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Generator Information

This is provided for informational purposes only, we accept no liability for errors or ommissions.

The Importance of Low Speed Charging Rate

Charging systems do not produce full power at low RPM, some drop off more than others. This can cause battery discharge at low RPM. Better low speed charging system output will help keep the battery fully charged which will improve battery and starter motor life.

Testing for overloaded charging system

If your electrical system uses more amps then your charging system can put out the voltage will drop causing battery discharge. When this happens it appears that the charging system has failed when in reality it may just be overloaded.

Testing System Voltage

The easiest and fastest way to determine if the charging system is operating satisfactorily is with a volt meter. Connect the volt meter across the battery before starting the bike. A fully charged battery should read 12.7 volts at 72degrees F. Anything less indicates a battery that needs charging or has other problems. Start the bike and allow it to run at a high idle for a minute or two to recharge the battery after starting if electrical stator was used. (Caution: take care not to blue the pipes). Turn on accessories used in normal operation (not turn signals or horn). If you get between 13.8 and 14.8 VDC (depending on model) you can assume the charging system is keeping up (as long as there is not a intermittent problem). If the voltage is too low, do a system output test and compare charging system output to system usage.

Determining Usable DC Amps (System Output Test)

We will define usable amps as the amount of DC amps the charging system can deliver while maintaining an acceptable voltage. If voltage drops below 13.8 the battery will not fully charge. By the time voltage drops to 12.7, you are drawing amps out of the battery, so any increase in ampere output after voltage drops to 13.8 will be considered unusable amps. To test system output, you will need a volt meter, an amp meter rated for higher amps then the system rated output, and a load dump. (i.e. carbon pile resistor or other variable load) Connect the volt meter across the battery. Connect the amp meter in series with regulator output (measure amps in wire from regulator to battery). Start motor and allow to warm-up enough to obtain a steady idle. Connect load dump across battery. (Add light bulbs, resistors or whatever your using for a load dump) until the volt meter reads between 13.8 and 13.5. Read amp meter. This is usable low speed amps. Now increase RPM to normal cruising speed (usually between 2500 & 3200 RPM depending on model and riding style) add load until volts drop to 13.8. Read amp meter. This is usable high speed amps. On some systems maximum amps will come at 4500 RPM or higher. Unless you cruise at 95+ DO NOT consider this as usable. USABLE DC AMPS SHOULD BE AT LEAST 1 OR 2 AMPS HIGHER THEN SYSTEM LOAD.

Determining Amperage Usage

  1. Disconnect your charging system. Put a battery charger on the battery to keep system voltage up.
  2. Use a clamp on inductive amp meter on battery negative cable.
  3. Start engine. and turn on all normal electrical accessories. Read amp meter. Add 2 amps for battery draw. This is normal usage.
  4. Turn on all accessories. Read meter. Add 2 amps for battery draw. This is maximum usage.

Note: Amperage usage should be at least 2 amps lower then charging system output.

Generator Output Tesing

(Standard 2 Brush Generators)

Voltage Test

With the generator mounted on the motorcycle or test drive stand and no wires connected to the A and F terminals, connect the red lead from a voltage meter to the A terminal. Connect the black lead to a god clean ground. Set the meter to 20 VDC scale and accelerate to spin the generator shaft 3000 RPM. The generator turns faster than the motor, on sportswear,1.3 times motor rpm, on the Big Twin 1.6 times rpm, Sportsters=2500, Big Twin=2000 for 3000 rpm generator speed. The voltage meter should read a positive 2-4 volts DC. If you get no voltage or a negative voltage, polarize the generator. (See Polarize section) Reversed voltage will be cured by polarizing. After obtain a reading of 2-4 volts positive on the meter leave the meter hooked-up the same and change to a 50 DC volt scale. Use a jumper wire and connect the F terminal to ground just long enough to read the meter. It should read a positive 40-50 volts. This shows with an open field circuit you have low voltage. With a closed field the voltage is high. It is the regulators job to keep the voltage at the proper level.

Amperage Output Test

(Standard 2 Brush Generators)

You will need an amp meter capable of reading +/- 30 DC amps and a 1 ohm load. Connect the positive lead of the amp meter to the A terminal. Connect the 1 ohm load between the negative side of the meter and ground. Spin generator shaft 3500 rpm, that is about 2800rpm on a sportswear tachometer and 2200 rpm on a Bog Twin. Ground the F terminal just long enough to read the meter. It should read positive 18-24 amps. If amperage is low check for other problems.


With motor off connect F terminal to ground and momentarily flash the A terminal with positive. Remove ground from F terminal..

NOTE: If you have properly installed a working regulator it will turn on and ground the F terminal.

Caution! Do not flash the "F" terminal or regulator damage may occur!!

Voltage test for Harley 12volt Generator

With the Harley 12volt Generator mounted on the motor and no wires connected to the B terminal connect a volt meter red lead to the B terminal. Connect the black lead to a good clean ground (you can use the bearing in center of end bell regulator for a ground). Run the motor and read the meter. Voltage should read between 16-17 volts DC.

Amperage Output Test for Harley 12volt Generator

The current regulation system on the Harley 12volt Generator is temperature sensitive, so the maximum allowable output changes with temperature. Use the test described in Determining Usable DC Amps but test amps immediately with motor cold. At 72 degrees F and a freshly started cold motor you should get 14-15 amps at 2500 RPM. As the generator warms up this will drop off. Cruising at 60 mph on a 80 degree day you can expect 12-13 amps constant.

The Harley 12volt Generator is polarized at the factory and should not require any further polarization.