What is a crankshaft sensor?
The crankshaft sensor is an electronic device that’s mounted
near a notched wheel on the crankshaft. It’s used to detect the exact location of the crankshaft in its rotation cycle and the speed of rotation.
There are two types of crankshaft sensors; variable reluctance and hall effect.
Variable reluctance (VR) crankshaft sensor
A variable reluctance crankshaft sensor is a transducer that detects the presence of ferrous objects passing near the sensor. The VR sensor is built with a magnet wrapped in a coil of wire. It is mounted in a stationary at a set gap away from a rotating ferrous notched ring. As the teeth on the rotating notched ring pass the sensor the teeth cause the amount of
magnetic flux passing through the magnet to vary. When the tooth is directly in front of the sensor, the magnet flux is at a maximum. As the tooth rotates away from the sensor, the flux drops off. So the signal the PCM sees is a typical AC analog sine wave. A VR sensor requires not power to operate.
A Hall Effect sensor contains a thin metal strip with a current applied to it. When a magnetic field passes in front of the sensor, the electrons are deflected towards one edge of the metal strip, producing a voltage gradient across the short-side of the strip (perpendicular to the feed current). The voltage change is detected and translated into a digital signal that’s sent to the PCM to determine fuel injection or spark timing.
The PCM uses input from both the crankshaft sensor and the camshaft sensor to determine fuel injector and spark timing.
What does a crankshaft sensor do?
A crankshaft sensor is used on a fuel injected engine to inform the powertrain control module (PCM) the exact of the crankshaft so the computer can operate the fuel injectors and spark plugs at the proper moment.
Where is the crankshaft sensor located?
The notched wheel can be mounted on the front of the crankshaft near the harmonic balancer, on the flywheel (manual transmission) or flex plate (automatic transmission), or right on the crankshaft itself. The notched wheel (also called a “reluctor ring” or “tone ring”) has evenly spaced teeth and a timing gap. As the tone ring rotates, the crankshaft sensor senses each notch and reports the movement to the PCM. That’s how the PCM determines the number of degrees of crankshaft movement AND the rate of rotation speed of the crankshaft. The timing gap helps the computer determine when a full revolution has occurred.
What happens when a crankshaft sensor fails?
A failing sensor can cause a no start, misfire, rough idle, hesitation, vibration, backfires, lack of power or engine stall. Crankshaft sensors are damaged by heat and impact from road debris, and wiring harness can be damaged by vibration, impact, heat and water intrusion. A crankshaft sensor can fail intermittently causing the poor operation symptoms listed above, or it can fail intermittently when hot but work fine when cold, or vice versa. When it fail totally, you will experience a “crank but no start condition.”
How to test a crankshaft sensor?
A VR type sensor can be tested using a digital voltmeter set to the AC scale. Connect the meter and crank the engine. You should see an AC voltage. However, this only tests the sensor’s ability to generate an AC voltage. The meter can’t measure amplitude and frequency. For that, you need a digital oscilloscope.
A Hall Effect sensor can only be tested with a digital oscilloscope.
Crankshaft sensor replacement cost
Crankshaft sensors usually cost less than $100 and the labor to replace the sensor usually is less than one-half hour. However, the cost to diagnose the failure can easily run upwards of $150, making the total repair cost around $300-$400.
©, 2018 Rick Muscoplat
Posted on by Rick Muscoplat