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Scope & Scan: The Ups and Downs of Scoping AC Sensors

Sunday, June 1, 2008 - 00:00

Two-wire Variable Reluctance (VR) position sensors have been with us for better than 30 years. However, testing these devices can still be problematic because of their unique AC voltage output, which rises above as well as below the chassis ground reference point.

If the AC signal sync pulse rises first or falls first does matter to the Electronic Control Unit (ECU) and to your test equipment. Note the two top waveforms in Figure 1 showing the AC signal from a Crankshaft Position (CKP) sensor with a sync pulse (used to synchronize crank and cam position signals). The red waveform is being probed with my positive scope lead connected to CKP "high" and my common lead connected to chassis ground. As you can see, the sync pulse falls first. If I connect my positive scope lead to CKP "low" and my common lead to chassis ground, the sync pulse seems to rise first.

If you expect the ECU to recognize this sync signal, the two harness connections must be made in the correct polarity. More than once I have seen drivability problems resulting from CKP wiring harness repairs that reversed the polarity.

The first time I learned how polarity could be a problem, I was using a scope to check a wheel speed sensor output to the vehicle's dedicated Analog to Digital (A/D) converter, and the signal looked good going into the A/D converter. I hooked up the second channel of my scope to see the output square wave of the A/D converter. Suddenly, I had no wheel speed sensor signal and no corresponding square wave signal from the A/D converter.

The AC signal went almost completely flat. It turns out that the factory schematic had the sensor connections "high" and "low" mislabeled. This means that when I hooked up the positive scope channel, the high side signal shorted to ground through the scope's common ground lead.

Amplitude Matters

When scoping a VR sensor, it is essential that you probe the circuit while the sensor is still connected to its ECU or external A/D converter. The ECU or A/D converter will act as a load or resistive current path to ground on the sensor's signal-output circuit. If the sensor signal is weak — due to a cracked magnet, degraded coil windings, high circuit resistance or excessive clearance between the sensor and target wheel — the output signal will suffer amplitude loss when connected to its intended load.

However, the same sensor may show perfectly good signal amplitude when disconnected from its intended load and tested with a scope. A good signal while the sensor is not connected to a load can lead you to conclude the sensor is good when in fact it is faulty.

In Figure 2, the waveform on the left is a CKP sensor probed while the sensor is still connected to its intended load, an ICM. As you can see, the signal output amplitude only measures 250mV peak-to-peak. Looking at the waveform on the right, we see that when the CKP sensor is disconnected from the ignition module, the signal output amplitude jumps up to 7V peak-to-peak. This same loaded and unloaded testing caution applies when using a DMM to compare the sensor's AC voltage output to a published DMM AC voltage output specification.

Too much signal amplitude caused by the sensor being mounted too close to the target wheel can also cause a drivability issue. Too much signal amplitude can cause a problem in the ECU sensor input interface, which can cause incorrect dwell time or ignition timing. This can happen either due to excessive noise generated by the sensor or due to incorrect input signal "Adaptive Gain" function in the ECU created by the excessive signal amplitude.

Adaptive Gain is a function built into the ECU to adapt to the VR sensor's varying amplitude, which is normal and caused by the varying speed of the sensor target wheel. The ECU must adjust its trigger set-point to compensate for the triggering amplitude occurring earlier as the target wheel speeds up.

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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