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Testing an alternator?

Plymouthfan

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Yet AGAIN it appears I have gone through another alternator.

I hooked up my battery and tried to start the Runner last week since it hasn't been run in a month. My battery was low, so I gave it a quick charge. Hook it back up, and it fires right up. I take a blast around the neighborhood and stopped for coffee. I get back in the car and nothing! I get a jump, it fires up and on the way home I notice it's discharging.

I'm assuming it's the alternator even though it's less than two years old. How can I test the alternator to be certain?

My Scamp eats starters.... this one eats alternators. :brickwall:
 

Big John

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IIRC, you have done the dual field alternator/electronic regulator conversion.

What you need to do is disconnect the field connection at the alternator from the voltage regulator and jumper it to ground. With the engine running, if the ammeter on your dash still shows "discharge" then its most likely the alternator. If it shows a high "charge" then its probably the regulator. If its just barely charging, it may be a diode in the alternator.

That's assuming all the rest of the wiring and especially the battery and connections are good.

I'd check the battery too. I use a cheap Harbor Freight load tester myself.

BTW, if you have a single field alternator, the jumper goes from the alternator field connection to the alternator output terminal.

Its covered here: http://69roadrunner.net/phpBB3/viewtopic.php?f=19&t=364

If you are sure its the alternator, chances are its the diodes. They are really not that hard to replace. Change the brushes out while its apart too. Unless the stator is bad (doubtful), or the bearings (you'd know LOL) there isn't much else that can go bad. Alternators are pretty easy to rebuild yourself.

Repair kits here: http://www.alternatorparts.com/chrysler_alt_repair_upgrade_kits.htm I've never bought anything from them, so I cant tell you much about them.. but it gives you any idea about price etc.
 

ACME A12

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Good post, Big.

This is the kind of stuff that I do so infrequently that I have to essentially reinvent the wheel (figuratively speaking) every time that I want or need to go through the diagnostic process.

I'm so glad that the resources are available herein for those rare ocassions and that some people (like Big) have a greater capacity to recall the info when needed... Just another reason to hang out here...

:thumbsup:
Ray
 

Plymouthfan

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OK, had a chance to work on this today and it's looking like the alternator may be bad.

I disconnected the field wire and grounded that terminal on the alternator. With the car running the gauge read discharge. I also spoke with someone else and he said I should bring the RPM's up while doing this. The gauge should increase and decrease (much like a tach) with a good alternator. Mine was steady at discharge.

The strange thing is when I first start the car the gauge reads a positive charge, as it warms up it steadies out at center. I took the car out for a spin and it was NOT discharging while driving. I let it sit a while and it fired right back up. The voltage at the battery with the car idling was 13.5 (I believe) and was 12.5 with the car off.

So where do I stand? Showing a charge at start up and discharge during the test has confused me...
 

Big John

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Hmmm.. Intermittent electrical problems are teh suck... The alternator test that shows the alternator as being bad, then it still charges has me baffled.

The battery voltage sounds a little low. If the alternator is charging, but not charging enough, its usually one or two of the diodes are shot in the alternator. That said, I would start looking at every connection in the charging system first especially the battery terminals.
 

Basketcase

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tell me what kinda coffee you loke, and we can use mine.........
 

Basketcase

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you're not going to let a little thing like distance stop you? :haha:
 

Plymouthfan

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LOL, maybe :crazy:

Just bought a new Battery Tender on eBay. I need to keep that puppy charged! It's hard to monitor it when I live a half hour from the car and the temp's are dropping again...
 

Plymouthfan

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Hmmm, I put the donor alternator in yesterday and had the exact same results when grounding out the field... it showed a discharge... but was charging when I started the car. :huh:

I brought the alternator home and I'm gonna have it bench tested locally. Could this be a grounding problem, maybe with the voltage regulator?
 

Big John

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Hmm... Maybe the ammeter is hooked up backwards? That would show a discharge when its actually charging.
 

Big John

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Let's try this...

Check the battery voltage with the car off. It should be 12 volts. Now start the car. Check the voltage, at idle and with the RPM up (~ 1500 RPM) it should be around 14 volts. Turn on your headlights, heater blower, stereo etc and check the voltage again at idle and with RPM up. It should still be about 14 volts.
 
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