Single Field Alternator Rebuild Help!

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threewood

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I have my original alternator in a box and finally got around to rebuilding it. I have the service manual but I need some of the info dumbed down a bit. It refers to special testing tools so I don't know what they are using.

1. How do you correctly test the diodes (+ and -) out of the case with a digital multimeter and what should the readings be?

2. How do you test the capacitor with a digital multimeter? i tried several settings and get nothing with leads hooked to either end.

3. Is the stator testable out of the case? Mine looks to be de-laminating in a few places and would like to test before I buy any parts.

4. Would it be less a pain in the butt to just buy a remanufactured alt and swap out the case for my original? The parts all seem to add up to $40 and if I need a capacitor it will be another $30+ as I don't see these available anywhere but nos on ebay. Rebuilt are $30 to $50

Thanks
 

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The problem with rebuilts is you "don't know" what you are getting, literally.

So far as I know, to test the diodes, you must isolate them. It's been along time since I had a Mopar apart to check out (EDIT I see in the service manual that you indeed must separate them.)

The problem with trying to check the stator, is that they are such low resistance, you can have a partial short across windings, and a continuity check might not show that up. One thing you CAN check, if the stator is isolated from the diodes, is check the windings to the core. Should be infinate.

Also try to "wiggle" the windings. They should not move. If they do, they probably will or have, rubbed the wires together and rubbed the lacquer insulation off. You can SEE this by running the unit at night

You can NOT check these big power diodes with an ohmeter. You must run considerable current through them, and check them under load. If you have some sort of battery load tester you can put them under a short duration 20A or so load. Measure the voltage ACROSS the diode during this high loading. Should be very little drop, less than 1 volt.

I don't think much of the "book" method of a test lamp. Again, you need to run quite a bit of current through them to really test them. Otherwise you will get "false positive" results

The rotor you can check, neither slip ring should show continuity to the core. If you hook the rotor in series with an ammeter to a good charged battery, it should (depending on year an amperage) draw somewhere around 2-5A. Most are around 3, less is OK. More than 5-6A indicates a shorted turn(s) in the rotor.

Caps can be difficult to evaluate with a digi meter because the meter is so high impedance. Generally you used to set the ohmeter to either a high R scale, or the one "marked" for cap test. When you hook up the leads, the mter will "jump" to a low R reading, then slowly decay to a very high value. If you reverse the leads, the reading should be similar. Some caps are large enough to give weird readings due to a little bit of charge remaining. The main thing is that it's not shorted (low R reading) or infinity, in which case the reading won't "jump" nearly at all
 
Wow, that's a lot to take in. I replaced the alt because it tested bad (cannot remember what the oreillys guy said it failed). I set my multimeter to diode and checked all 6 both ways. All the same except one which did not show OF.L the other way so one was letting ac through. The bearings and cups are easy, I guess I could get new diodes and brushes, put it together andve it tested. I really want to save the cases as they are DPCD marked and are in great shape.
 
So you did disconnect the stater before testing the diodes?

And if I ? understand you? You got one diode that shows the same readings both ways? That would be bad, I would think.
 
So you did disconnect the stater before testing the diodes?

And if I ? understand you? You got one diode that shows the same readings both ways? That would be bad, I would think.

Correct on both accounts. The capacitor has me at least concerned as I read on line that a bad diode can be caused by a battery being hooked up wrong or a bad capacitor. And they are not cheap for what they are.
 
Did the alternator work fine before it quit?

In my experience, the old mopar alternators were pretty much indestructible and when they quit it's because the brushes wore out. A good cleaning, lube and simple kit like a Quick Start 604 and possibly a drive end bearing would get one back in action. This is what most of the parts house "rebuilt" units have done to them and I think a more accurate term would be "serviced".
 
For a basic diode test since you have them out of the case, put the ohmmeter in ohms mode on the lowest scale; a good one will read something one way (the conducting direction) and not read anything the other way. When I say 'not read anything', the meter reading will not change from what it reads with the leads not connected. What the meter will read in the conducting direction depends on the meter itself; some will read a low resistance value, and others will read a low voltage value.

If one reads some low value in both directions, it sound like it is shorted.

As asked, what were the symptoms? Low voltage?
 
Oh man, $30-50 for a rebuilt one or $40 in parts and do it yourself? rebuilt all the way. 2 reasons: if it fails, you get another one for free, test it on their machine right out of the box. 2. Everything that wears gets tested (with the proper procedures) or replaced when they do a rebuild, and the assembly line operation just makes the time so short to do a rebuild that it gets passed along to you. I would only attempt a $3 brush replacement as most everything else seems to be good 95% of the time. I always have them test rebuilt starters after I got 3 bad ones in a row for a 351C out of the box. Seems the Ford starter is underbuilt for what it needs to do.
 
For a basic diode test since you have them out of the case, put the ohmmeter in ohms mode on the lowest scale; a good one will read something one way (the conducting direction) and not read anything the other way. When I say 'not read anything', the meter reading will not change from what it reads with the leads not connected. What the meter will read in the conducting direction depends on the meter itself; some will read a low resistance value, and others will read a low voltage value.

If one reads some low value in both directions, it sound like it is shorted.

As asked, what were the symptoms? Low voltage?


I'm tellin you, a guy cannot test high amperage rectifiers with an ohmeter. You MUST load them to test them. Years ago, I thought this up myself. Afterwards, I confirmed it in a Motors Manual. They tell you this very thing concerning testing the AMC / Motorola alternators. That's the only place I've ever seen this in print.

Now if they are "clear open" or "clear shorted" then yes, they would test bad. The PROBLEM is that the diodes can be damaged and produce poor output, yet test OK.
 
For a basic diode test since you have them out of the case, put the ohmmeter in ohms mode on the lowest scale; a good one will read something one way (the conducting direction) and not read anything the other way. When I say 'not read anything', the meter reading will not change from what it reads with the leads not connected. What the meter will read in the conducting direction depends on the meter itself; some will read a low resistance value, and others will read a low voltage value.

If one reads some low value in both directions, it sound like it is shorted.

As asked, what were the symptoms? Low voltage?

It was a non charge issue. Battery replaced, VR replaced and soon after failed. Took it in and they said it was bad. I didn't ask for specifics. Should have bought one right then. But I bought a Remy rebuilt one which lasted 1.5 years until it blew it's guts out on Friday.

I have one with a lifetime warranty from Autozone, easy enough to swap if if this one goes tango uniform. But I would like to have my original up and running as a spare and I had never rebuilt one.

9h, someone asked about brushes being bad. They still look to have plenty of life.
 
^^I should have mentioned earlier. If you are not concerned with "resto" appearance, it is far better to buy a newer isolated field, "squareback" which has better low / med RPM output, in general. Even on a 69, with the older regulator, you can ground one field terminal and use the 69 regulator
 
I'm tellin you, a guy cannot test high amperage rectifiers with an ohmeter. You MUST load them to test them. Years ago, I thought this up myself. Afterwards, I confirmed it in a Motors Manual. They tell you this very thing concerning testing the AMC / Motorola alternators. That's the only place I've ever seen this in print.

Now if they are "clear open" or "clear shorted" then yes, they would test bad. The PROBLEM is that the diodes can be damaged and produce poor output, yet test OK.
I knew you would say this Del LOL... that is why I said 'basic test'. Like looking for dead shorts or opens. BTW, my son's '65 'Cuda FSM refers to a form of load test.
 
It was a non charge issue. Battery replaced, VR replaced and soon after failed. Took it in and they said it was bad. I didn't ask for specifics. Should have bought one right then. But I bought a Remy rebuilt one which lasted 1.5 years until it blew it's guts out on Friday.

I have one with a lifetime warranty from Autozone, easy enough to swap if if this one goes tango uniform. But I would like to have my original up and running as a spare and I had never rebuilt one.

9h, someone asked about brushes being bad. They still look to have plenty of life.
OK, well it could be any part: windings, brushes, or diodes. Understood on the old DCPD stamp on the housing; I have a '62 Dart that is 98% original and so understand the 'rebuild the original part' thing. I rebuilt the head light switch!
 
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