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
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.
High-Performance IAT Sensor Trick 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%.
Throttle Position Sensors Three-wire potentiometer Five volts from PCM Signal return Ground
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.)
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
MAP Sensor Manifold Absolute Pressure (MAP) A decrease in manifold vacuum means an increase in manifold pressure. Compares manifold vacuum to a perfect vacuum.
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.
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.
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.
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.)
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.
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.
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.
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.
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
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.
Air Vane Sensor This is not a mass air flow sensor.
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.
Karman-Vortex This is not a mass air flow sensor.
Ultrasonic Karman Vortex Used by Mitsubishi in many vehicles. Very reliable. Early versions used LEDs and phototransistors, which were subject to dirt.
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.
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.
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.
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
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.
Visual Inspection Look for a very dirty filter. Look for fuzz on the sensing wire from fibers coming off of the filter paper.
Mass Air Flow (MAF)-False Air Usually affects operation in drive; may run OK if driving in reverse.
MAF Sensor Authority High authority sensor If in Doubt-Take it Out If the MAF sensor is disconnected, the PCM substitutes a backup value. If the engine runs OK with the MAF disconnected, then the MAF has been supplying incorrect information.
Oxygen Sensors Oxygen sensors react to the presence or absence of oxygen in the exhaust. The voltage signal is used by the PCM for fuel control. The mixture must switch from rich to lean for the three-way catalytic (TWC) converter to work.
Do Not Solder O2S Wires Universal oxygen sensors are often used when replacement becomes necessary. Many O2S “breathe” through the wiring itself. If the wires are soldered, then the sensor cannot detect outside oxygen. Use crimp and seal connectors if a universal sensor is used.
Oxygen Sensor Diagnosis If the fuel system is functioning correctly, the oxygen sensor voltage should fluctuate from above 800 mV to below 200 mV. As the oxygen sensor degrades, the range narrows. A sensor should at least be capable of reading above 600 mV and go below 300 mV.
O 2 Sensor Diagnosis (Continued) Use a digital multimeter (DMM) set to read DC volts and use the min/max feature. Back probe the signal wire and operate the engine normally while recording the readings using min/max. The maximum reading should be above 800 mV and the minimum reading below 200 mV.
O 2 Sensor Diagnosis (Continued) Check the average of the O 2 sensor readings: –If the average is above 450 mV, then the engine is operating rich or the sensor is skewed high. –If the average is below 450 mV, then the engine is operating lean or the sensor is skewed low.
Rich to Lean Should switch in less than 100 ms.
A cracked exhaust manifold can cause oxygen to be drawn into the exhaust upstream from the oxygen sensor. An ignition misfire can also cause a false low-oxygen sensor reading. Remember that the oxygen sensor looks at the oxygen in the exhaust, not the unburned fuel!
Antifreeze Contamination If the engine has had a blown head gasket, be sure to check or replace the oxygen sensor. The silicates can coat the sensor. Dexcool and other organic acid technology (OAT) coolants do not cause this problem.
Oxygen Sensor Authority The O2S is a high-authority sensor when the engine is operating in closed loop. Can add or subtract up to 25% from the base pulse width (Some vehicles even more) If the sensor is skewed, it can create a driveability problem. If in doubt, take it out. If the sensor voltage is not connected, the PCM will go into open loop.
High/Low Authority Sensors High-Authority ECT –especially when the engine starts and is warming up. O2S-while the engine is operating in closed loop MAF TP Low-Authority IAT TFT PRNDL KS
Position Sensors Two Types: –Analog sensors - such as magnetic or variable-magnetic sensors –Digital sensors - such as Hall-Effect magnetic-resistive or optical sensors.
The first magnetic sensors were called pulse generators (pickup coils).
The Soldering Gun Trick Hold a soldering gun near a magnetic sensor and the changing magnetic field around the soldering gun will induce a voltage into the windings of the magnetic sensor, The frequency will be 60 Hz. If used on a VSS and the ignition is on, the speedometer will read 54 mph (GM).
Hall-Effect Sensors Effect discovered by Edwin H. Hall in 1879. He discovered that a voltage is created if a magnetic field is exposed to an element. The voltage goes to zero if the magnetic field is shunted or blocked. Very accurate.
Hall-Effect Three-wire sensor –Power –Ground –Signal Output is a square wave. Very accurate and will work at lower speeds than a magnetic sensor.
Produces a square wave signal. Very accurate – can be used to show every 1 degree of crankshaft rotation. Optical sensors do not like light; must be shielded (sparks inside the distributor can cause problems if the sensor is not shielded).