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Scope & Scan: Some ABCs of CKPs

Tuesday, July 1, 2008 - 00:00

Unlike the Variable Reluctance (VR) sensors that output an AC signal, the output from a Hall-effect sensor is a square wave. This square wave starts from the sensor reference ground level to a positive DC voltage of typically 5, 8, 10 or 12 volts, depending on vehicle application. Hall-effect sensors, unlike VR sensors, always are supplied a power and ground in order to operate the sensor's internal electronics.

Even so, Hall-effect sensors, like VR sensors, are devices that are magnetically triggered. As such, sensor mounting distance from its intended target wheel is still a critical factor in proper operation. Distances as little as 1 millimeter can make the difference between the sensor generating a good signal and not generating a signal at all.

The Crankshaft Position Sensor (CKP) in Figure 1 is from a Jeep that recently had a new engine installed. After the installation, the new engine ran poorly and misfired. The normal CKP sensor pattern is eight thin pulses per each crankshaft revolution. I typically use the Camshaft Position Sensor (CMP) signal to quickly identify one complete engine cycle, which always consists of two crankshaft revolutions. Using this method of viewing, you can see that there is a wide extra defective pulse that regularly repeats right before the normally occurring fifth CKP pulse.

A regularly repeating defective pulse is a good indication there is a problem with the CKP-to-target wheel gap and not a problem with the sensor or its related circuitry. This vehicle has its CKP target wheel mounted to the flywheel. Each CKP pulse is generated as the holes in the target wheel pass the sensor. Upon inspection of the target wheel, it was found that the wheel apparently had been very slightly dented in during the engine installation, creating enough extra distance from the CKP sensor to cause the sensor to mistake it for another hole.

At times an engine can run so poorly and the speed of the crankshaft can vary so greatly that the position sensor will output wildly varying pulse widths. Under these conditions, it can be difficult to get the required two crankshaft revolutions seen on one scope screen, and at the same time get a steady enough crankshaft rotation speed to determine if the sensor, sensor circuitry or target wheel relationship is causing the varying widths displayed.

The pattern displayed in Figure 2 had only two of six cylinders firing consistently, causing the crankshaft rotational speed to vary wildly. A technique that can be used to separate out the possible causes is to disable the ignition system so that the crankshaft speed becomes steadier due to the influence of uneven combustion being removed from the equation.

In Figure 3, I disconnected the ignition module and removed the spark plugs to get a look at just the interaction between the sensor, sensor circuitry and target wheel without the influence of uneven crankshaft speed. I am using the CMP signal as a counting reference. As you can see, the CKP is failing, causing the signal to mis-trigger.

If you have a situation where there is no signal reference output from a Hall-effect sensor at all, you need to determine where the signal reference voltage for the sensor comes from before you can determine the cause of its absence. Typically, if the CKP sends its signal to an ignition control module, it is the ignition control module that supplies the sensor with a reference voltage that ultimately gets pulled low by the Hall-effect sensor. If the Hall-effect sensor is wired directly to the engine control module, then the engine control module supplies the reference voltage.

Always check for signal reference voltage while the engine is cranking so that the target wheel will be moving in and out of the magnetic field causing the signal reference to be grounded and ungrounded. If you check for a signal reference voltage with the target wheel stopped in the wrong spot, the signal may just be pulled low by the CKP as part of its normal function.

Of course if the signal reference voltage is missing, you must make sure the wire has not been shorted to ground due to a wiring defect. Not checking for this is a common mistake that can cause needless replacement of sensors and components.

Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.

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