Alternator field terminal question

[Kevin B]

I am using Nigel Calder's "Boatowner's Mechanical and Electrical Manual" to trouble shoot a charging problem and need clarification on what he is saying about how to test if the alternator (with an external regulator) field terminal is pos or neg (pg. 57 of the 2nd edition). With the meter connected between the field terminal and ground you measure the ohms. Calder states "A P-type alternator will give a reading near 0 ohms; an N-type will give a high reading". I am getting 6.5 ohms; is this what Calder considers "near 0"?ThanksKevin

 

[Gord May]

no

Don't have the book - but no - 6 Ohms is NOT near zero.Sounds like a poor ground, which could be (part of) your problem.

 

[Kevin B]

Gord, a follow-up question

First, thanks for the response Gord. Since I made the post I was checking my late '70s and early '80s auto manuals. It looks like the Fords and Chryslers of that era had negative field terminals.The Ford spec for this test states the reading should be 3 ohms, which seems to confirm your answer. Calder's book states "most external regulators are connected on the positive side of the field winding"; so this had me looking for and thinking positive.When you suggest a poor ground do you mean at the regulator side?And could I indulge in a second question: Calder's procedure to bypass the voltage regulator states you use a 12V test light to connect the field terminal to ground (for neg field) or bat pos (for pos field). But my Chrysler manual says to just use a jumper. Is there some better reason to use a test light?Thanks,Kevin

 

[Gord May]

< 1 Ohm ...

Regulators control voltage output by controlling field current, and are differentiated (P or N) by the exact location of the regulator on either the positive or negative side of the field coil.A “P-Type” Alternator places the regulator on the positive side of the coil, and has one of the brushes grounded.An “N-Type” Alternator places the regulator on the negative size of the coil, and, the brushes are un-grounded.Hence:A “P-Type” Alternator should have zero* resistance between the field terminal and ground. *In reality, there is no such thing a perfect connection, so there will be a small termination resistance - likely less than 1 Ohm. Higher readings might indicate high resistance internal connections.Since an “N-Type” Alternator has no grounded terminal (negative connects to regulator), there will be an ‘infinite’ resistance between field & ground. Caution: A typical multimeter is not suitable for measuring very low resistance values. Below a few ohms the accuracy is not very good. Below one ohm the accuracy is very poor. Test leads must be firmly placed a clean points of contact.HTH,Gord