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I have found the most common issue is a bad field with the second being a bad rear armature bearing and last being a bad speed control.
Most of the time slow speed or even running backwards is a bad field (part #2 on the breakdown) #1619PA5590
Remove the cap on the end of the motor & the 2 carbon brushes.
Remove the 4 screws holding the motor to the frame and pull it straight off.
Check the bearing on the end of the armature to ensure it is not frozen or stiff. If it is also check the rubber boot that sits over the bearing and the motor housing itself to ensure it is not melted.
I've worked on some table saws with problems like you describe, a couple needed a new speed control unit (Bosch PN 2610997306) in the motor. If all the wiring to and inside the motor is OK and not shorting out anywhere, that's the next part you should look at.
With the saw unplugged, turn the saw blade. If it turns easily with no hangups I would guess its the field. When mine went bad the blade went backwards very slowly. I replaced only the field and not the armature and it's worked fine for over 2 yrs.
Right now I have it apart to change an armature bearing which was completely frozen up.
that is a pretty unique problem you should return it to the manufacture this a sirious safety issue that the manufacture should be aware of and should put a recall on all those models before someone gets hurt.do not try to repair it youself. you cant buy fingers at the corner store........!!
This is unfortunatly a fairly common problem with the Bosch saws, the field coil has failed and needs to be replaced. If the saw is less than a year old it will be under warranty, if not this repair is still worth doing as it is alot less than a new saw. This repair is faily difficult and should be done by a Bosch service center, find one in your area and they will be able to help you.
You can find the 4000 manuals and parts diagram at bosch tools online. You will need a new armture Bnew bushes and possibly field. I would also replace the bearings and motor speed control circuit board.
Bosch does not sell the motor assembly.
Turn unit upside down and remove the Plastic body - approx 6 screws.
Thake a digital camera and take LOTS of pictures of dissassembly as you will need them later. Blow unit clean and work neatly keeping all parts in labeled bins for reassembly. Don't attempt this job when your pressed for time.
Before you start check to see if this machine is still under warranty!!
Call customer support listed on website if you run into a problem.
Good Luck !!
Replace the field coil. I'm going to bet its a Model 4000. I have done about 100 of these saws in the last couple of years and my customers always complain of the same problem, "the saw was running backwards"
Change the armature back bearing while you have it apart, they always seize eventually.
I had a customer bring me a saw with this same problem. All components tested OK on my growler. I went on a limb since the speed control and the armature tested OK. I replaced the feild since to me it seemed to be the only component that could cause the motor to run in reverse. Tool now works properly and have had no more problems with the tool.