Understanding Differences, Challenges Between Crankshaft Position Sensors - Engine Builder Magazine

Understanding Differences, Challenges Between Crankshaft Position Sensors

I don’t believe that I have ever written about watching out for
damaged as-cast reluctor rings, spark triggers or whatever other name
some manufacturer has given them to create their own
identity/ownership. It’s not as bad as the variable displacement
acronyms but still running a close second.  

ANYWAY. Before we talk about this issue you need to understand why
this area is so critical: it triggers the crankshaft position sensor
used on engines with distributorless ignition systems. The crankshaft
position (CPS) sensor serves the same purpose as the ignition pickup
and trigger wheel in an electronic distributor. It generates a signal
that the PCM needs to determine the position of the crankshaft and the
number one cylinder.

This information is necessary to control ignition timing and the
operation of the fuel injectors. The signal from the crank sensor also
tells the PCM how fast the engine is running (engine rpm) so ignition
timing can be advanced or retarded as needed. On some engines, a
separate camshaft position sensor is also used to help the PCM
determine the correct firing order. The engine will not run without
this sensor’s input.

There are two basic types of crankshaft position sensors: Magnetic Field (Variable Reluctance) and Hall Effect (Figure 1).
The magnetic type uses a magnet to sense notches in the crankshaft or
harmonic balancer. As the notch passes underneath, it causes a change
in the magnetic field that produces an alternating current signal. The
frequency of the signal gives the PCM the information it needs to
control timing. The Hall Effect crank sensor uses notches or shutter
blades on the crank, cam gear or balancer to disrupt a magnetic field
in the Hall Effect sensor window. This causes the sensor to switch on
and off, producing a digital signal that the PCM reads to determine
crank position and speed.

If a crank position sensor fails, the engine will die. The engine
may still crank but it will not start. Most problems can be traced to
faults in the sensor wiring harness. A disruption of the sensor supply
voltage (Hall Effect types), ground or return circuits can cause a loss
of the all-important timing signal, just as a crankshaft with a damaged
or missing cog/tooth in the trigger wheel of the as cast Reluctor ring (Figure 2) will cause problems.

There are many crankshafts with as-cast reluctor rings that have one of
the notches machined to identify number 1 cylinder at TDC. Because they
are part of the iron casting, these machined areas are very fragile.
Remember, cast iron is brittle and has very little elasticity.

What you see here is indicative of many crankshafts that have Reluctor
wheels to identify the number 1 cylinder.  If this small center lug is
somehow broken off or damaged so that it no longer provides a good
signal the engine will not run.  

I know this because of a course that I took in SHoKU (show’ coo)
(School of Hard Knocks University) when an engine was sent out and it
would not fire. So pay close attention to the way that you handle any
and every crankshaft that has an “as cast” reluctor ring on it because
the engine you may save could be the one that goes to a friend or a one
of your best customers.
figure 1Figure 2

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