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Making Sense Out of Sensors

Making Sense Out of Sensors. Jim Halderman Dayton, Ohio. We started communicating by writing on the cave walls. As the years went by we progressed. We invented tools to make communication better. Communication through Sensors.

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Making Sense Out of Sensors

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  1. Making Sense Out of Sensors Jim Halderman Dayton, Ohio

  2. We started communicating by writing on the cave walls.

  3. As the years went by we progressed. We invented tools to make communication better.

  4. Communication through Sensors The PCM uses sensors to learn what is happening in the engine.

  5. Sensor Basics • How does a computer “know” what is going on under the hood? • How does the voltage change inside the PCM if changes occur at the other end?

  6. Pull Up Resistors • The voltage changes at the sense terminal inside the PCM after the resistor.

  7. Pull Down Resistor • The voltage changes if the switch is open or closed.

  8. Resistor Network • Resistances can be used to simplify inputs. • Ford Manual Lever Position (MLP) is an example.

  9. What would happen if some resistance were added to the circuit?

  10. If the resistance increases, what will happen if you are driving at highway speeds in drive?

  11. Temperature Sensors • Use a semiconductor material that becomes more conductive as the temperature increases. • This is called a Negative Temperature Coefficient (NTC) thermistor.

  12. Temperature Sensors

  13. PCM • What will the PCM read if unplugged? • What will the PCM read if the wire going to the sensor is shorted to ground?

  14. ECT • ECT should read the same at IAT at key on engine off (KOEO).

  15. Quick and Easy Metric Temperature Conversion • Double the Celsius degree number • Add 25 • Should be close to the Fahrenheit temperature • Example: 50 degrees X 2= 100+25=125 • Actual= 122

  16. A GM Stepped ECT Switches at 1.0 volt (120°F).

  17. ECT Sensor • Is the sensor used in a stepped ECT circuit different from the sensor used in a one-step circuit? • How many wires are used in a two-step ECT sensor?

  18. ECT Sensor Authority • The ECT sensor is a high-authority sensor especially at engine start • Helps to determine the base pulse-width • Can account for up to 60% of the pulse-width determining factors

  19. Intake Air Temperature (IAT) • Similar to the ECT sensor except it has opening for the airflow. • Is used by the PCM to modify the fuel and spark timing program based on the temperature of the air entering the engine

  20. IAT Sensor Authority • The IAT is usually considered to be a low-authority sensor • However it is usually capable of causing the PCM to add up to 20% to the injector pulse-width if the incoming air is cold • The IAT can cause the PCM to reduce the injector pulse-width by as much as 20% if very hot air in entering the engine.

  21. High-Performance IAT SensorTrick • If a 10 K Ohm 1/2 watt resistor is used in the place of the sensor, the PCM will “assume” that the air temperature is about freezing (32 degrees) • This will cause the PCM to advance the timing compared to if the air temperature was warmer (4 to 8 degrees). • Will increase the pulse width up to 20%.

  22. Throttle Position Sensors • Three-wire potentiometer • Five volts from PCM • Signal return • Ground

  23. Testing the TP sensor • Great location to check for five-volt reference (Vref). • General Motors recommends checking for PCM ground voltage drop at the TP sensor. (There should be less than 0.035 volts between the TP sensor ground terminal and the negative terminal of the battery.)

  24. TP Sensor Waveform(Defective)

  25. TP Sensor Authority • The TP sensor can cause the PCM to command up to 500% (5 times) the base pulse width if the accelerator is depressed rapidly to the floor • Can cause the PCM to reduce the pulse width by up to 70% if the throttle is rapidly closed

  26. MAP versus Vacuum

  27. MAP Sensor • Manifold Absolute Pressure (MAP) • A decrease in manifold vacuum means an increase in manifold pressure. • Compares manifold vacuum to a perfect vacuum.

  28. Silicon-Diaphragm Strain Gauge Design MAP Sensor • Most commonly used. • Silicon wafer is exposed to engine vacuum. • This results in changes in resistance due to strain on the resistors attached to the wafer (called Piezo-resistivity). • Resistors are connected to a Wheatstone bridge and then to a differential amplifier, which creates a voltage in proportion to the vacuum applied.

  29. Silicon-Diaphragm MAP Sensor

  30. MAP Voltage • Normal engine Vacuum is 17-21 in. Hg. • MAP sensor voltage is normally between 0.88 volts to 1.62 volts (GM). • 17 in. Hg. is equal to about 1.62 volts. • 21 in. Hg. is equal to about 0.88 volts. • Therefore, a good reading should be about 1 volt.

  31. Capacitor-Capsule MAP Sensor • Used by Ford. • Uses two alumina plates with an insulating washer spacer to create a capacitor. • The deflection due to engine vacuum changes the capacitance. • The electronics in the sensor translate this into a frequency output.

  32. Ford Frequency versus Vacuum • KOEO…………. 156-159 Hz (0 in. Hg.) • Idle (sea level).. 102-109 Hz (17-21 in. Hg.) • WOT……………. 156-159 Hz (Almost 0 in. Hg.)

  33. Ceramic Disc MAP • Used by DaimlerChrysler. • Ceramic disc converts manifold pressure into a capacitive discharge. • The discharge controls the amount of voltage drop delivered by the sensor to the PCM. • The output is the same as the previously used strain gauge/Wheatstone bridge design.

  34. Ceramic Disc MAP

  35. MAP versus BARO • KOEO MAP should equal BARO. • Will vary with altitude and weather conditions. • The BARO reading is set at key on and updated if the throttle is detected to be at WOT and will update the BARO reading.

  36. Testing a MAP Sensor • Key on – engine off (KOEO). Voltage should be 4.6 to 4.8 volts at sea level. • Check for vacuum to the sensor. • Check the hose. • Replace the MAP sensor if anything comes out of the sensor.

  37. MAP Sensor Authority • The MAP sensor is a high-authority sensor on an engine that uses the Speed-Density method of fuel control. • If the exhaust is rich, try disconnecting the MAP sensor. • If the engine now runs OK, then the MAP sensor is skewed or giving the PCM wrong information.

  38. High-Performance MAP Sensor Trick • Insert a plastic vacuum fitting into the vacuum line to the MAP sensor • Use a hot straight pin and burn a small hole in the plastic fitting creating a small vacuum leak • Do not exceed 0.020 inch hole • PCM “assumes” a higher engine load and increases the injector pulse-width

  39. Air Vane Sensor • Usually contains an internal IAT sensor • Works similar to a TP sensor where the air vane is used to move a potentiometer • Airflow moves the vane, which causes a switch to close to power the fuel pump.

  40. Air Vane Sensor This is not a mass air flow sensor.

  41. Karman-Vortex • Named for Theodore Van Karman, a Hungarian scientist (1881 – 1963). • He observed the vortex phenomenon in 1912. • This type of sensor has proven to be very reliable and not subject to dirt.

  42. Karman-Vortex This is not a mass air flow sensor.

  43. Ultrasonic Karman Vortex • Used by Mitsubishi in many vehicles. • Very reliable. • Early versions used LEDs and phototransistors, which were subject to dirt.

  44. Pressure-Type Karman Vortex • DaimlerChrysler uses a Karman Vortex sensor that uses a pressure sensor to detect the vortexes. • As the flow increases, so do the number of pressure variations. • The electronic circuitry in the sensor converts these pressure variations to a square wave signal that is proportional to the airflow through the sensor.

  45. Mass Air Flow • A hot wire is used to measure the mass of the air entering the engine. • The electronics, in the sensor itself, try to keep the wire 70° C above the temperature of the incoming air. • The more current (amperes) needed to heat the wire, the greater the mass of air. • The current is converted to a frequency.

  46. MAF Sensor

  47. Normal MAF Readings • Use a scan tool to look at the grams per second. • Warm the engine at idle speed with all accessories off. Should read 3 to 7 grams per second. • GM 3800 V-6 should read 2.37 to 2.52 KHz. • If not within this range, check for false air or contamination of the sensor wire.

  48. MAF Sensor Diagnosis • If the MAF sensor wire were to become coated, it cannot measure all of the incoming air. • A normal warm engine at idle should be 3 to 7 grams per second. • Rapidly depress the accelerator pedal to WOT. It should read over: • 100 grams per second or • higher than 7 kHz or • 4 volts

  49. MAF and Altitude Reading Barometric pressure (BARO) is determined by the Powertrain Control Module (PCM) software at WOT. At high airflows, a contaminated MAF sensor will under estimate airflow coming into the engine, and therefore, the PCM determines that the vehicle is operating at a higher altitude.

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