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Discussion Starter · #1 ·
I'm having a bit of an issue that I can't figure out with my charging system, and it seems to be a little different of a situation from any other thread I've looked at. I just got a new 1980 rabbit pickup, and when I got it the PO was driving it around without a top mounting bolt on the alternator so it was shaking itself apart. That messed up the windings on the inside so I ordered up a new alternator. I've got it installed in the truck but it doesn't charge. BUT, it will start charging if you jump 12v to this M, or W, or motorola logo post the the suppression condenser (red circle).
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You only have to jump it for a split second and it will keep charging until you shut it off.

All grounds are new and connected properly (battery to frame and to trans mount, as well as alternator to the engine. The blue wire has no short to ground anywhere. The battery light on the dash comes on with the key, stays on once started, even with a blip of the throttle, and won't go out and start charging until jumping 12v to that post.

Anyone have any ideas of what the problem could be? Any help would be greatly appreciated! Seems like an issue with the exciter circuit but I can't find anything wrong with it, with the exception of something internal to the alternator. If I can't figure this out I might just run a jumper from the starter solenoid to this post on the alternator so it gets jumped just when cranking.
 

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might just run a jumper from the starter solenoid to this post on the alternator so it gets jumped just when cranking.
Seems like a solution to me. Perplexing issue and it would be nice to know why, but...
 
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I'm having a bit of an issue that I can't figure out with my charging system, and it seems to be a little different of a situation from any other thread I've looked at. I just got a new 1980 rabbit pickup, and when I got it the PO was driving it around without a top mounting bolt on the alternator so it was shaking itself apart. That messed up the windings on the inside so I ordered up a new alternator. I've got it installed in the truck but it doesn't charge. BUT, it will start charging if you jump 12v to this M, or W, or motorola logo post the the suppression condenser (red circle). View attachment 150346

You only have to jump it for a split second and it will keep charging until you shut it off.

All grounds are new and connected properly (battery to frame and to trans mount, as well as alternator to the engine. The blue wire has no short to ground anywhere. The battery light on the dash comes on with the key, stays on once started, even with a blip of the throttle, and won't go out and start charging until jumping 12v to that post.

Anyone have any ideas of what the problem could be? Any help would be greatly appreciated! Seems like an issue with the exciter circuit but I can't find anything wrong with it, with the exception of something internal to the alternator. If I can't figure this out I might just run a jumper from the starter solenoid to this post on the alternator so it gets jumped just when cranking.
Someone please correct me if I’m wrong but most alternators need a 12v ignition wire to ‘excite’ the coil inside, kickstarting the charging circuit. The alternator can spin, but until it gets the 12v excitement, it won’t provide any useful voltage to the battery. Once it’s spinning it can supply the voltage on its own, so it’s only needed for a second.
Some newer ones like by ACDelco (Tech Tip: Flash the Field | Delco Remy)have an auto-start function, but as far as I know, especially for one made to fit a 1980 vehicle, the 12v jump is standard.


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Discussion Starter · #4 ·
Correct me if I'm wrong, but isn't the blue wire in the plug the exciter wire? Doesn't it provide 12v from the indicator light, and once the alternator starts charging the light goes out because the exciter wire is no longer acting as a ground?
 

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Correct me if I'm wrong, but isn't the blue wire in the plug the exciter wire? Doesn't it provide 12v from the indicator light, and once the alternator starts charging the light goes out because the exciter wire is no longer acting as a ground?
I believe you are correct. If your alt light is on before you manually excite using the w terminal and off afterwards, then I believe that verifies your stock exciter wire connection.
 

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Discussion Starter · #6 ·
If your alt light is on before you manually excite using the w terminal and off afterwards, then I believe that verifies your stock exciter wire connection.
That's what has me scratching my head. The only thing that is a little suspect is the voltage of the exciter wire at the alternator plug with the key on. The battery is at 12.7v and that connector is only at 11.7v . Maybe the alternator really wants to see a solid 12.6v at that wire to start charging and the old wire with extra resistance just isn't quite delivering.

Thoughts?
 

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Correct me if I'm wrong, but isn't the blue wire in the plug the exciter wire? Doesn't it provide 12v from the indicator light, and once the alternator starts charging the light goes out because the exciter wire is no longer acting as a ground?
After familiarizing myself more with your specific car I see you’re definitely right about that, my bad.


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edit… just read that you’re getting voltage at the alt side of the exciter. I’ve replaced the entire wire with new 16ga and had no change. I thought the same about resistance.

Did the damaged alt charge properly? Just because this alt is new, doesn’t mean it’s good.

I’d do a continuity test from the exciter alt harness to the cluster harness. I’ve had very few alts that self excited.… most do require a throttle blip. I jump power at the cluster harness… 15 to exciter with an inline resistor. I like your solution more, but I’ll wait for you to test it!

-Todd
 

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Discussion Starter · #9 ·
Old alternator didn’t charge at all. Was spitting out lots metal dust it is was so hammered. I haven’t ruled out a faulty new alternator yet. I’ll try putting a full 12.6v on the blue exciter wire tomorrow and see what that does. If that does nothing I’ll try my jumper hack and let you all know if that solves the issue.
 

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gotta blip the throttle and have the blue wire hot, connected
this is typical MK1 MK2 behavior
should read over 13V, maybe 14+ at the battery, idling
good clean grounds?
Dont leave it to warm up without a blip to get it charging
rockauto rebuilds good selection
 

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I would take your alternator out and have it tested.

The Blue exciter wire in the plug has 12V on it from the Dash light when you turn the ignition on, this lights the light.
and applies power to the field coils to start charging. Once you blip the throttle, or if all the pulleys are clean and tight, the alternator starts to charge and the battery lamp is now being charged with a higher voltage on the back side because of the diode in the circuit. The reverse bias at a higher voltage cancels the lamp.

I would suspect the Voltage regulator on the alternator as you do have a voltage on the blue exciter, and placing 12V on the X or W is allowing the field coils to start producing.... IIRC the W is normally used to power a tach.

Once upon a time I went through 4 alternators from NAPA, before I got one that actually worked. (yep the last one I got I had tested prior to leaving the store.) lesson learned, test it prior to leaving. Two of the alternators were DOA with no voltage on after installation, the 3rd one was exceptional as it was putting 18V out... The last one was at 14V. I did have the last 2 tested the one with 18V was after I removed it, and the 14V was prior to installation.
 

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Use a test light to check the exciter wire. Volt meters are pretty much useless in load situations. It's possible the resister in the cluster is so high that there is not enough current to do anything. If there is enough current to light up a test light, then the circuit is probably good enough. 11.7v seems a bit low. I can't say what I've seen in the past, because I use a test light on loaded circuits.

I would be very cautious about hooking any wire up to the starter. Is it possible that the energized wire keep the starter on? Just something to think about.
 

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Discussion Starter · #14 ·
Alright! Got it charging!

I put a test light on the blue wire from the alternator and the light went out when I started it as is should and was charging 13.9V at idle, so I knew at that point the alternator was good. I checked the wire up from the connector by the battery to the fuse box and the light etc. Everything looked good. I checked the connector by the battery again and it did have a little corrosion, but not much at all. I soaked both ends of the connector in a vinegar and salt solution for a little while, neutralized it, and cleaned with some electronic parts cleaner and plugged it back it. I went to start it again and after a little blip of the throttle, viola! Dash light went out and all was good!

Thanks to everyone who chimed in. And I did decide the whole jumper from the starter idea would be a bad fix because it indeed does stay hot, which would cause the starter to stay engaged. Good catch Butcher.
 

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I learned a long time ago, don't use a volt meter on that kind of load. I had lots of arguments with 'Professional' mechanics on the merits of a test light. Never again will I get screwed. If all you got is a volt meter, make certain the circuit is plugged in.

Glad you got it worked out. I hate vinegar but salt and vinegar would be better suited for fish 'n' chips verses cleaning electrical connections.
 
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