One-wire (hot rod) alternator blues

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On my motorhome. 1990 VIN Econoline 350 460 FI gas. No idiot light-- just an ammeter.

Came with a dud alternator or wiring. Discovered via research that its "2G" alternator was horrible trash, trying to output all its power through two 1/4" spade lugs. Elected to fix it with a used 130 amp "3G" alt out of a 1994 Mustang V6. Bolted right up. Added a 6-8 gauge starter cable left over from my motorcycle and 100 amp breaker to wire it to my battery (+) terminal, and used the truck's 3-pin (plus stator) connection. No worky. Tested the A/S/I connections, everything had voltage where it should have per the schematic.

Alternator tested bad at autozone, FWIW.

Blessed with an extra pigtail from the donor vehicle, I made my own "idiot light" between Battery (+) and the field ( I ) connections, but still couldn't get this to work. Aggravated, I got a self-exciting regulator, installed it, and it worked. Sometimes it took more than a "blip" of the throttle to get going but it always worked, until two weeks ago. Made a eight mile trip to get inspected at my mechanic on battery power alone, then two miles into my return home the alt came to life.

Not really one to know what's going on in that little transistorized box, I found a tapped hole on the back of the alternator case and added an extra ground wire that went straight to the battery (-) terminal. It worked perfectly... for two days. Always engaged with just a little throttle blip. Then it went back to its old self. The mounting brackets & fasteners aren't rusty, FWIW.

There's a screw for testing on the regulator, so I added a wire and ring terminal. Grounding the wire full-fields the alt. Doing this, even for a second, also "tickles" the not-self-starting self-starting regulator and gets it going for the duration of the engine run.

So here we are, options:

The factory wiring, for whatever reason, hates life. I could rig a new wire from ignition (+) through a 500 ohm (?) resistor to the "I" Terminal on the A-S-I block of a "normal" regulator. Pros: It's normal, and if I need an alternator or regulator in the future I can get one at any junky parts store. Con: I've devoted a lot of diagnostic time in this direction without positive outcome.

I could add a temporary push button to the ground-test wire to energize the field, but it looks ghetto and might impact resale value. Con: If one forgets to do this they could drive off to a flat battery.

I could get some sort of cheapo timer module that grounds the test wire 5 seconds after starting for 1/2 second. Con: Things are really getting into the Rube Goldberg/ MacGyver weeds. And if the battery's healthy it could lead to an overvoltage situation.

I could get a mopar (!) external regulator and wire it up to the ground-to-test wire like the alt is completely unregulated. Con: Two regulators would be fighting each other, might let the smoke out of something.

Gimme some feedback!
 
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The alternator had a bad conventional regulator when you got it. You may regret having taken a lot of time to figure that out, but that is sunk cost now. I would run conventional. This also gives better regulation of the battery voltage if you hook up the Sense wire, which would be especially useful if the RV has an extra battery you can regulate at the battery isolator instead of at the alternator.
 

eljefino

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Hmm, I should add that my replacement used (3G) alternator and its original factory-correct regulator didn't play well with the truck's wiring either. That would be an amazing coincidence, but like Sherlock Holmes says, eliminate the impossible and you're left with the improbable.
 
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I would go back to the stock regulator setup. Really with this setup all you need is the ignition hot wire to tell the alternator to turn on. I could be wrong but I seem to recall the only purpose of the resistor was in case the bulb burned out your alternator would still charge. I have mine hooked straight to ignition power without a light or resistor and it worked all these years.
 

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The wiring of a 2G and 3G alternator is identical as far as the regulator is concerned. The only difference is the output terminal, a stud instead of a spade terminal.

So, if your 2G alternator was working with with the factory wiring...so should the 3G alternator. Unless something in the wiring got damaged in the process of installing the 3G alternator.
 

eljefino

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So, if your 2G alternator was working with with the factory wiring...so should the 3G alternator. Unless something in the wiring got damaged in the process of installing the 3G alternator.
I bought this in Bangor, ME in August 2021 with a 2006 Missouri inspection sticker on it. The thing was used as a kids' playhouse in the interim. The odds are ok that both the original alternator and wiring have issues just from age/ corrosion.

I shouldn't have chucked the "correct" 3G regulator that came with my 3G alt, as I'd like to do more diagnostics on it. But I don't feel like I could get it to work either with my #194 bulb "dummy light" or with straight +12v wiring. Used a schematic like CDX825s.
 
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I bought this in Bangor, ME in August 2021 with a 2006 Missouri inspection sticker on it. The thing was used as a kids' playhouse in the interim. The odds are ok that both the original alternator and wiring have issues just from age/ corrosion.

When you checked that you were getting 12V at the alternator, did you use a test light or a multimeter? A multimeter has the amazing capability of reading correct voltage through even the worst high resistance connections. I can get one to read 120V just by putting one probe in the hot side of an outlet, and touching the other probe and a grounded object. Meter reads 120V and I don't feel a thing.
 

eljefino

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When you checked that you were getting 12V at the alternator, did you use a test light or a multimeter? A multimeter has the amazing capability of reading correct voltage through even the worst high resistance connections. I can get one to read 120V just by putting one probe in the hot side of an outlet, and touching the other probe and a grounded object. Meter reads 120V and I don't feel a thing.
This is what I am thinking-- that there is some schmutz in my original +12v wiring.
 
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This is what I am thinking-- that there is some schmutz in my original +12v wiring.

Start tugging on wires and look for the non-waterproof butt splices with green powder coming out the ends. There are still "mechanics" who think it's acceptable to use those on underhood and undercar wiring.
 

eljefino

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Epilogue, ran a new I-wire straight from a switched igntion source, to a conventional voltage regulator, and it works flawlessly. Funny how I got involved in the "sunk cost" fallacy of a self-exciting unit.

The A-wire (voltage sense) supports enough current to run a 194 bulb, and always had, but I checked it anyway. No comment on how I didn't figure this out a year ago.
 
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read 1st 4 posts only.
As can B seen in the (RJM) schematics to alt (3G) is 'internally regulated". I got rid of my fender mounted*. The 3G puts out so much power I'd think you burned out some wires ('82/'93 era vehicle). A 150/175 amp mega sits ona 2 AWG line to the battery. The 3G is supperior to the 1wire (GM's similar era alt) you named and the red lined/striped green line is just 1 reason. I hada put in a diode (1 way switch of power, can only flow 1way). If the alt fails the dash idiot light goes on to let U no. Lets say the belt flies off. U can no B4 overheat (serpintine or H2) pump shared belt) /OR/ alt fails & U run dwn bat in the boonies - trouble. * isa '66 bronco.
 
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