ABS, Crankshaft and Camshaft Position Sensors Explained

What do ABS, crankshaft and camshaft position sensors actually do?

They all read a moving toothed wheel and turn it into a pulse the control unit counts. An ABS sensor watches a reluctor ring on each wheel hub for wheel speed. A crankshaft (CKP) sensor reads the trigger wheel on the crank for engine speed and piston position. A camshaft (CMP) sensor reads a wheel on the cam to tell the ECU which stroke a cylinder is on. Same job in three places: convert teeth passing a tip into a frequency.

The same pulse principle drives the speed pulse generators we stock for instruments and fleet senders, so the test methods carry across.

Inductive or Hall-effect: how do I tell them apart?

Count the wires and check what the tip faces.

An inductive (variable reluctance) sensor has two wires and no power feed. It is a coil and a magnet, and a passing tooth induces an AC voltage in the coil. Output rises with speed, which is why a worn inductive crank sensor can start the engine but cut out at idle, or the reverse. The VDO speed pulse generators in our range work on this inductive principle and put out a sine-shaped signal that climbs with rotational speed.

A Hall-effect sensor has three wires: a feed (usually 5V or 12V), an earth and a signal. It produces a clean square-wave that holds the same amplitude at any speed, so it reads down to almost zero rpm. Most modern crank and cam sensors are Hall; many ABS sensors are now active (Hall or magneto-resistive) for the same low-speed reason.

How do I test an inductive (two-wire) sensor?

You can test it cold with a multimeter, then live.

1. Set the meter to ohms and measure across the two pins. A typical coil reads somewhere between 200 and 2000 ohm. Open circuit or a dead short means the sensor is finished. Check the figure for the specific part, since ranges vary.
2. Set the meter to AC volts, unplug nothing else, and crank or spin the wheel by hand. You should see an AC voltage appear and rise with speed. No AC voltage while the coil reads in range usually points to the trigger wheel, the air gap, or metal swarf bridging the tip.

Air gap matters on inductive sensors. Too far from the wheel and the signal is too weak to trigger. Many tips sit at a fixed gap, but a bent bracket or a packed-out tip will drop the signal.

How do I test a Hall-effect (three-wire) sensor?

With the connector still on, back-probe the pins, ignition on.

1. Confirm the feed pin has its 5V or 12V supply and the earth pin is at battery negative. A missing feed is a wiring or ECU fault, not the sensor.
2. Back-probe the signal pin and rotate the wheel slowly. The voltage should switch between near zero and near the feed as each tooth passes. A signal stuck high or stuck low with a good feed and earth means a failed sensor.

A scope shows this best: a clean square wave for Hall, a growing sine for inductive. A ragged or clipped trace points to a damaged tooth on the trigger wheel rather than the sensor.

Why does a position sensor read fine cold and fail hot?

Heat opens hairline breaks in the coil or the internal semiconductor. The classic symptom is an engine that starts and runs cold, then cuts out once warm and restarts after it cools. If a sensor passes every bench test but the fault returns with heat, test it again at operating temperature, or replace it, because intermittent heat faults rarely show on a cold meter.

How to order or request a quote

Tell us the application or the part number, and whether the old sensor is two-wire inductive or three-wire Hall. We will match the correct sensor, confirm the connector and output type, and quote on single units or workshop trade quantities.