Alternator/Generator, does Ampage matter?

Tiny
MYGENERICEMAIL
  • MEMBER
  • 1996 GMC SIERRA
  • 5.7L
  • V8
  • RWD
  • AUTOMATIC
  • 160,000 MILES
My alternator/generator on my truck is starting to make grinding noises, so I am looking at replacing it.

The label came off the alternator/generator, so I have no idea what ampage its supposed to be. Someone told me to look on the code list in the glovebox, which I did, and to find the numbers that start with "K". One that matches the code list is K60 which is 100 amps. So I guess this is the standard one for this truck engine?

My question is, will it hurt the truck or the battery if I get a higher amp alternator/generator? Will anything over 100 amps work without problems, or does it strictly have to be 100 amps?

I got a new battery last year and don't want to damage it or the truck.
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Sunday, February 28th, 2016 AT 12:01 PM

5 Replies

Tiny
CARADIODOC
  • EXPERT
Think of the generator as a variable-displacement water pump. It won't move any more water unless there is a place for it to go. An AC generator will not develop more current than the electrical system needs. They are self-regulating in that respect. That means a larger generator will have a higher capacity, but it will not produce higher voltage, more current, or charge the battery any faster.

Where the problem can arise is during a full-load output current test with a professional load tester. That is the only condition where the generator will produce its maximum rated current. That can blow the fuse in the output circuit. There are usually multiple optional size generators that could have been installed at the factory, and the size of the fuse is selected based on which generator is installed. You can usually switch to a larger fuse along with the larger generator because the rest of the wire that is being protected can handle that higher current. Again, during normal driving, you aren't going to get more of anything. The generator just has a higher capacity.

Be aware too that starting with '87 models, this has been a real poor generator design. It is not uncommon to go through four to six of them in the life of the vehicle. Due to their design, they develop huge voltage spikes that can damage the internal diodes, voltage regulator, and interfere with computer sensor signals. It's the job of the battery to dampen those spikes. To reduce the chance of a repeat failure, replace the battery at the same time unless it is less than about two years old. As they age, and the lead flakes off the plates, they lose their ability to dampen and absorb those spikes.
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Sunday, February 28th, 2016 AT 2:02 PM
Tiny
MYGENERICEMAIL
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So is the fuse for the alternator/generator in the dash fusebox, or is it located somewhere else?

Are the fuses special order or can they be had at the local parts store?
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Sunday, February 28th, 2016 AT 2:08 PM
Tiny
CARADIODOC
  • EXPERT
That fuse is going to be bolted in the under-hood fuse box. Plug-in terminals would never handle that much current. You can find them at any auto parts store, but I prefer to get them from salvage yards.
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Sunday, February 28th, 2016 AT 2:22 PM
Tiny
MYGENERICEMAIL
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One last question.

The alternators/generators I find listed for my truck at the parts stores show the lowest amp at 105.

Is this going to affect the fuse? Or do I need to get a new fuse for higher amps?
Or is there just one standard fuse for these things?

Thanks for all the info, I appreciate it!
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Sunday, February 28th, 2016 AT 2:26 PM
Tiny
CARADIODOC
  • EXPERT
If you buy the lowest current-rated generator, you know the fuse will handle that. You can install any higher generator without touching the fuse. The fuse will only blow when its value is exceeded, and that will only happen during a load test.
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Sunday, February 28th, 2016 AT 3:28 PM

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