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16 to 17 volts at alternator output on 3 wire 10Si

orangeblaze

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Well, I usually don't get stumped while working on my 81 k5 electrical.
But last month my truck alternator started putting over 18 volts out.
I figured it was the voltage scenting wire that I ran to the hot terminal on the the firewall to help with voltage drop, not that.... So I then figured it was the internal voltage regulator in the alternator.

After a quick swap of voltage regulators it seemed to work again... For about 5 days.

So I decided to do a rebuild kit with new bearings, brushes, diode tri, and another voltage regulator.

Finished up tonight. Still putting out 17 volts, but only 12.1 at the battery!!! What the heck.

Any suggestions?
 

diesel4me

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Might be the battery isn't accepting a charge or has a shorted plate ?..or something isn't letting the charge flow back to the battery..

Where the "sense" wire is hooked up makes a difference--I have seen alternators on things like hot rods and tractors guys swapped on,that had the red wire on the #1 terminal on the plug "jumped" right to the rear stud ,and that caused them to put out maximum volts/amps..

I would think it'd be near impossible for the alternator to exhibit the exact same symptoms twice ,after replacing what you did...unless maybe the regulator bolt that has the insulating bushings is letting it ground out-,then it'll put out 100% all the time too--does the same thing as grounding that "test tab" on the regulator,you can see thru the back of the case..

Perhaps the #2 wire that excites the field isn't getting the proper voltage--that wire needs to have some resistance from the "ALT" lamp,or a shunt on vehicles with a volt meter,to regulate the output of the alternator..putting full 12V to that wire will make it full field and put out maximum amps & volts too..
 

black dawg

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If the big wire from the alt does not have continuity to the battery, you will have high volts at the alt post, and battery volts at the battery. With key off engine off, check voltage at big post on alt....should be battery voltage.
 

Fordum

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You have actually answered your own question. If you have a different voltage in one place than the other, there is not a good connection between the two. Either the ground or the hot wire is at fault.
The voltage regulator senses the voltage somewhere, and adjusts the output so that it is the correct voltage. In theory, it should sense the voltage at the battery. But usually its at a point in the circuit that is connected directly to the battery.
In your case, obviously the battery is not hooked firmly to the output of the alternator. And the alternator is sensing the voltage at the battery. So it keeps cranking up the output to try to get the battery voltage up to the proper level.

The thing to remember, is that the ground is part of the circuit as much as the hot wire. If the hot wire is good, but the ground is not connected well, the regulator cannot sense the correct voltage, since it senses it referenced to ground.
 

orangeblaze

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Last night I ran the rebuilt alternator to the local auto parts store to do a bench test.

The good news: it passed

The bad news: it passed

Unfortunately, I only get a green check for a passing grade with no voltage or amperage read out.

I am starting to think it might be an oily connection on the starter terminal where the 4 ga from the battery + and 10 ga wire from the output terminal on the alternator meet.

Also, I'm checking the braided engine to body grounding strap on the rear of the engine block tonight.
 

diesel4me

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I have added a ground wire from the alternator case to the negative battery terminal on a few old GM's and that solved an intermittent lack of charge problem..

The "bench test" a parts store does usually only tells you if it works at all--not specific outputs and maximum load output--few stores have a very good tester like a rebuilding shop uses...you can do the same test at home with a meter while its on the truck with the engine running...
If you turned on all the accessories like high beams,heater blower,wiper motor,you can see if its keeping up with the demand..

In your case I think the alternator isn't getting the proper "signal" to the sensing wire ,so it assumes the battery is low,and it cranks up the output to maximum to compensate..
 

diesel4me

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It says I dont have permission to view those posts....so I will have to guess by the titles of them that was the trouble ?..
 

orangeblaze

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Weird, sent from my phone this was the first pic. There was no options to set who can view images.

IMG_0840.jpg
 

orangeblaze

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This is the temporary wiring to see if the splice was the problem. I plan on using a heavy duty inline fuse like would see on a car amplifier in place of the fusible link.

IMG_0841.jpg
 

orangeblaze

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h142CFHG4-F-1.jpg
This is what I plan on using. Easy to see if blown and easy to replace
 

Blue85

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When you have plastic crimp connectors underhood, it's only a matter of time until this type of problem shows up. Do yourself a favor and replace the rest of them with proper splices before they go bad as well. Plastic crimp connectors are for trail fixes.
 

dyeager535

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h142CFHG4-F-1.jpg
This is what I plan on using. Easy to see if blown and easy to replace


What are these made out of? I'd be worried that they would become brittle and fall apart with underhood temps, *if* they aren't rated for constant high heat.
 

diesel4me

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You could use one of the "mega-fuse" fuse holder things like a Ford Taurus has behind the engine, that has a 50 amp blade style fuse in it...those hold up to high temps ,and can be found cheap at salvage yards..

My fears using a fuse instead of fusible links is one might "pop" and kill the engine,ligts,etc,and on a dark road that could be quite hazardous..fusible links tend to be "slow blow" and give you some warnings before one fails completely most of the time..but they do suck to replace...
 

orangeblaze

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I stuck with all new fusible links. 3 of them. Two coming from the starter and one by the terminal block on the firewall
77_80_chevytruck_fusible_links.jpg

Ended up using this diagram. I replaced everything in red.

I agree, the housing on the car audio fuse holder would melt or get brittle from the "under hood temps". Especially with headers!
 
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