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If your car stalls when it gets up to temperature or refuses to start, your crankshaft rotation sensor could have failed. You can read all about the faults and how to remove and refit this sensor by clicking here.

The failure of this sensor is well known and people online are very quick to correctly diagnose this fault.

However, there are some cases where the wiring to the sensor is faulty or the replacement sensor is faulty. This can make it very hard to diagnose. So, we need to check the sensor to see if it’s ok.

Coil Test

The crankshaft rotation sensor is a variable reluctance sensor (VRS). These have 2 connections. They are far more simple than the hall effect sensors that have 3 connections and internal microprocessors. The VRS only has 2 functioning parts. A copper coil and a permanent magnet.

To test the coil to ensure that the internal copper coil isn’t broken (corroded or burnt out) we need to check the resistance in Ohms between the 2 connector pins. So, get your multimeter, set it on Ohms and attach the probes to to the sensor connectors.

Keep any metal away from the sensor and take a reading. This reading should be static (not changing). On this particular sensor (not new), I’m getting a resting resistance of 653Ω (0.653KΩ). If you get no reading at all, the copper coil has failed and the sensor is junk.

Now try waving a ferrous metal object in front of the sensor. The Ohm reading should fluctuate.

If you get a fluctuation and the reading goes back to the original reading when you stop waving the metal object, the sensor coil is fine. The variation of the Ohms is because the coil is picking up the eddy current from the ferrous object.

Reluctance Explanation

Reluctance is a clever thing. The variable reluctance sensor contains a permanent magnet that creates a directional magnetic field. As the ferrous metal reluctor (a ring with teeth) passes in front of the sensor it interferes with the magnetic field which in turn affects the resistance of the copper coil.

So, the resistance increases and decreases rapidly as the reluctor passes in front of the sensor. The car passes 5 volts into the sensor and will monitor the voltage coming back out the other side.

What you get out of the other side is a sine wave. The frequency of the sine wave tells the car how fast the reluctor is passing in front of the sensor. On the crankshaft sensor set up, it reads teeth spinning around the clutch.
In this position there is a missing tooth to instruct the car of the top dead centre (TDC) position.

How Can The ECU Time The Engine With Only 1 Sensor?

This is a question that came to me a few years ago and it took me some time to work it out. Therefore, there must be at least 1 other person who is interested in knowing.

The crankshaft has to rotate twice (720°) for a full combustion cycle, however, the cam shaft only has to rotate once (360°)

The crankshaft has a clutch/flywheel with teeth all around except for 2 that have been removed. It’s the gap that the sensor sees as the crankshaft spins.

That means the crankshaft sensor sees the gap pass twice during each full combustion cycle because the flywheel rotates twice. However, it only uses 1 of those to inject fuel and fire the spark plugs.

But how does the car know which 1 of the 2 rotations it needs to use to do that? Imagine if it picked the wrong one, it’d be firing the spark plugs and the injectors in the wrong order. That wouldn’t work right?

Wrong

The engine fires the spark plugs for both rotations and initially it injects fuel for both rotations too.

1. The Spark (Wasted Spark)

The ignition system is designed to fire the plugs on both the compression stroke and the exhaust stroke.

On the compression stroke, the spark ignites the fuel.

On the exhaust stroke, the spark fires into empty air (the “wasted” spark). Since it fires every time the piston reaches the top, the ECU doesn’t actually need to know which stroke it is on to get the engine to start and run.

2. The Fuel (Initial “Blind” Injection)

This is where it gets interesting. To get the car started, the ECU initially operates in a “Batch Fire” mode. It sprays fuel into the intake ports based solely on the Crankshaft Position Sensor.

It doesn’t know if the intake valve is open or closed for a specific cylinder yet.
It just sprays, knowing that the fuel will “sit” behind the closed valve for a split second if it’s the wrong stroke, and then get sucked in when the valve finally opens.

3. Finding the “Phase” (Crankshaft Acceleration)

Once the engine is actually rotating, the ECU uses a method called Segment Timing or Crankshaft Speed Fluctuation “sync up” for more precise fuel injection:

When a cylinder is on a compression stroke, the starter motor (or the engine’s own momentum) has to work harder to push the piston against the air pressure. This causes a tiny, microscopic deceleration in the crankshaft speed.
When it’s on an exhaust stroke, there is no pressure, so the crank doesn’t slow down as much.

The ECU’ is fast and precise enough to measure these tiny differences in the “teeth” of the flywheel. Once it detects that specific “dip” in speed, it says, “Aha! That’s the compression stroke,” and it switches from “Batch Fire” to Sequential Injection for better fuel efficiency.

Clever!

Or lazy cost cutting. One of the two. It does mean is wastes a tiny amount of fuel and over-fuels the engine for a second. It also means that the engine can take more rotations to start unlike engines with a separate camshaft rotation sensor.

On the flip side, it also means there’s less to go wrong and when you time the engine, the camshaft can be 180° out of phase from how it was and the engine will still run.