A6 · General Electrical≈ 6 minPower off · isolated
01 The Concept
The ohmmeter's two jobs — and the one rule that keeps it honest.
The ohmmeter sends its own tiny current through a part to measure resistance, so it only works with the circuit dead and the part isolated. Two jobs: continuity — is there a complete path? (low ohms = yes, OL = open) — and resistance — does a component measure within spec? Coils, sensors, and resistors all have a spec window; too high means an open/high-resistance winding, too low means a short.
Power OFF, part isolated
Never ohm a live circuit — you'll get a false reading or damage the meter. And isolate the part: leaving it in the circuit lets parallel paths read lower than the real value.
Reading the window
OL / infinite = open. Near 0 = short or a plain wire. A component reads a spec range — compare to the service data, not to zero.
⚑ When ohms lie
A corroded connection can read near 0 Ω cold yet drop 1–2 V once real current flows. The ohmmeter's test current is far too small to reveal it. For a weak or intermittent complaint, trust a voltage-drop test under load (Module 00), not the ohmmeter.
02 The Ohmmeter Bench
Pick a part to test. Watch the reading — and try flipping the circuit power on.
Ω · Resistance
0.2
ohms
Circuit power
03 What If…
Reason first, then reveal.
?A connection reads 0.3 Ω on the ohmmeter, so a tech calls it good — but the circuit is still weak. What did they miss?
The ohmmeter's test current is a few milliamps — far too little to reveal a marginal joint. Under the circuit's real load, that 0.3 Ω can drop over a volt. Confirm with a voltage-drop test under load, not an ohm reading.
?You measure a coil's resistance while it's still wired into the harness and get a suspiciously low number. Why?
Parallel paths. Other components on the same circuit give the meter extra routes, so it reads lower than the coil alone. Isolate the part (unplug at least one side) before you trust a resistance number.
?An ignition coil primary reads OL. Spec is 0.4–0.8 Ω. Good or bad?
Bad — an open winding. OL means no path at all, so the primary is broken internally. A shorted coil would read below spec (near 0). Either way it's outside the window; replace it.
04 Practice
ASE-style. Pick an answer to see why.
05 Recap
Ohmmeter = circuit dead, part isolated. Never on a live circuit.
Continuity: low ohms = complete path, OL = open.
Components read a spec window — too high = open/high-R, too low = short.
In-circuit readings run low from parallel paths; isolate first.
Near-0 cold can still drop volts under load — use a voltage-drop test for weak/intermittent faults.