
1971 VW Super Beetle...sold it to make room for the Porsche.
Old 12-04-2017, 08:05 AM
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Nigel Tufnel Wannabe
 
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Blue exciter wire

Today I jumpered the blue exciter wire to 12 volts from the battery and the alternator was charging. I guess next step is to determine where the issue is. I am going to try a spare gauge cluster first. Any other suggestions? If I have to I'll run a second wire from the alternator to a 12 volt source temporarily. Does anyone know if the exciter wire is 12 volts? Not sure if I'll need to throw a resistor in there.
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1986 Porsche 944 Turbo Project.
2005 VW Jetta GLS
2000 Toyota Solara...boring to drive but good mileage.
1971 VW Super Beetle...sold it to make room for the Porsche.
Old 12-04-2017, 08:07 PM
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I had an issue getting my alternator to excite at startup and it was the factory alarm. I had not bypassed it correctly. May not be your issue, but it does run through the alarm.
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'87 Porsche 944 Turbo/LS1
Old 12-05-2017, 03:55 AM
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It could be that the OEM alternator self-excites easier than the one you put in. You may not have had a working exciter wire this whole time.

Look at the wiring diagram and trace it.
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Old 12-05-2017, 07:27 AM
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believe the mechanism is nominally 12V but the lightbulb provides a certain amount of resistance which the alternator uses to figure how much output it needs to make.

so you might be able to bypass it for testing by measuring the resistance of the lightbulb, then getting a radio-shack resistor of that value (or just use the lightbulb on a test wire) between the alternator and battery +.
Old 12-05-2017, 08:27 AM
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Dumb question- ground at the engine need a cleaning?
Wire : corroded under the insulation?

Time for a voltage drop test?
^^^^thanks djnolan

... asking for a friend....
Old 12-05-2017, 05:43 PM
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Nigel Tufnel Wannabe
 
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I have a good ground. I even ran a ground directly to the alt housing during testing.
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1986 Porsche 944 Turbo Project.
2005 VW Jetta GLS
2000 Toyota Solara...boring to drive but good mileage.
1971 VW Super Beetle...sold it to make room for the Porsche.
Old 12-06-2017, 06:27 PM
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I have a 1983 Porsche with the same issue. I thought the alternator was not working. Replaced it, and still no charge. Then I used my Powerprobe to quickly put 12V at the exciter wire. I think it is light blue with a black stripe or something. Its hard to tell from the picture. The alternator immediately started charging...

Where does this wire go after it leaves the alternator? I've tracked it back to right before the firewall, but the PO had it hooked (with a solder joint) to a solid blue wire that goes into a two pin connector. Since this does not look to me to be factory, tracing this down will probably be a waste of time.

I was thinking of hooking 12V through a diode to the alternator, but then how would it know to stop charging?
Old 12-08-2017, 07:56 PM
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The blue wire from the alternator is supposed to go to that two-pin connector. From the other side of that connector, the blue wire goes directly to the charge light bulb in the instrument cluster. The charge light bulb is an incandescent bulb which varies resistance with voltage, and is an integral part of the charging circuit.
Note that the charge light bulb must be incandescent and should never be replaced with an LED style bulb.
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Old 12-08-2017, 08:16 PM
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So if the bulb goes out, the alternator won't turn on? Wow. I guess I'll turn the key and look for the light. Is it obvious, like "alt"?
Old 12-08-2017, 09:05 PM
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1. Correct. Burnt out bulb (or no power TO the bulb) = no charge.
2. Turn the key to the first position and look at the left cluster gauge for a battery symbol (rectangle with + and - sign). Should be lit before you engage the starter.
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Old 12-09-2017, 04:06 PM
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I can't find a nice wiring diagram that shows this circuit, so I was thinking of trying this for trouble shooting the dash bulb:
Ground the wire that comes out of the two pin connector. Since it is already cut, this is easy. This should cause the light on the dash to come on when the key is in the ON position if 12V is going to the bulb.

I'm going to start digging into the alarm circuit...
Old 12-10-2017, 09:37 AM
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Yes. Good place to start (pardon the pun).
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Old 12-10-2017, 11:19 AM
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Way to freaking cold to work on the car today, even though it was in the barn. Snow in Atlanta GA...

I have a biz trip that will keep me out from under the hood until Saturday. Hopefully my beautiful wife will let me spend an hour or two working on it then. I will keep you posted on what happens. I hate it when I follow a thread and you're left hanging.
Old 12-10-2017, 10:20 PM
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I am having a similar issue on my 911sc/1983. The battery light never came on. One day with the cold snap there was not enough power to start. Recharged the battery to 12.4 and started the car. Was only getting 11.4 with engine running. Took alternator to a rebuilder who said it was fine but was not getting exciter. While I had the alternator out I touched the blue wire to the chassis and the light went on. Installed a new Valero alternator and the light is on continuously, regardless of engine revs. I have checked the grounds and they seem fine. The po installed an alarm system that I disconnected because the fuel pump was not getting power. I think somewhere between the disconnected alarm and the blue light is the problem but struggling to find..
Old 12-12-2017, 08:51 PM
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The alarm relay cannot just be unplugged, it has to be bypassed. Look on here or the other site for the pins to jump for your year car. I think Clarks Garage has instructions on the early model.
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Old 12-13-2017, 04:58 AM
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Thanks. Upon further investigation there are concerning modifications including cut wires left open.
Old 12-13-2017, 10:03 PM
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Quote:
Originally Posted by DeadPorsche944 View Post
Way to freaking cold to work on the car today, even though it was in the barn. Snow in Atlanta GA...

I have a biz trip that will keep me out from under the hood until Saturday. Hopefully my beautiful wife will let me spend an hour or two working on it then. I will keep you posted on what happens. I hate it when I follow a thread and you're left hanging.
Finally a break in the weather. I got a Porsche dash bulb and hooked it in between the exciter wire and a switched 12 V source. The car charges nicely now. I think it also needs a diode, cathode towards the alternator, in series with the lightbulb. This would cause the light to go off when the battery starts charging. For now, I have a light under the hood that is always on when the car is on. ;-)

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Old 12-28-2017, 12:39 PM
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The light under the hood should only be "ON" when the parking lights or headlights are switched on AND the hood is open.

That under hood light bulb is controlled by a mercury switch within the bulb holder.

You may have to "adjust" the bulb socket so it tilts correctly. I just did this because mine wouldn't come on (now it does).
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Old 12-28-2017, 12:45 PM
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Quote:
Originally Posted by onZedge View Post
The light under the hood should only be "ON" when the parking lights or headlights are switched on AND the hood is open.

That under hood light bulb is controlled by a mercury switch within the bulb holder.

You may have to "adjust" the bulb socket so it tilts correctly. I just did this because mine wouldn't come on (now it does).
I'm afraid you misunderstood. Instead of using the light in the dash for the alternator, I used a spare bulb hooked to the exciter wire on the alternator. Until the voltages match, the light will be on.

The bulb under the hood has been gone since before I owned the car.



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Old 12-28-2017, 08:31 PM
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