13. Figure 55–4 The transistors between steps usually occur at a temperature that would not interfere with cold engine starts or the cooling fan operation. In this example, the transition occurs when the sensor voltage is about I volt and rises to about 3.6 volts.
24. Figure 55–7 A chart showing the voltage decrease of the ECT sensor as the temperature increases from a cold start. The bumps at the bottom of the waveform represent temperature decreases when the thermostat opens and is controlling coolant temperature. Continued
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32. NOTE: Some engines use a throttle - body temperature ( TBT ) sensor to sense the temperature of the air entering the engine, instead of an intake air temperature sensor. In a warm engine stopped in very cold weather, when the engine is restarted, the ECT may be near normal temperature such as 200(F (93°C) yet air temperature could be –20°F (–30°C). In this case, the engine requires a richer mixture due to the cold air than the ECT would indicate. Continued
33. If the intake air temperature sensor is defective, it may be signaling the computer that the intake air temperature is extremely cold when in fact it is warm. In such a case the computer will supply a mixture that is much richer than normal. If a sensor is physically damaged or electrically open, the computer will often set a diagnostic trouble code (DTC). This DTC is based on the fact that the sensor temperature did not change for a certain amount of time, usually about 8 minutes. If, however, the wiring or the sensor itself has excessive resistance, a DTC will not be set and the result will be lower-than-normal fuel economy, and in serious cases, black exhaust smoke from the tailpipe during acceleration. Poor Fuel Economy? Black Exhaust Smoke? Look at the IAT
59. Figure 55–15 Checking voltage drop between TP sensor ground and a good engine ground with the ignition on (engine off). Reading greater than 0.6 volt (600 mV) represents a bad computer ground. Figure 55–14 Checking the 5-volt reference from the computer being applied to the TP sensor with ignition switch on (engine off). Check Power and Ground Before Condemning a Bad Sensor - Part 2
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64. Figure 55–16 (a) As an engine is accelerated under a load, the engine vacuum drops. This drop in vacuum is actually an increase in absolute pressure in the intake manifold. A MAP sensor senses all pressures greater than that of a perfect vacuum. Continued
65. Figure 55–16 (b) The relationship between absolute pressure, vacuum, and gauge pressure.
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75. Figure 55–19 A waveform of a typical digital MAP sensor. See the charton Page 638 of your textbook.
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77. Figure 55–20 A ceramic-disc-type MAP sensor showing the substrate and the circuit. Continued
79. A MAP sensor measures the pressure inside the intake manifold compared with absolute zero (perfect vacuum). For example, an idling engine that has 20 inches of mercury (in. Hg) of vacuum has a lower pressure inside the intake manifold than when the engine is under a load and the vacuum is at 10 in. Hg. A decrease in engine vacuum results in an increase in manifold pressure. A normal engine should produce between 17 and 21 in. Hg at idle. Comparing the vacuum reading with the voltage reading output of the MAP sensor indicates that the reading should be between 1.62 and 0.88 volt or 109 to 102 Hz or lower on Ford MAP sensors. Therefore, a digital multimeter (DMM), scan tool, or scope can be used to measure the MAP sensor voltage and be used instead of a vacuum gauge. Use the MAP Sensor as a Vacuum Gauge
80. See the chart on Page 639of your textbook. NOTE: This chart was developed by testing a MAP sensor at a location about 600 feet above sea level. For best results, a chart based on your altitude should be made by applying known vacuum, and reading the voltage of a known-good MAP sensor. Vacuum usually drops about 1 inch per 1,000 feet of altitude.
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87. NOTE: A MAP sensor and a BARO sensor are usually the same sensor, but the MAP sensor is connected to the manifold and a BARO sensor is open to the atmosphere. The MAP sensor is capable of reading barometric pressure just as the ignition switch is turned to the on position before the engine starts. Therefore, altitude and weather changes are available to the computer. During mountainous driving, it may be an advantage to stop and then restart the engine so that the engine computer can take another barometric pressure reading and recalibrate fuel delivery based on the new altitude. See the Ford MAP/BARO altitude chart for an example of how altitude affects intake manifold pressure. The computer on some vehicles will monitor the throttle position sensor and use the MAP sensor reading at wide-open throttle (WOT) to update the BARO sensor if it has changed during driving.
88. Continued See the chart on Page 640 of your textbook. NOTE: Some older Chrysler vehicles were equipped with a combination BARO and IAT sensor. The sensor was mounted on the bulkhead (firewall) and sensed the under-hood air temperature.
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93. A defective vacuum hose to a MAP sensor can cause a variety of driveability problems including poor fuel economy, hesitation, stalling, and rough idle. A small air leak (vacuum leak) around the hose can cause these symptoms and often set a trouble code in the vehicle computer. When working on a vehicle that uses a MAP sensor, make certain that the vacuum hose travels consistently downward on its route from the sensor to the source of manifold vacuum. Inspect the hose, especially if another technician has previously replaced the factory-original hose. It should not be so long that it sags down at any point. Condensed fuel and/or moisture can become trapped in this low spot in the hose and cause all types of driveability problems and MAP sensor codes. Visual Check of the MAP Sensor
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107. Figure 55–27 A Karman Vortex airflow sensor uses a triangle-shaped rod to create vortexes as the air flows through the sensor. The electronics in the sensor itself converts these vortexes to a digital square wave signal. Continued
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114. Figure 55–28 Carefully check the hose between the MAF sensor and the throttle plate for cracks or splits that could create extra (false) air into the engine that is not measured by the MAF sensor. Continued
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128. The owner of a 1996 Chevrolet pickup truck complained that the engine ran terribly. It would hesitate and surge, yet there were no diagnostic trouble codes (DTCs). After hours of troubleshooting, the technician discovered while talking to the owner that the problem started after the transmission had been repaired, yet the transmission shop said that the problem was an engine problem and not related to the transmission. A thorough visual inspection revealed that the front and rear oxygen sensor connectors had been switched. The computer was trying to compensate for an air–fuel mixture condition that did not exist. Reversing the O 2 S connectors restored proper operation of the truck. The Chevrolet Pickup Story
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131. Oxygen sensors are numbered according to their location in the engine. On a V-type engine, heated oxygen sensor number 1 (HO2S1) is located in the exhaust manifold on the side of the engine where the number one cylinder is located. Where is HO2S1? Figure 55–34 Number and label designations for oxygen sensors. Bank 1 is the bank where cylinder number 1 is located. Continued
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139. Some manufacturers such as GM have the computer apply 450 mV (0.450 V) to the O 2 S signal wire. This voltage is called the bias voltage and represents the threshold voltage for the transition from rich to lean. This bias voltage is displayed on a scan tool when the ignition switch is turned on with the engine off. When the engine is started, the O 2 S becomes warm enough to produce a usable voltage and bias voltage “disappears” as the O 2 S responds to a rich and lean mixture. What happened to the bias voltage that the computer applied to the O 2 S? The voltage from the O 2 S simply overcame the very weak voltage signal from the computer. This bias voltage is so weak that even a 20-megohm impedance DMM will affect the strength enough to cause the voltage to drop to 426 mV. Other meters with only 10 megohms of impedance will cause the bias voltage to read less than 400 mV. Therefore, even though the O 2 S voltage is relatively low powered, it is more than strong enough to override the very weak bias voltage the computer sends to the O 2 S. What Happens to Bias Voltage? - Part 2
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144. Figure 55–37 Testing an oxygen sensor using a DMM set on DC volts. With the engine operating in closed loop, the oxygen voltage should read over 800 mV and lower than 200 mV and be constantly fluctuating. Continued
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147. Figure 55–38 Using a digital multimeter to test an oxygen sensor using the min/max record function of the meter. See the chart onPage 649 of your textbook.
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155. Figure 55–42 Using the cursors on the oscilloscope, the high- and low-oxygen sensor values can be displayed on the screen. Continued
156. Figure 55–43 When air–fuel mixture rapidly changes, look for a rapid response. Transition from low to high should be less than 100 ms. NOTE: GM warns not to base diagnosis of oxygen sensor problems solely on scope pattern. Varying voltage output of can be mistaken for a fault in the sensor itself, rather than a fault in the fuel delivery system.
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164. CAUTION: Do not spray any silicone spray near the engine where the engine vacuum could draw the fumes into the engine. This can also cause silica damage to the oxygen sensor. Also be sure that the silicone sealer used for gaskets is rated oxygen sensor safe.
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