Determining How Many Alternator Poles You Have

The above is a question we get routinely From Diesel engine owners. Why on Earth would one need to know this? Unlike a gas engine, there is no electrical/ignition pulse for a tachometer to count on a diesel engine. For this reason, Diesel engine tachometers either use a flywheel sensor or the alternators AC pulse to determine engine RPM.

In order to Calibrate a diesel tach you need to know how many Poles the alternator has and the pulley ratio. You would also need a digital photo tach to confirm the tach is correct. Any time you change an alternator or pulley the belt ratio can change and throw your tach off…

This article however,  is only about determining the number of poles. Alternator regulators such as the Wakespeed WS500 need this input data and a new regulator will be out soon that also requires this data (NDA cannot discuss).

What is a Pole? Alternators use a rotor inside a stator to generate energy. The rotors are called “claw-pole rotors” and each claw is either a North or South.I could delve into alternator science here but  this is just about determining how many poles your alt has.

There are a few ways tdeterminethe number of poles.


1- Ask the manufacturer; Notice, I did not say “merchant” as most resellers will have no clue.

2- Count Them

Front Fan Alternators

For alternators With a single front fan;

#1 Remove pulley & fan. Most alternator pulley nuts are a 15/16″ socket. It is best to use an impact gun to remove this nut. Some penetrating oil can help but Please don’t over do it or you can ruin the front bearing.
#2 Look through the front housing, with good lighting, and count the number of poles.

#3 Once you know this number double it. This is your total number of poles.


#4 Re-install front pulley and torque to 70-80 ft pounds (use a torque wrench).

Here’s Why you double what you see through the fan opening;

Dual & Single Internal Fan Alternators

Alternators that use dual or single internal fans have the fans physically bonded to the rotor poles but they can still be counted. The nice thing with internal fan alts is you don’t often need to remove the pulley  but with some you may need to pull the rear rectifier/regulator cover. The rotor below is a DIF (dual internal fan.

The rotors look like this;

Getting a look inside these alts is more difficult but ,with good lighting they can be counted;

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