T&T: Alternators - P or N

[Terry Neill]

On a recent trip my 32 Volt alternator stopped charging. After  
returning home I did some troubleshooting using Nigel Calder's "Boat  
Maintenance" as a guide.

As I followed the logic tree I arrived at the point where I had to  
determine whether my alternator was a 'P' or an 'N' type: that is,  
whether the external regulator was connected to the Positive or to the  
Negative side of the field winding.

I did as Calder said and shut down the engine, turned off the ignition  
(sic) and the batteries and disconnected the field wire from the  
alternator. I then measured the resistance between the field terminal  
on the alternator and ground. It was 145 ohms.

Calder says "A P type alternator will give a reading near 0 ohms; an N  
type alternator will give a high reading."

My question is: what does he mean by 'high' and 'near zero'. Relative  
to what? I guessed that 140 ohms was a high reading and 'near zero'  
would be a few ohms (maybe under 10).

So I decided I had an N type alternator.

The rest of the logic tree suggested that the external regulator was  
at fault and I'm now trying to locate one with a suitable form factor,  
but I'm troubled by my decision re: high or low ohms.
Anyone able to help?

By the way, the external regulator seems to be well made for marine  
use but carries no identifying marks whatever, merely one circled W  
which appears to be a quality control stamp.

Terry
Tamarack