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Discussion Starter #1
Well for probably a year my '91 B250 Van with 5.2L and auto-trans has on occasion decided to stop charging. It occasionally just seemed to drop a single phase; it was barely discarching at all. Finally, it went completely out while driving and gave the check engine light. Funny thing is, it just randomly starts working again. I suspected the gauge until I got the check engine light several times. The last few weeks it became consistant, to the point where the battery drained enough to slow the starter down and almost fail to start. The battery took a charge no problem after that, but still no charging and the check engine light.
Since the problem was intermittent I saw no reason to try and bench test the alternator, so I ponied up the $$$ and got a new alternator (since the regulator is inside). Well, it's still not charging, dang the luck. I have 12.6 volts across the battery posts with the key off and 12.25 volts with the engine idling and all accesssories off (except for that piercing check engine light, dangit).
So, assuming the replacement alternator is actually good, where else can you actually look? The fuses are good, the harness at the alternator was corrosion free, and the regulator is inside the alternator. I can only imagine the key switch or ECU having any say as to whether the alternator is on or off, but I'd love some input from knowledgable folks.
Thanks,
b-z-
 

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A '91's alternator is regulated by the ECM.

They still used fusible links on this year too. A fusible link is a length of wire with a different type of insulation.
http://www.whiteproducts.com/fusible-faqs.shtml

Follow the cable from the alternator (+) stud to the battery, It will probably be tucked behind the brake booster.

When fusible links blow, they are supposed to stretch out and be obviously blown.

21 year old fusible links can develop very high resistance and be completely intermittent, yet the insulation still appears fine

I'd wager this is your issue.

Put your battery on a charger overnight. Never rely on the alternator to fully charge a dead battery unless you are driving on the interstates for 8+ hours.
 

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You can verify the above fusible link fault by meauring voltage across the alternator output stud /battery and it should certainly be a lot higher than battery voltage.
 

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Also check the condition of the field wires to the alternator, as they can be intermittent if strands are broken inside.
 

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Discussion Starter #5
This sucker???


What a pain in the butt to find. I thought it would be the quick way as my alternator is on the opposite side from my battery (plus the ominous radiator fan). I'm going back out to fish around for some long enough wire to test from alernator to battery. There are several different lines coming off the positive battery cable though. One of them has a quick disconnect and maybe that's the one I need. Would be an easy check that way compared to reaching to the alternator and battery at the same time. Very hard to chase the wires though as they run over the top of the engine and go forward and back, and the looms are covered in decades of road grime and that wierd fabric type tape that's decomposing and brittle.
Well, back at it, The fusible link looks way to clean/sealed/unmolested to swap it without testing.
Thanks for the input folks.
b-z-
 

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Discussion Starter #6
Well I just ripped through the insulation at both ends of the fusible link (it makes a lopp in the harness and I'm not 100% which end is which). With the engine at idle I'm seeing 12.18V from either end of the link to the negative battery post. I was expecting nothing or 13+ volts. I get 12.3v going across the battery posts with the engine idling. What gives? Am I testing this thing wrong? I thought going from the output wire of the alternator (before the fusible link) to the ground stud on the battery would show me alternator output, but 12.18V from a brand new alternator? Makes no sense to me.
b-z-
 

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This should be a two wire field alternator. There should be about a 3-5 volt difference between the voltage of the two wires if things are working correctly. If they read about the same (about 12v), then the regulator is not working. If they are very low, there is a problem leading up to the alternator.
 

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Discussion Starter #8
Update:

Replacing the fusable link seems to have worked. Honestly, it stopped charging once and gave me the CEL about the 2nd-3rd time I drove it after replacing it. The next time, and ever since, I have not seen a charging problem at all. I reckon the problem is fixed. It's been awhile now.
b-z-
 

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AGREED! Why in the world auto makers decide to make connections and terminations alongside the battery is beyond me. Placing an ECU near a battery is sheer madness. Checking the VOLTAGE at the main alternator battery stud is a shortcut to find if the alternator has a lot of lead in its pencil and no one to write to.

But in this type of situation BE CAREFUL checking voltage, Many years ago I got knocked on my rear when I touched an older squareback alternator stud with the engine revved a little. A meter revealed a hundred five volts. The regulator was reading low voltage and full fielding the alternator and the output voltage had nowhere to go. It would have instantly blown out a test light. With the engine OFF, there MUST BE battery voltage at this alternator output stud. Double checking by using a meter with the engine running would have revealed the OP's source of problem quickly.
 

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Discussion Starter #11
Yes I should have done further testing on the alternator. Chalk it up to laziness and the difficulty of reaching the alternator from under the hood. Should have just taken the doghouse off. Cost me $110 or something like that for the alternator. Maybe that was before the core charge refund, I don't remember. Either way, that van was up for sale (got a '95 B2500 to replace it) and it started making rear end noises out of nowhere yesterday. Looks like I'll be starting another thread soon; I've never had to deal with a differential in any way. Will sell it as a mechanic's special though if I have to pull the axle. Maybe I'll luck out and it's a wheel bearing. Can't mess with it for another couple days.
b-z-
 
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