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Alternator not charging, exciter wire question

I have a 87 944 Turbo/LS1 swap that has a non charging alternator. The alternator has tested fine and the bulb has been swapped out with a know good bulb ,so, that leaves the wiring at the blue wire. The other wires are getting the proper voltage.

The alarm has been bypassed by bridging 1-4 and 7-8. I had cut the wires and put jumpers in the slots. Searching reveals that the alternator wire goes through the alarm. I reconnected the wires to the original spec with the jumpers in place.

Still no alternator light with the bulb check, key in position two and nothing at the blue exciter wire.

Does anyone have any other ideas?

Old 09-02-2011, 07:14 PM
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No specific info but the field wire (excitor) should have b+ with key on. Do you have voltage here? If not, apply b+ and test.

Swap to gm 1 wire. No field charge needed.
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Old 09-02-2011, 09:21 PM
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Thanks, I already have this alternator and it is a high output unit. Besides, now I am totally obsessed with getting the alternator light on the dash to work properly. I may have found the issue. The alarm grounds the blue wire when activated, I just opened my door and the alarm went off. If the alarm box is pulled it may fix the issue.
Old 09-03-2011, 05:36 AM
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Let us know.
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Old 09-03-2011, 06:38 AM
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The bypass fixed the alternator charging issue, but the bulb still doesn't light with the bulb check or during start up.

Last edited by Arthropraxis; 09-03-2011 at 07:12 PM..
Old 09-03-2011, 07:13 AM
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The bulb might have a bad ground at the dash. Dash/gauge grounding issues are common on these cars. Oh, and all you have to do to excite the field (if the sense wire is not connected) of any alternator is blip the throttle once.
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Last edited by wild man; 09-04-2011 at 06:40 AM..
Old 09-04-2011, 06:35 AM
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Came across this thread and have a similar question. My new battery is dead in the mornings. After I boost the car it runs fine and all lights air con work good. So I am assuming the alternator is good. Ideas??? I never arm the alarm. The car gets rarely driven. 1987 944 n/a
Old 09-04-2011, 09:49 AM
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I don't even have the key to activate my alarm and it was on. So, the bypass worked well for me. I have taken the instrument panel apart and put it back together a couple of times as well as switching bulbs to see what works. The alternator bulb never lights up. I have a 470ohm resistor in line. I may take that out to see if it allows the alternator light to light up.
Old 09-04-2011, 05:34 PM
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" The bulb might have a bad ground at the dash. Dash/gauge grounding issues are common on these cars."

Only problem is that particular lamp isn't grounded. The way it works is that one side is connected to a switched source of 12+v. The other side is connected to the internal regulator on the alternator. When the alternator isn't charging, current flowing through the lamp will find ground through the field coil, which is what establishes the bootstrap magnetic field. But once the alternator is charging, the lamp will "see" 12v on both leads. Since the difference in potential is zero, the bulb goes out.

If your resistor is in series with the lamp, then the lamp may not be getting enough voltage to produce light. The alternator will still bootstrap, as it only needs a tiny flow of current to get started. The resistor should be wired in parallel to the lamp, so that it provides an alternate path for bootstrap current if the bulb blows.
Old 09-05-2011, 09:01 PM
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The resistor is in series. The alternator seems to be charging fine. It was recommended with this alternator to have a 3W bulb or a 470ohm resistor to turn it on. The voltmeter bulb is smaller than 3W so, it may be better just to keep the resister in place.
Old 09-06-2011, 05:42 AM
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That's a weird setup. The lamp is unlikely to light with the resistor in series. The idea of a resistor in parallel is to bypass a blown lamp. In series, it's just another failure point and you lose the usefulness of the indicator.

The addition of series resistance will reduce voltage to the alternator trio. While this is "extra soft" on the trio, it's unnecessary as the lamp already provides sufficient resistance. However, with the alternator is operating well, let it be if you're happy without the indicator.

Last edited by mxfrank; 09-06-2011 at 06:50 PM..
Old 09-06-2011, 06:42 PM
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Well, I have gotten pretty obsessed with making the alternator light come on. The harness isn't wrapped yet. I will just cut out the resistor and see if that works. Thanks, for the help.

Old 09-06-2011, 07:38 PM
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