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DIY Auto Service: Permanent Magnet and Hall Effect Sensor Diagnosis and Testing

Updated on March 21, 2016

Crankshaft Position Sensor

The internal tone ring for the Crankshaft Position Sensor (CKT) is part of the crankshaft.
The internal tone ring for the Crankshaft Position Sensor (CKT) is part of the crankshaft.

Speed and Position Sensor Introduction

There are many areas of the vehicle where the speed of rotation needs to be measured. On the electronic engines, speed and engine position are important to fuel injection timing. Some of the speed sensors are; wheel speed (WSS), vehicle speed (VSS), transmission input speed (TIS), transmission output speed (TOS), crankshaft speed and position (CKT) and camshaft speed and position (CMP).

Two types of speed and position sensors used in today’s vehicles; the Permanent Magnet Generator and the Hall Effect Sensor. The Permanent Magnet Generator or Induction Pulse Generator is widely used for Vehicle Speed Sensors, Wheel Speed Sensors, Transmission Speed Sensors and Crankshaft Speed Sensors. The Hall Effect Sensor is used as an accurate speed and position sensor for crankshaft and camshaft position and speed.

The overview of vehicle computer systems is available on a my previous Hub. DIY Auto Service: Automotive Computer System Operation

Truck and Trailer Systems Textbook

PM Generator Operation

A magnet surrounded by a fine wire collects the magnetic lines of force to generate an Alternating Current (AC). As the tone ring rotates with the shaft or wheel, the high and low points are closer and farther away from the magnet. When the surface is closer, a magnetic field is at its strongest. As the high point moves away, it distorts (bends) the lines of force across the magnet. As the lines of force cross the fine wire, AC current is produced.

External Flywheel Tone Ring

This tone ring and crankshaft position sensor are locate behind the flywheel. The wide notch signals a given engine position.
This tone ring and crankshaft position sensor are locate behind the flywheel. The wide notch signals a given engine position.

Permanent Magnet Generators Testing

A good DVOM will be needed to accurately test the resistance and AC output of the sensor. Permanent Magnet Sensors are a two wire sensor. To test the sensor, follow the steps below.

  1. Disconnect the sensor lead.

  2. Set the voltmeter to OHMs.

  3. Measure the resistance of the sensor across the two pins.

  4. Check against the manufacturers specifications. Typically 600 to 3,000Ω.

  5. Check the strength of the magnet by picking up a large paper clip. If the magnet is cracked inside the sensor, the magnetic field will be weak.

  6. To check the sensor output, turn the DVOM to AC Volts.

  7. Rotate the wheel or whatever speed you are measuring.

  8. Place the meter leads across the sensor and measure the AC voltage output.

  9. Typically, if the shaft is rotated at about one turn every 2 seconds the output should be around .7 to 1.0 Volts AC.

  10. If the sensor passes the tests but still has a problem in the circuit, retest at the computer. Use the wiring diagram to locate the pins to test at the computer. Using a T-pin, carefully slide the T-pin into the signal wire connection.

  11. If the wiring has a problem, you may have to replace the harness.

  12. Clear the trouble codes, road test and recheck to confirm the problem is fixed.

Service Tip: Never repair the leads coming away from a Permanent Magnet sensor. This wiring has a shield built in to protect the signal from radio or magnetic interference. Another method of shielding the signal is to twist the pair of wires.

Permanent Magnet Sensor Test

Two tests are available for two wire Permanent Magnet Sensors. AC voltage output and resistance (Ohm's) tests.
Two tests are available for two wire Permanent Magnet Sensors. AC voltage output and resistance (Ohm's) tests.

Permanent Magnet Sensor Problems

The PM Generator style speed sensor produces a very low AC voltage. This voltage would be very easy to have outside interference affect the signal. The loss or interference of the signal could cause; the engine to stop running, the ABS to not work or the speedometer to quit working.To protect the signal, the wires are shielded or twisted to reduce the interference. The wheel speed sensor harnesses typically have shielding built into the harness and should not be repaired only replaced. Twisted wires should be kept twisted as one wire acts like the shield to the other.

Clearances can be an issue between the tone ring and the sensor magnet. Too much clearance and the magnetic field will be weakened. Too little clearance and the sensor could be damaged by the tone ring striking the sensor.

Damage to the sensor magnet could occur if the sensor was hammered on or even just dropped. This could crack the magnet in the sensor which would result in little or no output.

The wiring and connectors could be damaged or corroded. Because this type of sensor produces such a low voltage, a bad connection voltage drop could significantly affect the sensor reading.

Hall Effect Sensor Signal and Voltage Test

As the engine or component is rotated, the signal should go High and Low as the tone ring (windows) pass.
As the engine or component is rotated, the signal should go High and Low as the tone ring (windows) pass.
The Hall Effect Sensor is sent a source voltage. Test the source to ground.
The Hall Effect Sensor is sent a source voltage. Test the source to ground.

Hall Effect Sensor Testing

To test a Hall Effect Sensor you will need an accurate DC Voltmeter set to the 20 Volt scale.

  1. With the sensor plugged in insert a T-pin in the signal wire cavity of the sensor plug.

  2. Bar the engine over or rotate the shaft the sensor is sensing.

  3. Watch the voltmeter.

  4. As it rotates the signal voltage should go back and forth from low voltage to the specified voltage sent to the sensor. Some Crankshaft and Camshaft Tone Wheels have an ODD Notch to signal an engine position.

  5. If there is no signal, unplug the sensor and check for source voltage at the connector.

  6. Check the ground by powering one meter lead and testing for ground with the other.

  7. If the power and ground are good, replace the sensor.

  8. If the sensor has a correct high/low signal but still has a code, use the wiring diagram to identify the Hall Effect sensor signal wire at the computer. Retest for the signal. If there is no signal, repair the harness. If there is a signal at this point but no indication of a signal from the computer, the computer is bad.

  9. Repair the problem and check to see if it works.

  10. Clear the trouble codes, road test and recheck to confirm the problem is fixed.


Service Tip: When testing a Hall Effect Sensor with 5 Volt supply, hook a DC Voltmeter to the signal wire at the Cam or Crank sensor. Bar the engine over and watch the meter. The meter will read 0 then 5 volts DC as the engine is turned.

Hall Effect Sensor Problems

Because the Hall Effect Sensor reports the position and speed of the component, loss of the signal could cause; the engine to stop running or the speedometer to quit working. Most of these problems will also show up as a "Trouble Code" stored in the computer system.

One of the problems that may develop is a clearance problem. The sensor must be within a specified clearance to the tone ring it reads. Typically this clearance is about .020 to .040". Some sensors are adjustable, while others are not. Some adjustable sensors use shims and others may screw into the housing. The crankshaft and camshaft position sensors that are sealed with an O-ring should never have silicone sealer applied as this can act like a shim.

Wiring and connection problems can occur due to the routing and condition of the harness. The connectors are designed to be weather tight to keep out dirt and moisture. If the seal for the connector is damaged or missing, moisture could cause a corrosion problem. A bad connection may cause an intermittent bad connection to set up the problem. Test and repair the problem using the factory recommended procedures for the best results.


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