The saw had been making a chattering, rattle-ly noise (bad bearings or field windings) concurrently with the motor dropping in RPM's (bad motor speed control, bearings, brushes, armature or field winding) and there was sparking visible through the motor dust shield ( bad brushes or armature). On dissasembly, the brushes weren't too bad, the armature appeared fine, but the tail bearing was worn and discolored as was the field winding. When I was messing around with the saw, trying to figure out what was wrong, I briefly disconnected the speed control, but that had no effect on the speed or chattery noise, so I figured that the speed control wasn't the culprit. When the speed control fails, the speed either goes too high or the saw actually runs slow or backward.
These saws (and the newer 4100) have a history of motor problems, with the field winding and electronic motor speed control being the most common points of failure. If you go to an online tool parts supplier like toolpartsdirect- Bosch 4000 saw you will see a list of the parts available, as well as the most commonly ordered parts. Very helpful. I ordered all of the most commonly replaced parts, because we all know what happens if we only get what we think that we need........
|Have you looked under your saw lately?|
|Just a 4 screws to remove the case|
|Keep track of the parts|
|3 jaw puller (the bearing came off easy)|
|I prefer the one on the left|
|Fancy tools, very sophisticated|
|New field winding|
|Don't do this!|
Here is a really bad wiring photo. I'm having computer issues at the moment......