Vehicle Speed Sensor: Testing and Inspection

Permanent Magnet Vehicle Speed Sensor:

BACKGROUND AND APPLICATION INFORMATION:
Vehicle Speed Sensors can be AC signal generators like the one above, or they can be digital Hall Effect, or even digital optical sensor. The magnetic AC-type, like the one above, is an analog sensor. They are used primarily because they are very reliable, perform adequately, and are inexpensive to manufacture.

These sensors can be found mounted in transaxle final drive cases, transmission tail housings, differential housings and the like. They are two wire sensors, but in a few cases their two wires are encased in a shielded, wire-braid conduit. This is because their signal is somewhat delicate, in electronic terms, and is susceptible to Electromagnetic Interference (EMI or RFI) from other electronic devices on the vehicle. EMI and RFI can distort the signal's Critical Dimensions and create an "electronic communication" breakdown causing a driveability or other problem.

CONNECT AND SETUP THE DIGITAL STORAGE OSCILLOSCOPE (DSO):
1. Connect the "COM" probe to the sensor wire labeled (-) or "low" in the wiring diagram.
2. Connect the "CH1" probe to the sensor wire labeled (+) or "high" in the wiring diagram.
3. Set the voltage scaling to 5 V/div. Alter as needed later.
4. Set the time base to 2 ms/div. Alter as needed later.
5. Select "GND" coupling and position the trace on the division line in the center of the DSO display.
6. After ground position is set, select "AC" input coupling.
7. Select "NORMAL" acquire mode.
8. Set the "TRIGGER" mode to "AUTO."

EXERCISE THE SENSOR
Raise the drive wheels off the ground to simulate driving conditions, or use DSO lead extensions to enable waveform viewing while driving.

Good Vehicle Speed Sensor Pattern (Typical):

EXPECT THIS WAVEFORM RESULT
As the wheels begin to spin, the flat line at the center of the DSO display will start oscillating above and below zero volts, getting bigger and bigger with vehicle speed. A waveform very similar to the sample should appear. This waveform was logged (saved to software) at about 30 MPH. Unlike some other AC waveforms, like crankshaft or camshaft sensors, Vehicle Speed Sensors (VSS) that are AC signal generators make waveforms whose shapes all look and behave very similar. Generally, the oscillations (the "ups and downs" in the waveform) will be very close to mirror images of each other above and below the zero level mark (symmetrical). The amplitude of the VSS's AC signal should increase with increasing vehicle speed. The higher the MPH, the taller the waveform. And, as MPH increases, frequency increases, meaning that more oscillations show up on the DSO display. What the diagnostician is really looking for is assurance that the critical dimensions of amplitude, frequency and shape are all intact, repeatable, regular and predictable. This means that the amplitude of the peaks has to be sufficient, the time between pulses repeatable, and the shapes repeatable and predictable. Ragged peaks or spikes may be caused by the sensor magnets hitting the sensor's reluctor wheel, which could be caused by a worn out sensor shaft bushing or out-of-round transmission component. Missing peaks may be caused by damaged or missing reluctor teeth.

The 5 Critical Dimensions

IMPORTANT NOTE: Peak to peak voltages will vary a little from one type of sensor to the next. Also, since an integral part of the sensor is a coil, or winding, their failures can be very temperature or vibration dependent. In most of these cases the waveform will become much smaller or very deformed, and a DTC (Diagnostic Trouble Code) may set at the same time the waveform takes on new shape. In general though, the most common type of VSS failure is where the sensor won't generate a signal at all -- it's completely inoperative. Not much to look at, but when driving the vehicle, if the waveform stays flat, check the DSO and sensor connections, make sure the circuit isn't grounded, make sure the proper parts are spinning (plastic gears are not stripped, etc.), maker sure sensor air gap is correct, then condemn the sensor.


FOR MORE INFORMATION
Description of Automotive Signals
Diagnosis Using A Labscope
Labscope Quick Reference Appendix