The Hall-Effect Sensor

Many of today's computerized engine control systems are using Hall-Effect sensors, also called Hall-Effect switches, to sense crankshaft and camshaft speed and position. These switches vary in design but are similar in operation. The main differences lie in the voltages at which they work, physical configuration and location on the engine.

The Hall-Effect sensor is a very accurate way for a computer to "see" the exact position or measure the speed of a spinning shaft. Most designs utilize a shutter which passes through an opening in the sensor. The opening has a magnetic field passing across from a permanent magnet to the electronic switch. When the shutter passes through the magnetic field,

it is interrupted and a change in voltage is sensed by the computer. With the shutter in the opening, the voltage falls to near zero. With the shutter out of the opening, the voltage rises to the specified voltage level. This voltage is usually equal to battery voltage on GM, Ford and many Chrysler engines. However, on Chrysler 3.3L, 3.8L and 3.5L engines, the EC sends out an 8 volt power supply and receives a 5 volt-0 volt signal back on the sensor output wire. Some Hall Sensors use a moving magnet attached to a timing chain sprocket (GM) or notches in the flex plate (Chrysler) to generate a signal.


                                                             General Motors 



General Motors, Ford and Chrysler all have engines that utilize dual Hall Sensors in one assembly. Chrysler Turbos have two separate sensors mounted to the distributor base with their own separate three wire connectors. GM and Ford engines utilize dual Hall Sensors in one assembly with one 4 wire connector. Both of these sensors share the 12 volt power wire and the ground. The other two wires are the 12 volt-0 volt output wires for the two sensors. GM also uses separate crank and cam sensors in conjunction with Distributorless Ignition and Sequential Fuel Injection timing.



                                                 3.8L non-turbo camshaft sensor 

Ford vehicles primarily use Hall-Effect sensors to signal the Thick Film Ignition (TFI) module. This is called the Profile Ignition Pick-up (PIP) signal and is relayed to the Electronic Control Assembly (ECA). The Ford DIS system utilizes a second sensor (in the same assembly) to identify number one cylinder. This is called the Cylinder Identification signal (CID).
                                                                     Ford 






 

Chrysler began using Hall Sensors with the introduction of the Omni/Horizon 1.7L engine back in 1978 and continued using it in later models. The sensor was located in the distributor and the shutter was part of the rotor assembly. This type worked off of a 12 volt circuit and sent the signal to an Electronic Ignition Module or to a Spark Control Computer. Later, during the 80's Chrysler introduced the DIS system and the Hall Sensors changed locations and design. A crank sensor, located in the bell housing, reads engine speed as 3 groups of 4 notches pass by the sensor. The cam sensor, located in the front cover reads cam position as notches on the cam sprocket pass by. These Hall Sensors work from an 8 volt power supply and the output is a 5 volt-0 volt signal to the Engine Controller (EC).
Chrysler 


 

                                                 crank sensor 3.3L/3.8L 

All Hall-Effect sensors use three wires to do the job. One wire carries the power voltage and a second wire supplies the sensor ground. Both are supplied by the ignition module or computer. The third wire is the "toggle". This wire is the sensor's output to the computer. The voltage rises, usually to power voltage, and falls to near zero with the movement of the shutter as explained earlier. The signal is a square wave and therefore needs no analog to digital conversion to be read by the computer. The computer measures the time between the pulses and calculates RPM for fuel and timing requirements.

Testing a Hall-Effect sensor is simple if you understand them and have a digital volt-ohm meter or a lab scope. The most difficult part of a test would be accessing the sensor's wires. All technicians should acquire a good set of jumper wires in order to probe the Hall Sensor connectors without damaging the circuit.
                                             Typical Hall-Sensor Circuit 
 



To function correctly, a Hall-Effect sensor must have power voltage and ground applied to it. If the power, ground, or signal wire are open, the sensor can't operate. A short to ground on either the power wire or the signal wire also eliminates the RPM signal.

Mass Air Flow (MAF) Sensor

The Mass Air Flow Sensor is probably the best way to measure the amount of air an engine takes in (engine load). This sensor not only measures the volume of air but also compensates for its density as well. Ford, GM, and many imports are using engine control systems based around this sensor.

 


There are two common designs of MAF sensors used in today's vehicles. One produces a variable voltage output (analog) and the other produces a frequency output (digital). In either case their operation is similar. Both outputs can be measured by a scanner or a digital volt/ohm meter (dvom) that can measure frequency.

Both designs work on the "hot wire" principle. Here's how they work. A constant voltage is applied to the heated film or heated wire. This film or wire is positioned in the air stream or in an air flow sampling channel and is heated by the electrical current that the voltage produces. As air flows across it, it cools down. The heated wire or film is a positive temperature coefficient (ptc) resistor. This means that it's resistance drops when it's temperature drops. The drop in resistance allows more current to flow through it in order to maintain the programmed temperature. This current is changed to a frequency or a voltage which is sent to the computer and interpreted as air flow. Adjustments for air temperature and humidity are taken into consideration since they also affect the temperature of the heated wire or film.

 

GM (Bosch) Hot Wire MAF Sensor 

Humidity always affects the density of air since humid air is denser than dry air. No other compensation is therefore needed for this factor. Air temperature affects density since colder air is more dense than warmer air. Many systems use an air temperature sensor to compensate for this factor since similar amounts of air can enter an engine at different temperatures. Some MAF sensors use an internal "cold" wire to send ambient temperature information to the computer. Some use an intake air temperature sensor in the manifold or the intake piping. This sensor is almost always ntc in design (negative temperature coefficient). That is, it's resistance goes up as air temperature goes down. This "thermistor" works just like a coolant temperature sensor and usually has identical resistance to temperature values. By the way, these values are very different from manufacturer to manufacturer and are available in most repair manuals. They are also programmed into scanner software.




Ford Hot Wire MAF Sensor


Now, as we discussed, the MAF sensor sends either a variable voltage or a changing frequency to the computer. The computer is programmed to accept this information when the car is running in any mode. For example, idle rpm will send a low voltage or low frequency and a high revving engine will send a high voltage or high frequency to the computer along a specific wire (the MAF signal wire). If the signal is not present when it should be and within a programmed parameter, say high voltage at high throttle opening, the computer will set a code.

Vehicle Speed Sensors

The PM (permanent magnet) VSS is usually located on the transmission case in the speedometer cable opening. It will either replace the cable (for electronic speedometers) or mount on the transmission in series with the cable (for mechanical speedometers).

The permanent magnet vehicle speed sensor (VSS) consists of a permanent magnet generator, which produces a pulsing (ac) voltage when it spins. Three miles per hour is approximately the speed at which the ecu recognizes its output. The voltage level and number of pulses increase with vehicle speed.

Since ECU's cannot read ac voltage easily, a buffer module is often used to amplify and convert it to a digital (on/off) signal which can be read easily by the ecu. The buffer module also sends the signal to the electronic cruise control and electronic speedometer systems.

Newer GM, Ford and Chrysler ECU's amplify and convert the ac signal to digital internally and no buffer module is needed.

The computer uses VSS input to operate the following:

1. The idle air control valve

2. The canister purge system

3. The torque converter clutch

4. Cruise control

5. Distance traveled

6. Deceleration

The PM (permanent magnet) VSS circuit can be tested using a scan tester or digital volt meter while driving the vehicle. It also can be checked for proper resistance using a digital ohmmeter. A typical resistance value would be 300 to 700 Ohms*. It can also be tested for proper output by connecting a digital volt meter across the two terminals on the VSS while turning the drive shaft. A typical specified output would be 1.5 to 6.5 volts at 20 mph*.

* Specifications vary. See service manual for correct specifications.




Another type of VSS is the optical VSS. This type is used by GM. It is located inside the speedometer head and is attached to the back of the instrument cluster. It consists of a photo cell and L.E.D. mounted in a housing, which is permanently wired to the buffer circuit (the buffer/L.E.D./photocell is an assembly). A spinning mirrored reflector with two blades completes the system.

The L.E.D. is lighted any time the ignition switch is turned on. As the speedometer cable turns, the reflector blades pass through the light beam from the L.E.D. twice each revolution. The light beam is reflected back to the photo cell. The photo cell generates an electrical signal to the buffer, which indicates the vehicle speed. The buffer then sends a digital (on/off) signal to the computer which interprets the number of pulses per mile as vehicle speed.

Caution: As simple as this system seems to be, trouble codes associated with VSS problems utilize other sensor inputs. Refer to troubleshooting guides for proper diagnosis!