ELECTRICAL REBUILDER’S ASSOCIATION—Technical Document CS201407A www.electricalrebuilders.org “Just the Brushes” BY NATHAN UNGER Be so often, an anxious customer will suggest that maybe his unit needs “just the brushes.” Of course, you and I know that if you want to make a good quality rebuilt unit, you cannot fully depend on the customer’s diagnosis. But in this case … Some time ago, a customer and friend of mine asked me to check out his Harley Davidson generator. He said that it was a 32E from a 48-57 Rigid. When I took it apart, the internal wiring looked most like the model 48 diagram in the ERA website Technical Article Library, “Harley Davidson Generators – 6-Volt Models.” It was a 6-volt, counter- clockwise, 15-amp, 3-brush unit. “It just doesn’t seem to be putting out enough,” the customer said. Being a mechanic himself, he had tried to determine what the problem was, but everything looked fine to him. So he brought it to me, hoping that I could figure it out. He didn’t want to buy anew unit, because he didn’t trust “Chinese made units” (sic). So, first of all, I checked all of the insulation on the brush holders, the armature, and the field coils with a megger—all good. The commutator looked good, the brushes looked good, the fields had correct current draw, there was equal voltage drop on both sides. (I even peeled back a wee bit of the cloth tape on each one just to see if the varnish on the wire was in good shape.) I put the armature on the growler, and it also passed the test. So now I had to look deeper. Everywhere the current goes through a rivet, a crimp, or a bolted or soldered connection, or where it passes from one kind of metal to another, it has to be measured for resistance. The idea is to pass a limited amount of current through the piece that you are testing, and then measure the voltage across it. If you want to do the math, Voltage drop / Amps = Resistance. Since the resistances we rebuilders work with are often in milli-ohms or less, it makes sense to own a milli-ohms tester. If you want to make a really sensitive tester, one that can measure resistance as low as one micro-ohm (give or take), check the ERE newspapers archives for “Diagnosing Slow Cranking Starters On The Bench” November 2012. That meter set-up can be built for around $100 plus minimal time and effort. I measured all of the solder connections on the commutator and the terminals of the generator. Hmm … Then I tested the brushes—right from the carbon part to the tab. Wow! Did I ever discover something! At the point where the brush leads go into the brush, they were poorly connected. Because of heat, those resistances became even worse when the generator was producing current. When I selected new brushes, I double-checked the resistance of each new brush just to make sure … (Believe it or not, not all new brushes are created equal. Ask me about that sometime!) I assembled the generator. It charged properly. (So did I!) The customer is so happy about it that he keeps bringing it up, even two years later! To him, that had been such a mystery. He couldn’t believe that it was “just the brushes.” Nathan Unger is the owner of ASI Rebuilders, Abbotsford, British Columbia, Canada. Boa Figure 1—A # 32E Harley Davidson Generator Figure 2—Brush set for the Harley Davidson generator Figure 3—This is the device | used to search for my customer’s problem. An ERA Publication. ©2014, Electrical Rebuilder’s Association. All rights reserved. No portion of the guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consent of the Electrical Rebuilder’s Association. Page 1