Electrical - Fresh Insight Needed

[Copper]

I decided to measure the crank shaft "pulley" as well. It is also smaller in diameter. So that adds to the problem.

6.5 vs 7.0

It's been a while since I worked for NASA. I'll try and do the math tonight and see what that means in rpms and compare it to the test card that came with the alternator.

It's become a matter of curiosity at this point.

[USN78W150]

pulleys

Copper,

I measured the pulleys on my truck, an unmodified, unmolested '77 W200 M885 USAF truck. It has 53,000 original miles, and was just auctioned off by the Air Force last year. The alternator pulley is 2.69 inches in diameter. The crankshaft pulley is a triple position pulley. the outer two positions are 7 1/4" diameter, with the innermost position measuring 7" in diameter. Of course the alternator runs off of the innermost position. I hope this helps you. If you don't have the correct crank pulley, there is a wrecking yard near me that has a tremendous selection of correct pieces for your engine.

[Copper]

USN78W150,

Thanks for the measurements, and the offer of help. I don't remember any of the local wrecks having an engine, but I'll check. Maybe I can turn up something local.

If anyone is interested in the pulley phenomenon ...

It may not be a perfect comparison, but since my 84' is working with the same type of alternator from the same parts store, but with a double pulley and it actually has more features, I'm going to use it as a model of "normal" operations for the electrical system.

The 84' has a 7" crank pulley and a 2 3/4" alternator pulley. At idle speed of 750 rpms, that turns the alternator at 1910 rpms.

The 77' has a 6 1/2" crank pulley and a 3" alternator pulley. At idle speed of 750 rpms, that turns the alternator 1628 rpms.

That doesn't seem like much difference until you look at the "Computer Tested Alternator" card that came with it. NAPA double checked it for me, and said it was running almost identical on their equipment.

1628 rpms produces about 17 Amps.
1910 rpms produces about 43 Amps.

That's a big difference, more than twice the amperage.


A 2 1/2" pulley on my 77' alternator would bring me up to 1949 rpms.

Reversing the math, I should get comparable results by increasing my idle to about 875 rpm. If that works, I'll look for a smaller pulley. It'll probably have to wait untill this weekend.

[KRB64]

Very interesting. Glad you solved the mystery scooby.

Just goes to show that you do need to pay atention to the small stuff huh.

[Copper]

Well, I finally checked my theory. If I increase the idle to 800 rpm, the problem is almost unnoticable. If I go up to 850 there is no trace of a problem. I'd have to say that confirms a 2.5 inch pulley would do the trick.

I went to the parts store today to see what / if NAPA could do to help me out. There was an "experienced" fellow there that said he had seen it before. He said there used to be several sizes and he thought they switched to a 2.69 inch (metric conversion) double pulley as a median size to reduce shelf stock. He said it worked fine for most of them, but there were a couple years where the crankshaft pulley was smaller and it caused a low voltage problem at idle. The 3" single pulley is even worse.

I tried swapping alternators with my 84. It has the 2 3/4 (maybe 2.69). It did help, but there is still a noticable drop when I have everything on.

I guess I'll have to take off the pulley and measure the shaft diameter and see what I can find.

Thanks again for all the input.

[Copper]

As it turns out, I had some difficulty finding a 2.5 pulley. I ended up talking to a different "parts technician" at NAPA, who after listening carefully to my saga decided I must have the wrong alternator.

He dug up an old catalog and low and behold there was an optional alternator that was not listed in the computer. It has a 2.5 pulley and according to the test card should put out about 45 amps when my truck is idling at 750 rpm as opposed to the 15 or so I was getting from the other alternator.

Go figure ...

[n1265]

And what is the NAPA part number for this alt ?

[Copper]

And what is the NAPA part number for this alt ?

He gave me back my dirty box, so I just don't know.

PME 20152 is the one you don't want. It has the 3" pulley.

I ran into an old mechanic at the salvage yard while looking for another part. I think he said he was 84; I hope I'm doing that good then.

He told me the old small pulley setup was 45 amps. When I told him it had 10 gauge wire from the alternator, he said "yup, more kids have burned them @#$% things up putting 65 amp alternators on them". He said the 45 amp puts out more at low rpms, but maxes out sooner.

I don't know, it makes sense. They did ask me if I wanted 45 or 65 amps. Of course I went with more power. It's got me wondering, but I'm not going to get another alternator to find out.

I seem to be doing alright now.

The one I ended up with is a double groove pulley. The pulley is painted black (I'm guessing recycled). It measures about 2.55, but the belt rides recessed about .05. I had to tap the spacer to the outside and add a washer to get the belt to line up.

There are a lot of single pulley alternators on the NAPA web site that are considered compatable. Unfortunately, they don't list pulley diameter. I bet one of them is the "real" correct alternator. You'll just have to get the sales person to pull them all and look.