Hi

I have charging problems and I wonder what kind of resistance diodes should have. I dismantled alternator and checked diodes when they were apart. They are working correctly as they should, but I do not know, what kind of resistance there should be. One electrician said that normally when diode is broken , it's leading both ways or not leading at all.

Also I wonder, if there is any way to meter before voltage regulator, if alternator is working,

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Sorry, my english is not very good

Have you determined if it's the alt for sure? You could tell through the ammeter if it's working or not. How you do this is turn everything in your car on while idling, the pointer will go towards the left. If when you rev the engine the pointer goes right then the alternator is working. If everything appears right in the alternator and it's charging then the alt might not be the problem, you might check the wiring from the alternator. You might aslo check how tight the alt. belt is, if it's too loose then you'll have bad charging.

If you're not an electrician then it may be better if you believe the alternator's a problem to put it back together go to Schucks(or your place of choice) and get one. When you have a core it's not too bad, something like 60 dollars. This way you know for sure the stuff's working and you don't accidentally hurt a good core alternator. I like to play with my car's wiring because it's simple, but when you get into things with diodes, resistors, and such I back off. Anyway, this only applies if you don't know electronics, if you do then going through the alt. might be good for you.

Also, check your voltage regulator. It may be causing resistance. I've upgraded mine to a solid-state regulator and it's helped some, and that was when my previous one was in good condition. If your's is in bad condition you may have a problem their.

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My car may be slow to sixty, but at least it's been reliably slow to sixty for more than thirty years.

I'm quite a sure that it is alternator. This is now other alternator that was in car before, about two years ago, when it first time stopped charging. Now the current alternator also stopped charging.

That's why I opened the first one, just to at check what is wrong. I'm not electrician, but I do not see anything difficult here. I only do not know, how I can be sure that diodes are gone. especially when they seem to be working like they are meant to work.

Of course the regulator may be broken also. But anyway battery is not charged at this time. And that's very easy to find out with voltmeter, and especially when there is only 8-9volts when engine is running.

60$ sounds cheap, but problem is that i'm other side of the world. Here these alternators are rare and much more expensive. But we have Nokia's here in finland and we can call help when car leaves us on the road

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Sorry, my english is not very good

When the alternator is charging, there is a magnetic field at the rear, center of the housing. You can do a quick check by holding the tip of a screwdriver near the housing. If it's charging, there will be a strong pull.

I've never checked diodes while they are in the alternator, but with the meter set on ohms, there should be a high resistance in one direction and a very low resistance when you reverse the leads.

It's fairly common for those diodes to burn out and they should be relatively cheap, if you can find them in Finland. If you live near a larger city, I would think you could find a rebuild shop that can rebuild yours. They might even have substitute diodes if the original parts can't be found.

Usually the first thing to fail is the brushes, but I suppose you already checked them for wear.

Joug,
The diodes should be open in one direction and greater than 500 Ohms in the other direction.
You can check the alternator current output by disconnecting the wire at the field terminal at the back of the alternator, and than use a jumper to connect the field terminal on the alternator to the alternator output terminal. This will cause the alternator to put out about 5 amps less than its rated current output with the engine running at about 2200 RPM.
Bob D

Didn't notice you were in Finland, guess parts would be hard to find

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My car may be slow to sixty, but at least it's been reliably slow to sixty for more than thirty years.