Cam and Crank Sensors

Cam and Crank Sensors... PAGEREF _Toc135836909 \h 1 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F0054006F0063003100330035003800330036003900300039000000

The VRT Sensor PAGEREF _Toc135836910 \h 1 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F0054006F0063003100330035003800330036003900310030000000

Hall Effect Sensor PAGEREF _Toc135836911 \h 3 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F0054006F0063003100330035003800330036003900310031000000

Photo Cell Sensor PAGEREF _Toc135836912 \h 25 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F0054006F0063003100330035003800330036003900310032000000

Magneto Resistive Sensor PAGEREF _Toc135836913 \h 34 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F0054006F0063003100330035003800330036003900310033000000

Duplicating a Symptom... PAGEREF _Toc135836914 \h 52 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F0054006F0063003100330035003800330036003900310034000000

An Alternate Use for Cam/Crank Signals. PAGEREF _Toc135836915 \h 56 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F0054006F0063003100330035003800330036003900310035000000

Conclusion. PAGEREF _Toc135836916 \h 77 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F0054006F0063003100330035003800330036003900310036000000

Speed and position sensors have many uses on todays vehicles. You will find them as ABS wheel speed sensors, transmission turbine speed sensors, vehicle speed sensors, but perhaps the most important are the cam and crank position sensors. These two sensors are used to determine the engine speed and the position of the camshaft or crankshaft. The PCM uses this information to determine when to fire the spark plugs and when to spray the fuel injectors.

There are several types of cam and crank sensors that work in different ways. They include the Variable Reluctance Transducer, which works much like a standard magnetic pick-up, the Hall Effect sensor, the Photo Cell sensor and the Magneto Resistive sensor. Learning to view and analyze these sensor signals on a scope is a key factor in diagnosing drivability problems.

The VRT Sensor

First, lets examine a VRT type sensor. These sensors can have two or three wires connected to them. If a VRT has three wires, one of these wires is only a shield wire used to keep electrical noise from interfering with the signal. A VRT works like a magnetic pick-up found in many older distributors. It generates its own voltage and needs no external power supply. A ferrous reluctor (toothed gear) passes by the sensor to generate an AC voltage signal. As a tooth approaches the crank sensor, the magnetic field fluctuates causing the sensor to begin producing a voltage. The voltage will rise to different levels depending on the speed of the reluctor. On some vehicles, the VRT may only reach 2 or 3 volts while cranking the engine at 250 RPM, but may reach 30 volts or more while revving the engine to high RPMs. The voltage then begins to fall and reaches the 0 volt mark in a downward direction just as the tooth is perfectly aligned with the sensor. As the tooth moves away from the sensor, the voltage continues down past zero to negative voltage. Take a look at the capture below to better understand the pattern generated.