Circuit testingBeginner

Resistance Test

Measures the resistance of a disconnected component or wire against a known specification.

What this test tells you

Whether a component's internal winding or element matches its specification. It is a component test, not a circuit test — resistance measurement cannot tell you how a circuit behaves under load.

When to use it

Checking an injector, coil, solenoid, heater or temperature sensor against a figure
Confirming a suspected open or shorted winding
Comparing one component against its identical neighbours

Safety

The circuit must be de-energised — measuring resistance on a live circuit damages the meter and gives nonsense
Disconnect the component so the meter does not read through parallel paths
On ignition components, allow any stored energy to dissipate

Vehicle setup

Ignition off
Component connector unplugged
Component at a known temperature if the specification is temperature dependent

Meter setup

Tool: Digital multimeter
Function: Ohms
Range: Auto range, or the lowest range that covers the expected value
Sockets: Red in VΩ, black in COM

Practise it — interactive simulation

Starter solenoid winding

Isolate the component, then measure its winding resistance.

GND — Battery → BPOS
BPOS — Ignition switch → IGN
IGN — Connector C21 → S1
S1 — Pull-in winding → S2
S2 — Earth E14 → GND

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RED Component terminal 1

BLACK Component terminal 2

Polarity does not matter for a passive resistance measurement

RED Either component terminal

BLACK Component body or case

Insulation check — should read open circuit on most windings

Procedure

Short the probes together and note the lead resistance, often 0.1 to 0.3 Ω
Subtract that lead resistance from your measurements on low-value components
Disconnect the component fully
Measure across the terminals and record the value
Compare with the specification, and with an identical component on the vehicle where possible
Check terminal to case for a short to the body

Normal result

Within the manufacturer's stated range at the stated temperature. Typical examples: a petrol injector around 12 to 16 Ω on a saturated driver, a relay coil around 60 to 120 Ω, and a coolant temperature sensor changing predictably with heat.

Fault result

Open circuit (OL) means the winding is broken. A value well below specification means shorted turns. Continuity to the case means a failed insulation.

Interpretation

IfReads OL or infinite

ThenThe winding or element is open

NextReplace the component

IfReads much lower than specification

ThenShorted turns — the component will draw excess current and may have damaged its driver

NextReplace, and check the controlling driver or circuit for damage

IfReads within specification but the component does not work

ThenThe component is electrically intact; the fault is in supply, ground, control, or mechanically inside it

NextMove to a power and ground check, then check the control signal

IfContinuity between terminal and case

ThenInsulation failure to the body