Saturday, July 20, 2013

Bosch 4000 table saw fix (momentum lost and found)

So it's been 10 days or so and the parts for my Bosch 4000 table saw have finally arrived.....time to bring the beast back to life. As you may remember, it had been making peculiar noises, off and on, for a few days prior to me actually deciding to do something about it, rather than burning the thing up. I have been known to actually be proactive, on occasion, regarding maintenance but generally the opposite is true. That must explain my ability to fix things. Practice!

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........

underside of Bosch 4000 table saw repair
Have you looked under your saw lately?
Here is where we left off. First remove the brushes and the three screws holding the dust cover in place. Take note of the wiring connections and separate them.

motor case disassembly Bosch 4000 table saw
Just a 4 screws to remove the case
Motor case removed Bosch 4000 table saw
Keep track of the parts
Tail bearing removal
3 jaw puller (the bearing came off easy)
Old/new bearing comparison
I prefer the one on the left
Using a socket to install a new tail bearing
Fancy tools, very sophisticated
Many light taps. I used a socket to keep from damaging the new bearing race.

New field winding Bosch 4000 table saw
New field winding
Motor re-assembly
Don't do this!
The tail bearing sits in a rubber bushing/seal cup, probably to allow for slight miss-alignment. I first put the rubber cup on the bearing, then put the case back in place, but the cup felt like it was getting in the way. Instead, I put the cup into the motor case bearing housing, THEN slipped the case over the armature and all was good. Also, the windings would have rubbed against the armature so they needed to be carefully (hahah) adjusted with a pliers (bent out of the way). This makes me a bit skeptical about the longevity of the repair or the overall quality of the parts. And these were real Bosch parts BTW.

No sparks!
Test run. A very few sparks and no chattery noises and the speed was good too. It looks like it was only the motor field winding that was causing the noises and speed fluctuation (although the bearing did need replacing, too). We'll see how long it lasts. My fingers are crossed.

Here is a really bad wiring photo. I'm having computer issues at the moment......


  1. I'm also rebuilding my bosch400 motor I've got it all back together but am confused by the soft start controller wiring. The parts diagram only shows two wires but there's actually 3 on the controller. Do you happen to have a clear photo showing all the wiring under the dust shield??

    1. Can't stoopid computer(cats!) broke. I don't know about clear, but here we go...

      Wire comes in as Black to controller blue.
      Wire in White to motor field white.
      Large white on controller to motor brushes(spade terminal on the motor, anyways).

      Hope that helps, I admire those who fix, or at least try. I'll try to add a photo.


    2. And controller black to motor black. I think that makes three wires.......hmmm. Yep, three. Well, three on the control board, that is.

  2. I'm working on rebuilding my Bosch 4000 table saw motor. I'm still trying to get it apart. I've removed the motor casing cap, the wire nuts, the brushes, and the four screws securing the motor casing. I'm having trouble removing the motor casing. Any suggestions?

    1. It's been a bit, so please forgive me if this is less than clear, but....

      It sounds like you got everything. I would give the motor casing (the plastic body) a few gentle taps, just to be sure that it has separated from the metal half of the motor, and not stuck together by corrosion/sawdust etc. Now, if memory serves, the tail bearing unit nearest the brushes sits in a rubber cup, which isolates the bearing from the plastic case. On my saw, the rubber cup had deteriorated/hardened and had a fairly good grip on the bearing, so I too was a tad leery about separating the case, but anger prevailed ]:-!

      If the case hasn't gotten so hot that it melted, it should just be a matter of popping the bearing free from the rubber cup. You've got screws, brushes, and wiring out of the way. There shouldn't be anything else really holding it together. I used a couple of large screwdrivers, one on either side of the case, and popped the plastic case free.

      My fingers are crossed, and I bet you'll get it. Let me know if I can help.


    2. I got the motor casing removed and separated the field windings from the casing. I inspected the bearing. It turns and isn't discolored. Everything loooks in good shape. I cleaned off any sawdust from the inside of the motor.

      I originally started working on it, because of arcing at the brushes and fluctuations in RPM speed.

      I by-passed the speed control and I don't get arcing, but it trips the breaker after about 3 seconds. I re-assembled everything (including speed control) the saw runs, but with lots of arcing and small RPM fluctuations. Do you think it's the speed control and not the field winding?

  3. I would suspect the field winding. That sounds to be exactly like my saw was behaving. I didn't trip breakers, but there was LOTS of arcing and small speed fluctuation. I bypassed the motor speed control, but to little effect. I replaced the field winding, and it worked perfectly thereafter. I ended up replacing the speed control and brushes when I sold the saw, but that was just because I had them on hand. Sort of a"piece-of-mind " thing for the next owner. I think that, all told, the total came to about $100 w/shipping, for the field, bearings, brushes and bearing cup. The field alone was around $50, not too bad considering how much these darn saw cost. I will always view them with suspicion, I'm afraid. I feel that my saw was responsibly used. I do miss the nice fence, though.

    Fingers are crossed,


  4. Thanks for all your help! I'll let you know how it runs after the new parts.


  5. I too am working on one of these Bosch saws. Mine is a 4100 and the motor started making noises so I tore into it. The saw started making noise and then started going backwards slowly... The brushes have scoring on them and the armature has VERY light signs of rubbing. I ground tested the armature, 180 degree tested the armature and tested the plates on the armature and the armature passed all tests. The field magnetized and pulls a screwdriver quickly and holds while powered up. The only device I don't really know how to test is the speed control board. I have 120VAC coming in to it but only 10VAC coming out and I don't see a step down transformer therefore I believe it may be bad but I am hesitant to bypass it and put 120VAC directly to the motor but I think that is what it needs. How did you bypass the speed control board? Any information would be greatly appreciated.

    1. Hi Jim!

      It sounds like you have a pretty firm grasp of the inner workings of that motor. To be honest, I can't remember exactly how I performed the bypass. To the best of my recollection, it amounted disconnecting all of the leads to the controller and then just jumping the white lead directly to the motor....... If you imagine the simplest possible option, that would be what I would've tried, haha!

      My concern for these types of bypass tests is the danger of inadvertently frying an electronic component. As we are REMOVING the electronics from the equation, just giving things a quick short term jolt of electricity, to see what happens, shouldn't cause significant damage to the other (more elementary) electric components. At least that was my thought process.

      That said, if your motor is running backwards......... I'm guessing that the motor/speed controller is bad. They are a common replacement item it sounds like, unfortunately. It looks like they are currently $37 at TPD (link above), so not too bad price-wise.

      Best of luck to you and sorry that I wasn't of more help, but if you have any other questions.... I'm here.

      Thanks, Jason

  6. I am going to buy miter saw. I read many website and I see Dewalt DW175 which suitable my need. I am going to buy this site . But I don’t know where is the best seller about miter saw. Give me your opinions

  7. Okay guys I could really use your help I have found a Bosch 4000 in really great shape they guys wants 150 for it only problem is the motor will only turn very slow and makes humming noise. Any ideas on what could be wrong and how much it would take to fix I don't want to be more the 250 to 300 in a working saw please help!!

    1. Hey John,

      Sounds like the electronic motor speed control, not too expensive, so it might be worth a shot. I listed some old prices in there somewhere, but you can shop around.

      Best of luck

  8. Tremendously valuable information,thank you all very much.Now, how do I replace the front bearing.Will removing the gear housing from the guide pins give me access?


  9. I just picked up a bosch 4000 that has that rattling noise when getting up to speed. Then every once in a while while using you can hear it just a little. I think I will check out the bearings. This was a great post

  10. I was cutting some 2x2 fir with my Bosch 4000 today and the motor started to labor on one of the pieces. Instead of slowing the speed rate, I sped it up and the motor stopped. I turned the switch off, waited about 15 minutes and turned the switch back on, but the motor doesn't move. Is the problem likely with the speed control. If so, before I order a new control, is there a way to test it?

  11. I was cutting some 2x2 fir with my Bosch 4000 today and the motor started to labor on one of the pieces. Instead of slowing the speed rate, I sped it up and the motor stopped. I turned the switch off, waited about 15 minutes and turned the switch back on, but the motor doesn't move. Is the problem likely with the speed control. If so, before I order a new control, is there a way to test it?

  12. What wires are connected to the brush holders?


Like all of us, I am figuring this out as I go, so when you see something that is incorrect or flat out wrong (and you will!), let me know. This is a learning process. Real people and names, please. Constructive comments and questions are very welcome, but hate speak/politics are not! Life (get one!) is too short.

Thanks, Jason