Mutli-Tool: Won't Turn on

The armature, located inside the field, is the power-producing part of the motor and the only moving part of the motor. If your multi tool will not power up, it could be cause by a defective armature. To check the condition of the armature, and to perform a few tests, you will need to remove the armature from the multi tool.
Remove the collet nut, the collet and the front cap. Remove the brush caps on the side and take out the brushes from the housing. Separate the two halves of the housing and then take out the power cable connectors and screws holding down the motor assembly. Pull out the motor and separate it from the switch and circuit board and then slide the armature out of the field. Clean the armature with electrical contact cleaner and wipe dry. Inspect the armature for burn marks and melted insulation on the winding. Replace the armature, if these signs are present.
There are also three tests you can do to check the condition of the armature. The first test is a continuity test. With the armature removed from the unit, stand it up on a bench. Attach one end of the ohmmeter to the shaft, for ground, and the other to the bar on the commutator. Move the probe to each bar on the commutator, checking all the bars. If the ohmmeter shows a reading on one of the bars, this will indicate a short and a bad armature.
The second test is a bar-to-bar test, determining resistance. Attach one probe to the bar on the commutator and the other probe on the bar right beside it. There will be a specific reading for this, depending on the design of the armature. But, most importantly, there is not a large fluctuation here between the values, indicating a fault. If there is an increase in resistance, this will indicate there is a broken, or burned out, wire in the coil. If the resistance drops, then there is a short. Move around the commutator, checking each bar.
The third test is the 180-degree test. Attach the two ends of the ohmmeter on the commutator bars, directly across from each other. There will be a specific set of readings for this, depending on the design of your unit. Again, what is most important is that there is not a large fluctuation in values, which would indicate a fault. If there is an increase in resistance, that would indicate a broken, or burnt out, wire. If the resistance drops, that would indicate there is a short. Continue this test on all the bars. If an armature fails any of these test, it is recommended that you replace it.
Slide the armature into the field. Connect the switch and the circuit board to the motor and reinstall the motor assembly into the housing. Reinstall the hold-down screws and the power cable connector. Reassemble the housing back together and reinstall the front cap, the collet nut and collet, followed by the brushes and caps.

If your multi tool will not power up, check the brushes. The brushes wear out because they are a wear item as they ride on the commutator in order to supply power to the armature. To check the condition of the brushes, remove the brush caps and pull out the brushes. Now, examine the brushes for any wear. Brushes under 1/8 of an inch are considered worn and need to be replaced. Also, if you notice any broken, cracked or chipped brushes, that will also require replacement. Install new brushes into the housing and reinstall the brush caps.

The field is an electromagnet used to generate a magnetic field in an electro-magnetic machine. The field is placed around the armature and the two, together, make up the motor. To inspect the field for damage, such as burn marks and melted insulation, and to perform tests, you will need to take apart the multi tool to access it.
Begin by removing the collet nut, collet and front cap. Remove the brush caps and the brushes. Take out the housing screws and separate the two halves of the housing. Remove the power cable connector and then all of the screws securing the motor and cable connector. Lift out the motor and separate the motor from the switch and circuit board.
Slide the armature out of the field and then clean the field with electrical contact cleaner and wipe dry. Inspect the field for burn marks and melted insulation. Replace the field, if any of this is present.
You can check the condition of the field by performing a couple of tests. Using a multimeter, you can check the field for continuity. Set the multimeter to an ohmmeter and place a lead on each end of the coil in the field. You will have to perform this test for each coil in the field. If the reading is infinity for one, or more, of the coils, then the field is bad and requires replacement. If there is a major variance between the coils, then the field needs to be replaced.
Slide the armature into the field. Connect the switch and the circuit board and place the assembly in the housing. Reinstall the screws to secure it in place and reinstall the power cable connector. Rejoin the two halves of the housing back together. Screw the front cap back on and reinstall the collet nut, followed by the brushes and caps.

The power cord supplies power to the tool. If the power cord becomes damaged, in any way, it may not be able to conduct the flow of electricity needed. Inspect your power cord for cuts, frays, burn marks or anything else out of the ordinary. A damaged power cord is also a safety hazard as it can harm you and cause a fire.
To replace a power cord, remove the collet nut, the collet and the front cap. Remove the brush caps and brushes. Separate the two halves of the housing by removing all of the housing screws. Remove the power cable connector and then the screws securing the motor assembly in the housing. Now, lift out the motor. Separate the switch from the motor. Insert a small pick next to the wire and then pull the wire and pick out together.
On the new cord, you may have to strip ¼ of an inch of the insulation off of the wire, if it is not pre-stripped. Push the wire ends into the switch and reconnect the switch to the motor. Place the motor assembly back into the housing and then secure the motor and cable connector in the housing. Reassemble the housing. Install the cap, the collet and collet nut, followed by the brushes and the brush caps.

The switch controls the power to the motor and, on some models, the amount of power for the variable speed. If the switch wears out, or breaks, it will need to be replaced. Remove the collet nut, collet and the front cap, followed by the brush caps and brushes. Open the two halves of the multi tool by removing the screws that secure it together. Remove the cable connector and the screws that secure the motor assembly in place. Separate the motor and switch. Remove the wires from the switch by inserting a small pick into the wire holes and pulling out the wire and pick together.
Now, you are going to test the switch by connecting an ohmmeter to the switch. Insert the leads of the ohmmeter into the wire holes to check for continuity. The ohmmeter should give you a reading in the ON position and no reading in the OFF position. For models with variable speeds, the ohmmeter reading should change as you adjust the setting of the variable speed. If you do not receive a reading in the ON position, or the variable speed settings, then replace the switch. Install the wire ends into the switch. Reconnect the switch to the motor and install the motor assembly back into the housing. Secure the cable connector and the motor assembly. Reassemble the housing back together and reinstall the brushes with the caps.

The brush caps hold the carbon brushes in place. If a brush cap is damaged, it may not be able to hold the correct amount of pressure on the carbon brush, to keep it in contact with the commutator. Therefore, the power will be disrupted to the armature. Check the condition of your brush caps – if you notice any damage, replace them. Caps can be removed and install with a screwdriver.

Some multi tool models have a circuit board that controls the variable speed and power to the motor. If this part fails, it can stop the motor from working. To check the condition of the circuit board, remove the collet nut, the collet and the front cap. Then, take out the brush caps and the brushes from the housing. Separate the two halves of the housing. Take out the power cable connector and screws, securing the motor assembly. Now, pull out the motor and separate it from the switch and circuit board.
Examine the circuit board for burn marks; if it has burn marks, the circuit board is defective and needs to be replaced. Install the new circuit board with the switch and motor. Circuit boards connect differently on all models, including some that do not have circuit boards. It is a good idea to take a photo, so you can later use it as reference to reconnect the wires. Also, some circuit boards just plug in. Reattach the hold-down screws and cable connector. Assemble the housing back together with the front cap, collet and the collet nut and then reinstall the carbon brushes with the caps.