Alternator Pulley Size and Effect on Output.108A vs 120A, Different Rotor?

Factual Questions
1

All the ALT’s I have ordered have the wrong size pulley and they all say 108A on the front half of the case where the original one says 120A.

The parts guy says he can put the bigger pulley on for me. (Big deal I can do that too, I expect the part to be right as it comes). But won’t this slow the ALT down thus causing a lower output?

Also, the 108A part makes me believe they are not using a Rotor from a true 120A ALT; maybe the faster (smaller pulley) makes up for the difference by spinning the Rotor faster.

What’s the Straight Dope here?

Thanks

2

I’ve noticed once or twice that a cheaper rebuilt alternator from the retail box will have a smaller pulley than the higher priced part from a better parts store. This makes me think that the smaller pulley lets the rebuilders pass marginal alternators without having an unacceptably high return rate.

3

Oh, you wanted me to reply to the OP!

The 108 amp alternator may very well be a rebuilt (or used but repainted) 120 amp unit which the remanufacturer labeled to reflect a lower QA standard.

Admittedly, I’m not in the automotive industry and this is just my personal hunch.

4

I’m certainly not an expert on alternators, but my advice is to go ahead and use the 108 A alternator with the correct size pulley. The lower RPM is likely a non-issue; I could adjust my car’s engine idle down by 20% and I’m confident the alternator would still charge the battery with no problem. And lots of different cars (with lots of different idle RPMs) probably use the same (or similar) alternator, yet they never seem to have a problem charging the battery.

5

I agree, it does not seem as though the variation in RPMs should be of primary concern. But if that is a way of passing a substandard part - that it not cool.

I have read that the CS144 ALT, uses a different rotor. That is a critical part.

6

I would be very suspicious if the rebuild came marked 108A when the factory one was 120A. I also think they used cheaper parts… Besides the rotor which may or may not be different, the parts that mostly limit output are the rectifier bridge and regulator. Both parts that can be bought in various levels of quality.

Have you checked at a different parts house that may carry a different rebuilder? My last alternator was actually marked 85A where the factory one was marked 70A. It has worked great.