Interesting alternator information/revelations

[boney]

1983 CJ7 -v-belt alternator issues.

I received this Jeep a handful of years ago with a bad alternator. Replaced it with whatever Napa Auto Parts said should be on the car. It was plug and play- that is, it fits the wiring in the car perfectly and I didn't look back. Until I realized that it doesn't charge at idle- and as I turn on lights and the HVAC fan the voltage drops. Mostly I've just dealt with it over the years because 1-it hasn't caused an issue and 2- it doesn't get driven every day.

So I dove into it today because I'm tired of thinking about it while driving.

The voltage drop with electrical load is because of the old and likely tired wiring and connections in the loom. With the RPM's up, the battery is getting 14.1V with everything on. At the dash it drops to about 13... To be addressed at a later date...

The alternator appears to be in good shape. There does not appear to be any issues with the diodes (as best as I can test with the volt-meter) and it is grounded well and the connections on the positive side of the system have no resistance.

It appears from research on the 'nets, that an alternator needs about 2400 RPM to produce power and that peak production happens at 6000 RPM.
With a crank pulley of 6.75 inches and alternator pulley of 2.5 inches, the ratio between the two is 2.7:1.

If the crank is turning at 750 RPM (roughly idle) the alternator is turning at about 2025 RPM.
I need 900 at the crank to turn the alternator at 2400 and 2200 RPM on the crank to reach 6000 at the Alt. But I don't have big electrical loads, so it's unlikely I'll ever need the full capacity of the alternator.

If I were to replace the alternator pulley with a 2.25 incher, then the alternator would be at 2250 RPM at idle. (3:1 ratio). That would put me closer to where I'd like to be... I haven't found any 2" pulleys available otherwise I'd go with that and have 2530-ish alternator RPM at idle. (3.375:1)

I'm curious if anyone else has found themselves down this rabbit hole and what solutions they've found tried.

 

[CJ]

Good post! Your found out one of the crazy things about what it takes to get the excitor circuit triggered on alternators. Unless you idle your Jeep 24/7 it's almost never an issue. One wire Delco types take even more RPMs.
Personally, I have never worried about it but I understand your concern. Most alternator shops have hundreds of old pullys laying around if you want to shop a little.

 

[hole]

When mine died I went with a GM CS144, which was probably overkill. CJ make a good suggestion, as long as you have a alternator shop nearby. Might try wrecking yards too for the pulley.