ELECTRICAL BASICS · DRILL 5 OF 5
How to test a contactorCoil, main contacts and auxiliaries, with a multimeter.
Lock it out, set the meter to ohms, then put the probes on a real contactor: the coil, each pole, and the auxiliary contacts. Find the bad pole.
A contactor can fail three ways: the coil burns open, a main contact burns or pits, or an auxiliary contact stops changing over. A meter on ohms finds all three, as long as the power is locked out and proven dead first.
Tap two terminals to place the probes. The meter shows what you’d read on the bench.
Contactor test quick reference
Power locked out and proven dead first (live–dead–live). Meter on Ω.
| Test | Good | Bad |
|---|---|---|
| Coil, A1 to A2 | A steady reading within the maker’s range (tens to hundreds of ohms on small coils) | OL = open coil. Close to 0 Ω = shorted turns |
| Main contacts at rest, L1–T1, L2–T2, L3–T3 | OL (open) | A reading at rest = welded contacts |
| Main contacts pushed in | Close to 0 Ω on all three | One pole much higher than the others = burnt or pitted |
| 13–14 (NO) | OL at rest, close to 0 Ω pushed in | No change = faulty auxiliary |
| 21–22 (NC) | Close to 0 Ω at rest, OL pushed in | No change = faulty auxiliary |
Then test it live, with voltage
- Hold Start and measure across A1–A2. Rated voltage there but no pull-in: a bad coil or a stuck mechanism. 0 V: the problem is upstream, in the control circuit.
- With the contactor pulled in, voltage on L1–L3 but not on one of T1–T3 points at a burnt main contact.
Some contactors have electronic coil modules that don’t give a plain ohms reading, and some can be pushed in through a test slot. Follow the maker’s instructions for your part number.
Common questions
How do you test a contactor coil with a multimeter?
Lock out and prove the circuit dead, set the meter to ohms, and measure across A1 and A2. A good coil gives a steady reading in the maker’s range. OL means the coil is open; close to 0 Ω means it’s shorted. For an exact value, lift one coil wire so nothing else is in the reading.
What should a contactor coil read in ohms?
It depends on the coil voltage and size. Small coils are often tens to a few hundred ohms. Compare with the maker’s data or a good contactor of the same part number. OL or close to zero is a fail.
How do you test contactor contacts?
With the power locked out, measure each pole (L1 to T1, L2 to T2, L3 to T3). At rest they should read OL. Pushed in, they should read close to 0 Ω. One pole much higher than the others is burnt or pitted.
What does OL mean when testing a contactor?
OL means “over limit”: no path. It’s right for open contacts at rest, and it means a burnt-open coil when you measure A1 to A2.
The coil reads good but the contactor won’t pull in. Why?
Measure voltage across A1–A2 while Start is held. No voltage means the fault is upstream: a fuse, Stop, Start, the overload’s 95–96 contact or a loose wire. Rated voltage there with no pull-in points at the contactor itself.
YOUR NEXT STEP
Try it on a real panel
Level 1 puts you at a motor starter panel: lock it out, prove zero energy with the meter, wire the control circuit and run the motor. It's free.
Training practice only. Always follow your site's written energy control procedure, your employer's authorization rules and the meter maker's instructions on real equipment.