Author Topic: volt gauge or alternator problem  (Read 1553 times)

Offline Henry

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volt gauge or alternator problem
« on: February 02, 2019, 05:14:56 PM »
Hi:
My truck is a 1976 C-20 2WD with 350 and TH400, A/C. All is OEM and stock except I have a Delco Remy 12SI 78amp alternator instead of the 10SI 63amp  alternator the truck originally came with.

What has been bugging me for a long time is that the voltmeter gauge is running right on the edge of the red (high side) so I decided to see if I have a problem. Since one or two of my instrument lights have been flickering or out, I replaced the flex circuit, and the instrument lights are all ok now. At the same time, I decided to check out the voltage gauge. I hooked it up to a DC power supply and ran 12V across the gauge...the needle on the gauge registered perfectly vertical on the 12V hashmark. Then I increased the voltage until the needle just touched the beginning of the red, which my power supply indicated I was providing 16 volts. So then I back the voltage down to 14 volts and my gauge needle is right on the hashmark between the 12V hashmark and the 16V hashmark at the start of red. So I think my gauge is good and pretty well calibrated.

I took some measurements of the 12SI alternator, and at idle, the voltage between the alt B+ terminal and the alt bracket was 14.4 volts. There was no voltage difference between the B+ terminal and alt bracket and the F2(red) terminal and alt bracket. Grounding looked good with 0 volts when checking battery neg terminal and alternator bracket. So I decided to take the alternator out and take it over to Autozone to check on their machine. Their machine checked it out as good. Their printout said the output checked 14.92 volts with an acceptance criteria of 14.20 volts min and 16.00 volts max. Just for kicks, I asked them to set their machine up to test it as if it was a 10SI alternator and the results were exactly the same.

So can anyone explain what is going on here? Do I have a bad alternator? Why is my voltage gauge reading 16 volts right at redline all the time? It reads reads consistently at the edge of the redline regardless of idle or high rpm and regardless of cold or hot.

I put this 12SI alternator in many moons ago and I cannot remember why I replaced the original 10SI with this one...but I do remember there was a time my voltage gauge read just at the 14 volt hashmark and not redlined as it is now...I cannot remember if this was before or after installing the 12SI alternator. Do I need to put a 10SI 63 amp alternator back in my truck?

Any thoughts would be appreciated,
Henry

Offline VileZambonie

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Re: volt gauge or alternator problem
« Reply #1 on: February 03, 2019, 05:51:01 PM »
It doesn't matter if it's a 10si, 12si, CS or whatever. Just check the actual voltage at the gauge or cig lighter or fuse with it running. If it differs from the gauge, replace the gauge. 
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