Pulley RPM MC8

rcbeam · July 02, 2011, 05:25:52 PM

I made a previous post about loosing prime, and in that mentioned that my newly installed alternator does not work. Probably should not do two subjects in a single post. The P/O removed the 50DN and replaced it with a 75amp belt drive, being driven off the street side rear (fly wheel end) pulley. The old alternator quit working, and I have been fooling with it and bad wiring for months. I replaced it with a 150amp Prestolite alternator. Today I finally finished it, got it all hooked back up and started the bus. No voltage output from the new alternator. So I've been thinking what in the world could be wrong. I double checked my wiring. The only odd thing about this new one is that is has an IGN terminal and needs to have 24v applied to it when the switch is on to excite the alternator. I had 24v at that terminal. Then I started thinking about the pulleys and RPM's. I did not get to run the engine long enough to do any more checks, but I wonder if I speeded up the engine if it would have then put out voltage, which would indicate a RPM problem. The engine ended up stalling and I suspect that I've lost prime. So can't work on the alternator issue until I get that fixed.

In the mean time... Does anyone know the RPM that the small (cam shaft I think) pulley turns on the street side rear (flywheel end) of the engine at idle?

wildbob24 · July 02, 2011, 05:33:09 PM

Russell,

Camshaft RPMs are the same as engine RPMs, so the pulley should be turning whatever your engine idle is set at, hopefully somewhere around 600-650 rpms.

Bob

JohnEd · July 02, 2011, 06:02:09 PM

It should have produced some voltage even if it were low. Does it have an internal or external regulator. Hope it is "external" cause you want the alt to be sampling the voltage AT THE BAT PLUS TERMINAL and adjusting the alt output voltage accordingly. Even the internal regulated models can have their sense line run out of the alternator for that purpose. I don't know if they make CW and CCW rotating units but intuition tells me they do....been wrong before.

John

zubzub · July 02, 2011, 08:13:03 PM

As I looked into this recently I will throw in my 2 cents.
Alts don't care which way they turn but the fan on the alt does. they can have either CW CCW or "neutral/universal" fins on the fan.
Pulleys and fans can be interchanged on most models to suit your needs.
I believe if you have fully charged batteries and you are not providing a load, you may get the alt to act like it is not charging. turn some lights on etc to wake it up. Also rev it up a bit, sometimes alts have something inside that keeps them asleep 'til they hit a certain rpm, then they charge all through the rpms.
Somewhere there must be the charts for different alts and what the cruising rpm on them should be. You then do a little math and find the right (close) pulley.
Hopefully you only need to change one pulley, but of course 2 pulleys (drive and alt) may be required. A good alt/starter rebuilder shop has all this info and the bits...
Have fun.

luvrbus · July 02, 2011, 08:50:53 PM

Camshaft drives only turn one direction on a 2 stroke right or left turning engines only time to worry about direction is a pulley on the crankshaft on a lefty engine that is the only part turning the opposite direction of a right handed engine

good luck

robertglines1 · July 03, 2011, 11:05:52 AM

I have one running off mine on my 8v92 and it does just fine at a idle. for house system. bob

rcbeam · July 03, 2011, 01:16:20 PM

I did some research online on pulley ratios. Measured the driving pulley and it is 5 inches, with the driven pulley 3 inches. If the bus idles at 600 (I have no way to determine what it idles at) then this is right at 999 rpm for the alternator. I went to the prestolite site and looked up the spec on the alternator and the cut-in rpm is 1400, so I guess under 1400 it just rides along. I played with some numbers and figured that if I can change this rear cam pulley to a 7 inch that would put me right at 1400 rpm @ idle of 600. Also plugged in the numbers for 2200 rpm, figuring that would be the outside max the engine might ever do and that is still under the max continuous rpm for the alternator so I'm ok there.

Now to see if I can take the pulley off the engine and if there is room for a 7 incher.

The more I do the more I learn.

Do I understand what I'm doing or is my line of thinking all wet?

JohnEd · July 03, 2011, 03:17:40 PM

Russel,

You make perfect sense to me. That may be a problem LOL I would try for 7 1/2 or 8 cause you will not be doing a sustained 2200. Hills and at the top of a shift. Your alt is over engineered, no doubt. Would an 8 incher over rev the alt at 2200? What you are setting up with the 7 inch would be marginal at 6oo rpm engine. If you have dead bats with that spec you would not be able to charge the bats at idle at all or the out-put would be weak. I would want to be able to sit and charge the bats or at least not drawn them down with the inverter running an AC or the HW heater.

I hope they make those pulleys in any size you can imagine.

Good luck,

John

rcbeam · July 03, 2011, 05:08:44 PM

John: Thx for the reply. I only figured up to a 7 but might be able to use an 8 and not overspeed the alternator, however I am not sure how much room I have where the cam pulley is located... will have to check for bolts, etc that would determine max pulley size. Like you i hate the idea of the alternator not being able to put out current at an idle and right now, my high idle is all screwed up.

I googled "pulley ratio" and found several sites... took the first one and it had a neat ratio calculator... just plug in numbers and it tells you want you need to know. http://www.csgnetwork.com/pulleybeltcalc.html

Russell

luvrbus · July 03, 2011, 05:10:44 PM

Russell, I don't think you are going to find a 7 inch pulley for that drive DD didn't make a large assortment of the auxiliary drive pulley sizes you may can find a larger pulley that will bolt on to the existing pulley where the 3 bolt holes are for pulling the pulley.
I would be looking for a smaller pulley for the alternator.
A 8v71 uses 5 inch front and rear on the cam drive the 8v92 uses a 6 inch I have a 6 inch blower drive if it that will help you that pulley is not a common pulley just found at any supply house it is a DD item.
Me personally I would be a little afraid of changing to a pulley on the back larger than the front one on the cam they are the same size for balance the best I can figure

good luck

lostagain · July 03, 2011, 05:51:08 PM

You can get yourself a Chinese hand held tachometer at Harborfreight for less than a hundred bucks. Good enough to see what your iddle is. Or to set your iddle or your max no load rpm speed.

JC

JohnEd · July 03, 2011, 08:18:45 PM

I hate to think you are stuck....not with the minds we have here. Clifford seems to have the solution with using a smaller pulley on the driven end. Not quite the same and the belts wear faster. Might explore "ganging" the belt(s) with the power steering or AC or compressor or?

You are welcome. Didn't do much but share your concern for having some charge at idle. Hope that can be resolved to your satisfaction.

John

rcbeam · July 07, 2011, 06:18:40 PM

Clifford: Never thought about a different size putting things out of balance, but you may have a point. With the driven pulley being only 3 inch, not sure how much smaller I can find, but that may just be my only option.

Thx everyone for your opinions. This is what makes this group so great.

robertglines1 · July 07, 2011, 06:32:03 PM

As you know our diesel don't like long idle periods so charging while doing that shouldn't be a great concern. should it?

Chopper Scott · July 07, 2011, 08:52:08 PM

You have to run the engine enough rpm to turn the alternator fast enough to exicte it to make it charge. If the alternator is a one wire type it won't charge until it gets turning fast enough. If you start it and never get above idle it's not going to. Rev it up and then let it idle. I have a 24 volt and a 12 volt alternator on my rig and they both act the same. They just need some rpm's to excite them and then they work great.