Diagnosing charging system problems part 2


This article covers the basic operation and troubleshooting of the model 58 to the model 65B. These generators where used from 1958 to 1984.

The first two-brush generator used on a Harley Davidson appeared in 1958. It was appropriately enough called the model 58. The model 58 six volt two brush generator was used from 1958 to 1960. In 1961 the bearing on the commutater end was upgraded from a bronze bushing to a needle bearing. This made the model 61. In anticipation of the electric starter, the electrical system was upgraded from six to 12 volts in 1965. This was accomplished by changing the field coils and armature, which made the model 65. The model 65 was only made for six months at which time the brush plate was rotated 180 degrees to put the positive brush on the topside. This was done to keep the brush insulator dry. The paper insulator used back then would swell causing the brush to stick. The A and F terminals swapped positions to keep the current in the field coil flowing in the proper direction. This generator is called the model 65A. It stayed on big twins until 1969 and Sportster until 1981. From 1982 to early 1984 the Sportster used the 65B. The 65B was made in Japan. It shared no parts with the other generators and is now obsolete. To the best of my knowledge there are no parts available for the 65B. Cycle Electric Inc still manufactures the models 61 six volt and 65A twelve-volt generators for restoration purposes. We highly recommend our DGV-5000 generator with built in regulator for normal use. It is a complete charging system with a two-year guaranty. This eliminates trying to figure out if you have a regulator or generator problem. Just hook up one wire and go. See the applications chart on the generator page for options.

Generator Out Put Testing

(Standard 2 Brush Generators)

In this section I will present two output tests. The first is a quick and dirty voltage test, which can be done with a simple voltmeter. The second is an amperage test, which is a better indication of the generator's ability to produce full power. The amperage output test requires an ammeter and a large and load dump resister. This is the test we use at the factory.

Voltage Test

With the generator mounted on the motorcycle disconnect the wires from the “A” and “F”terminals. Connect the red lead from a voltage meter to the “A” terminal. Connect the black lead to a good clean ground. Set the meter for a 20-volt DC scale and accelerate the motor to about 2500 RPM. The voltage meter should read a positive 2-4 volts DC. If you get no voltage or a negative voltage, try polarizing the generator. (See polarizing section) Polarizing will cure reversed voltage. After obtain a reading of 2-4 volts positive on the meter; leave the meter hooked-up the same and change to a 50-volt DC scale. Use a jumper wire and connect the “F” terminal on the generator 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. If you get no voltage or low voltage the generator has a problem.

Amperage Out Put Test

(Standard 2 Brush Generators)

You will need an ammeter 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 Sportster or 1957 and earlier big twin and 2200 rpm on a 1958-69 big twin.  Ground the “F” terminal just long enough to read the meter.  It should read positive 18 to 24 amps.  If the amperage is low, inspect the generator for other problems.

Note: A 16 ampere-hour or larger battery can be used as in place of the 1-ohm resister but care must be taken to prevent current from flowing from the battery to the armature. This can damage you meter. It is always risky to connect an ammeter to the positive side of a large battery.


With the motor off connect the “F” terminal to ground and momentarily flash the “A” terminal with positive.  Remove ground from “F” terminal. If you have properly installed a working regulator it will turn on and ground the “F” terminal in which case you will not need to use an external ground.

Caution! Do not flash the “F”terminal. This will damage the regulator.