First, the tale, then two questions below.
I am in Mexico, and a cousin asked me to look at the alternator on his quarry compressor that he bought last week. It is a 1975 Atlas Copco compressor, with probably a Deutz diesel four cylinder engine and a large pump, looks like piston style, to run compressed air stone drills. That diesel has obviously been rebuilt, it runs smoothly and steadily.
It looks like a standard car 12v. alternator, but very small compared to the alternators we find on newer cars. I suggested it looked as if he could put in a modern alternator with an internal regulator, since there is a lot of room there, and it might be cheaper than trying to find an exact replacement.
The ground wire is gone and the ground terminal has no continuity to the metal body, though the battery negative terminal does.
The wiring to the A terminal is good, has 12 volts at all times.
The external regulator has 12 volts on one terminal when the key is on. This terminal is jumpered to a second terminal, and the third terminal is connected to the alternator field wire.
There is no voltage at the field terminal on the alternator. So, though I suppose the regulator might not be properly grounded, I am guessing the regulator is bad, too.
***Here is my first question. Should I not be able to measure low resistance from the field terminal to the ground terminal on the alternator through the field winding, since the regulator is external?
My meter is a Protek 340A analog clamp meter. Ohms is battery powered, of course. On X10, I should be able to see resistance of several thousand ohms, but it doesn’t move from infinity.
***Now, my second question. I would say the pulley on that alternator is a bit smaller than on his 1975 Chevrolet pickup, which would mean a newer alternator with a larger pulley might run a bit slower. I don’t think there is any drain on that battery once the starter is finished, so four or five amps from that alternator should keep the battery up to snuff, since it runs for hours at a time.
Would a modern alternator produce at least 4 or 5 amps while running 10 or 20% more slowly, if there is indeed that much difference in pulley size?
Of course, when he gets a new one, I will suggest he see if he can swap the pulley.
Thanks for any information. I realize this is not a car, but that compressor does have four wheels and the alternator is clearly a car type system, even to the key switch system.
Just guessing here since I’m not familiar with rig at all, but does this unit have a volt meter or light mounted on a console panel? Most fields are energized by 12 volts being provided through either a gauge or lamp.
You could try running a 12 volt jumper to the field terminal, touching a screwdriver to the alt. pulley, and noting if it has a magnetic attraction. If it does, then one would assume there is a wiring, meter, or lamp problem in the circuit.
That’s not real scientific but it can work.
I don’t think you’ll have a problem with output by running a larger pulley. One of my tinker toy cars here has a set of underdrive pulleys on it that I think??? provides a 15% reduction in alternator, water pump, power steering speed, etc. and there has been no problem at all with the electrical system or failure to keep the battery up; and this car is gasoline powered with a number of electrical items that are functional when it’s running.
Don’t know if any of this will help you out or not, but maybe… 
Often an open field winding problem is simply the slip ring brushes worn down too short to make contact with the slip rings.
Thanks on the answer on different pulley size. I thought that, but things here cost so much that I wanted a second opinion before I essentially spend HIS money by recommending an action.
On the meter issue, there is a current meter. I did not have time to see if it is even wired in or not. In this case, there is voltage at one side of the external regulator when ignition is turned on. So continuity is there for field voltage, except it doesn’t make it to the field winding. If I understand B.L.E. he implies the field winding should measure something, since he refers to an open field.
IF the amp meter does not work, I will probably rely on a voltage reading to make sure a replacement alternator is working or not. That doesn’t tell exact amperage, but creates confidence it is working if the voltage does up a significant amount.
It would be very hard here in the rural area to find replacement parts for an unknown old generator. So, my recommendation will be to try to find a modern alternator with internal regulator, if he can find one that will physically mount in there and line up with the belt.
Thanks very much.
"The ground wire is gone and the ground terminal has no continuity to the metal body, though the battery negative terminal does. "
If I understand what you’re saying here correctly, you’re saying that there is no continuity between the ground of the alternator and the negative side of the battery. If this is in fact the case there is no reference voltage for the voltage regulator to work with, and there’s no possible way the alternator can charge the battery.
Try running a ground wire from the body of the alternator to the negative side of the battery, or at least to the chassis ground.