Post by pamcopete on Aug 12, 2008 13:59:20 GMT -5
Well, we all know how important it is to have a proper Ohmmeter reading on the rotor and brushes. Sometimes we get a high reading and assume the brushes are bad, and that may in fact be the case, but even with new brushes, it's possible to get a high reading.
When the engine stops, it may turn back wards a bit, especially if it stops on the compression stroke. This can have the effect of "tilting" the brushes onto their trailing edge which means that a much smaller part of the brushes are in contact with the slip rings and it will result in a higher reading. As high as 15 to 20 Ohms. Of course, when you replace the brushes, they are flat on the slip ring, and you will get a normal reading, like 5 to 8 ohms for the brushes and rotor .
So, try rotating the engine forward (counter clockwise) by hand, using either the kick starter or a 17MM socket on the rotor nut, and take another measurement. You may be surprised to see what was a 15 to 20 Ohm reading suddenly become 5 to 8 Ohms.
Excessive tilting of the brushes can be due to a worn out brush holder that doesn't hold the brushes perpendicular to the slip rings.
Of course, the same thing can happen in the other direction after installing new brushes. Tilting of the brushes will result in a smaller area of contact and you may not initially get 14.5 Volts at the battery until the brushes have worn to suit the "tilt" from the holder. So, ride a few miles before doing your final battery check.
It's also possible to get a high reading on a cold engine. Condensation can form on the slip rings which can greatly increase the measured resistance, so if you initially get a high reading on a cold engine, start the engine and warm it up to operating temperature and then take another reading.
A more dynamic way to measure the brush and rotor resistance is to unplug the regulator and check the brush resistance with the engine running. Remember, that on pre '80 engines, the inside brush is still grounded. Doing it this way will give you the "true" running resistance of the brushes and rotor, which will probably be about 10 ohms, because the brushes do not make perfect contact with the total circumference of the slip rings when they are in motion and there is always some residue on the slip rings, even if it is just the early stages of corrosion on the slip rings..
The outer brush travels further than the inner brush , so the outer brush collects more residue than the inner, so you can sometimes reduce the resistance by cleaning the outer slip ring, which is accessible without removing the brush holder, with a pencil eraser .
I think I'm going to drill a hole in the stator frame so I can use an eraser on the inner brush as well without removing the brush holder.
When the engine stops, it may turn back wards a bit, especially if it stops on the compression stroke. This can have the effect of "tilting" the brushes onto their trailing edge which means that a much smaller part of the brushes are in contact with the slip rings and it will result in a higher reading. As high as 15 to 20 Ohms. Of course, when you replace the brushes, they are flat on the slip ring, and you will get a normal reading, like 5 to 8 ohms for the brushes and rotor .
So, try rotating the engine forward (counter clockwise) by hand, using either the kick starter or a 17MM socket on the rotor nut, and take another measurement. You may be surprised to see what was a 15 to 20 Ohm reading suddenly become 5 to 8 Ohms.
Excessive tilting of the brushes can be due to a worn out brush holder that doesn't hold the brushes perpendicular to the slip rings.
Of course, the same thing can happen in the other direction after installing new brushes. Tilting of the brushes will result in a smaller area of contact and you may not initially get 14.5 Volts at the battery until the brushes have worn to suit the "tilt" from the holder. So, ride a few miles before doing your final battery check.
It's also possible to get a high reading on a cold engine. Condensation can form on the slip rings which can greatly increase the measured resistance, so if you initially get a high reading on a cold engine, start the engine and warm it up to operating temperature and then take another reading.
A more dynamic way to measure the brush and rotor resistance is to unplug the regulator and check the brush resistance with the engine running. Remember, that on pre '80 engines, the inside brush is still grounded. Doing it this way will give you the "true" running resistance of the brushes and rotor, which will probably be about 10 ohms, because the brushes do not make perfect contact with the total circumference of the slip rings when they are in motion and there is always some residue on the slip rings, even if it is just the early stages of corrosion on the slip rings..
The outer brush travels further than the inner brush , so the outer brush collects more residue than the inner, so you can sometimes reduce the resistance by cleaning the outer slip ring, which is accessible without removing the brush holder, with a pencil eraser .
I think I'm going to drill a hole in the stator frame so I can use an eraser on the inner brush as well without removing the brush holder.
