If the 83388-09 reverse motor for Tri Glide dies of natural causes, the brushes would be the problem. If no outside interference like pollutants or low voltage gains exposure to the motor the brushes would be the weakest link. This is partially due to design, but mostly an unavoidable scenario in all electric motors. As a DC voltage is applied to evoke rotation of the armature through the brushes, a small arc is unescapable.
THE PROBLEM : This arc is a wear factor. Minute pieces of the brush are being burnt up or destroyed as each part of the commutator bar passes over the insulation that separates them and moves to the next bar. Also, significantly less but definitely present, is a wear factor to the copper in the armature commutator itself.
To compensate as much as possible during the design phase, engineers call for a carbon / copper mixture of the brushes to increase resistance, which would serve to dampen the damaging effect of the arc. However conversely, carbon, which is the only other element readily available and cost effective, also has somewhat of a softening effect when mixed in a copper alloy. The friction of rubbing on the armature causes decomposition of the brush itself by friction.
HOW WE COMPENSATE : In short, the brushes need to have carbon to reduce the arc, but it makes them soft and they turn to dust easily. The exact, precise composition of the brush itself is the secret to the best performing, yet longest life expectancy of the motor. Also, equally important to combat premature brush failure is the brush spring tension and precise placement on the brush.
The spring metal must be of the utmost quality to withstand various thermal cycles with occasional exposure to overheating. Also, extreme caution and experience with the exact location of the contact area between the spring and the brush is a big help. The spring should not ride exactly in the middle of the brush after assembly. The brush should contact the spring slightly off center on the opposite side of the rotation of the armature so that the spring will slide toward the center line of the brush as the brush wears down and moves down the brush guide assembly.