When an electric motor stops running, use your ohmmeter or
multimeter to check the electrical system. With an ohmmeter, you
can check all grounds; measure the resistance of the motor windings
and overload protector; test the capacitor and insulation; and
troubleshoot the starting switch.
Some ohmmeters provide a true ohm value reading (typically models
with digital readouts/auto scale). Others, such as analog models
with needle indicators, feature numerical ranges that reach from R
x 1 — in which the meter indicates the actual value in ohms
— to R x 100K (100,000 times the indicated value in
When using a variable-range ohmmeter for testing and
troubleshooting, follow the meter maker’s instructions
regarding range selection for each test. If your ohmmeter
doesn’t have the exact range indicated, use the next higher
range. Here, the evaluations are done with analog meters.
Whenever electricity is involved, safety must come first. Prior to
conducting any tests, review your safety precautions and disconnect
all motor leads from the power source.
Also, perform a visual check of the electrical leads, wires,
terminals and contacts, checking for any burned, cut, pinched,
frayed or disconnected leads or wires. The following procedures
outline test and troubleshooting actions you can take for common
Check: Set the analog ohmmeter to the highest
range. Attach one probe to the ground screw and touch the other
probe to all electric terminals on the terminal board, switch,
capacitor and overload protector.
Troubleshooting: Any ohmmeter reading of less than
infinity indicates a ground. If any contact is
grounded, check and repair all external electrical leads. If the
ground is in the stator, you should replace the motor. Retest the
grounds until no readings register on the ohmmeter.
Check: Set the analog ohmmeter on R x 1. Discharge
the capacitor by shorting across the terminals with an insulated
screwdriver and compare the following readings with the ohmmeter.
Note: Leads can be different, depending on the manufacturer and
Assuming the leads function as follows:
L1 = one main winding lead
L2 = second main winding lead
L3 = third main winding lead
L4 = phase, auxiliary or “start” winding lead, then the
resistance between L1 and L2 must match the resistance between L2
and L3. And the resistance between L3 and L4 must match the
resistance between L1 and L4.
Troubleshooting: If the resistance reading for either of
the tests differs, check the external leads for repairs. The
indicator may point to “open” for shorted windings,
which will require rewinding or replacing the motor.
Check: Set the analog ohmmeter on R x 1K.
Discharge the capacitor by shorting across the terminals with an
insulated screwdriver. Attach one ohmmeter lead to each capacitor
terminal. The ohmmeter needle should move rapidly to the right,
then slowly drift to the left. Note: This test only applies to
analog meters. If a digital meter is used, readings should start
low and rapidly increase to maximum value. You also can use a
capacitor meter to see if it is open or shorted.
Troubleshooting: Replace the capacitor if:
The capacitor doesn’t register an ohmmeter value; or,
the ohmmeter reading moves to 0 and stays there.
The ohmmeter reading remains at a high value, which indicates
an open circuit within the capacitor.
The capacitor’s bleed hole looks stressed or shows
signs of corrosion.
Check: Set the analog ohmmeter on R x 1. Check the
resistance between the overload protector terminals. Resistance
between terminals 1 and 2 (disc) should be approximately 0.
Resistance between terminals 2 and 3 also should be approximately
Troubleshooting: Replace the overload protector if
either resistance value exceeds 1 ohm.
Check: Set the analog ohmmeter on R x 1. Attach
one lead to each switch terminal; the ohmmeter reading should be 0.
Flip the rotating switch (governor or actuator) weight into the
running position; the ohmmeter reading should be infinity —
defined as high resistance/more than 1,000 ohms. Visually check the
stationary and rotating switches when the motor is running; switch
contacts must be closed when the motor is at rest and open when the
motor reaches about two-thirds of full speed.
Troubleshooting: To discount a faulty starter
switch, bypass the switch and repeat the above tests. Clean the
points with a file or fine sandpaper. Replace the starting switch
if it continues to be faulty.