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Alternator wont charge battery

ChevyTruckGuy

By ChevyTruckGuy
in Ask A GM Technician

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ChevyTruckGuy

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my battery indicator light went on on my 1990 Chevy Silverado pickup 5.7 350 v-8....so i went to and got a brand new battery and installed it..i start the truck up and the battery indicator light still stayed on...i checked te voltage between the + and - terminals with te truck on and i got 11.44 volts...should be 14.4 when the truck is on right? So i put one lead of my voltmeter to the bat terminal on te alternator and the other to ground and got 0 volts...so i thought the alternator was screwed..I went and got a new alternator and installed it...fired the truck up again and got the same thing..but the voltage from the alternator bat terminal to ground was 11.34 volts and falling..so i exchanged the alternator, got it tested first to see if it wrked..it passed...so i got the 2nd alternator installed. I also installed some 0 gauge wire to the bat terminal and connected it to the positive terminal of the new battery with an inline fuse..tested for continuity..the wires good...so i start te truck again and let it idle for about 15 minutes..checked the voltage again and now its at 10 volts and falling..truck died...i installed another battery to just get me home but its not recharging like it should...HELP!!!!!.....what should i look for..te alternator works fine according to the test equpitment, and the battery is brand new....i tested all connections, terminals, etc and they are good...am i missing something? there is a 4 prong terminal that hooks to the alterator that only has 1 wire running to it...i tested for continuity on the 2nd prong and it works..the other 3 terminals dont give me any signal....hope im explaing everything ok...i just dont know why the alterator ist carging te battery..even with some wire directely connected to the battery and the bat terminal on the alternator...

 

 

 

 

 

 

Mike

GM-Tech Senior Enthusiast

GM-Tech

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Do you know if you have the old SI generator or the newer CS type? On the older, SI generators, the front and rear of the case is separated by maybe 3/16" - 1/4" by the stator frame. On the new, CS type, the separation is much wider... maybe 3/4" - 1" and it has a ribbed appearance.

ChevyTruckGuy Enthusiast

ChevyTruckGuy

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it looks like this....

 

 

http://www.partsamerica.com/ProductDetail....type=11&ptset=A

 

 

the plug that it has is a 4 prong plug..

 

 

Mike...the thing is..when im driving down the road sometimes i see the volt gauge go up to 14.4 volts..then goes back down back to 11 or 12 volts...is there a loose connection somewhere im missing? does the ecm send voltage in the plug turnng on the alternator? i really dont know what the plug actually does...

 

Mike

GM-Tech Senior Enthusiast

GM-Tech

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I don't know if there's a loose connection somewhere you're missing. That would be up to you to find out! :driving:

 

Though you have a 4 cavity connector, there should be only one, sometimes two wires in that plug. No the ECM does not regulate the alternator on 90's era vehicles. Newer ones, yes.

 

The large red wire that's on the stud on the back of the alternator goes to the battery, though indirectly. On newer vehicles, it goes directly to the battery. On yours, well I don't recall exactly where... it may go to the starter or a junction block on the firewall. Follow it and make sure it's good and secure everywhere. It probably has a fusible link built into the end of it where it connects. That could be a problem if it's not in good shape.

 

The wire(s) on the 4 prong plug come from the indicator light (brown wire I think) or if no light, then a resistance wire, and ignition power (red?).

ChevyTruckGuy Enthusiast

ChevyTruckGuy

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ok so there should be a fusible link on the wire from the bat terminal on the alternator back to te starter or junction block....i believe its 12 or 14 gauge wire..ill trace it and see what i come up with..ill take a ohm meter and see if iget continuity through the whole wire...

 

yesterday i added a 0 gauge wire with a fuse from the bat terminal on the alternator going str8 to the positive terminal on the battery...shouldnt it still charge the battery since electricity takes the path of least resistance anyway and ive got it going str8 to the positve terminal on the battery?

 

will a bad starter cause this also? i wouldnt belive it would since te truck still fires up...

i cant seem to find fusible links anywhere at any parts store around here...can i replace them with inline fuses? making sure they math the ratings of the fusible link?

 

 

thnx for all your help..you've been a life saver...now only if i can get my truck to recharge itself..lol...j/k

 

 

Mike

GM-Tech Senior Enthusiast

GM-Tech

Posted
Posted
yesterday i added a 0 gauge wire with a fuse from the bat terminal on the alternator going str8 to the positive terminal on the battery

 

That would work. No, a bad starter wouldn't cause a charging problem.

ChevyTruckGuy Enthusiast

ChevyTruckGuy

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ok..i think ive found the problem...i have a question too...

 

 

is there supposed to be 2 fusible links on the alternator to junction box wire? it looks like there was one put there not from factory...it was inline with a really bad soldering job, only holding on by 2 strands of wire.....when i tested for continuity it gave me a complete circuit but i didnt remember that te ohm meter doesnt give as big a load on the wire as the alternator does....

 

 

thnx for all your help!!!

 

 

 

Mike

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