Ch. 26

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600 CHAPTER 26 condition is a warm engine being stopped in very cold weather. In this case, when the engine is restarted, the ECT may be near normal operating temperature such as 200' F (93" C) yet the 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 seem to indicate. Testing the Intake Air Temperature Sensor If the intake air temperature sensor circuit is damaged or faulty, a diagnostic trouble code (DTC) is set and the malfunction indicator lamp (MIL) may or may not be on depending on the condition and the type and model of the vehicle. To diagnose the IAT sensor follow these steps: ' , After the vehicle has been allowed to cool for several hours, use a scan tool and observe the IAT and compare it to the engine coolant temperature (ECT) . The two temperatures should be within 5" F of each other. sensor and the wiring. If the IAT is screwed into the intake manifold, remove the sensor and check for damage. ' :',: :: Check the voltage and compare to the following chart. Manifold/lntake Air Temperature Sensor Temperalute vs. Resistance and V0ltage Drop (Approximate) Poor Fuel Economyt Black Exhaust Smoket Look at the lAT. lf 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 :hat is much richer than normal. lf 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 B minutes. lf, 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. . ftfAil{lFCILUl ieffS&LUTE PT{ESSURH The manifold absolute pressure (MAP) sensor is used by the engine computer to sense engine load. The typical MAP sensor consists of a ceramic or silicon wafer sealed on one side with a perfect vacuum and exposed to intake manifold vacuum on the other side. As the engine vacuum changes, the pressure difference on the wafer changes the output voltage or frequency of the MAP sensor. See Figure 26-7. See the following chart that relates engine load, engine vacuum, and MAP. See Figure 26-8. I oc OF 0hms Voltage Drop Across Sensor -40 -40 100,000 4.95 -8 +'18 15,000 4.68 0 32 9,400 4.52 10 50 5,700 4.25 20 68 3,500 3.89 30 86 2,200 3.46 40 104 1.500 2.97 50 122 1,000 2.47 60 140 700 2.00 70 158 500 1.59 80 t/o 300 1.25 90 194 250 0.s7 100 212 200 0.75 Figure 26-7 This MAP sensor is installed on the bulkhead with a vacuum hose attached that runs to the intake manifold. Some MAP sensors are attached directly to the intake manifold.
Computer Sensor Operation, Diagnosis, and Service 601 I MEASUBED IN UNITS OF POUNDS PER SOUARE INCH PSI {KPal 20 PSI 10 PSr ATMOSPHE R IC PBESSU RE --\ O PSIG {PSI GAUGE} MEASURED IN UNITS OF INCHES OF MERCURY (lN. Hg) (mm Hs) ENGINE VACUUM AT WIDE OPEN THROTTLE (w.O.T.) 10 lN. Hg ENGINE VACUUM AT IDLE 20 lN. Hg PERFECT VACUUM Figure 26-8 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. lf lt's Green,It's a Signal lflire Ford-built vehicles usually use a green wire as the signal wire back to the computer from the sensors. lt may not be a solid green, but if there is green somewhere on the wire, then it is the signal wire. The other wires are the power and ground wires to the sensor. Engine Load Heavy (w0T) Manifold Vacuum LOW (almost 0 in. Hg) Manifold Pressure High (almost atmospheric) MAP Sensor Voltage Signa I High (4.6-4.8 V) ?€6EF.: A MAP sensor and a BARO sensor are usuallv i 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 pressurejust as the ignition switch is turned to the on position before the engine starts. Therefore, altitude and u'eather 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 wili 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. See the Chrysler MAP : sensor chart. Light (id le) High (17-21 in. Hs) LOW (lower than LOV/ (0.8-1.6 V) Ford MAP/BAR0 Altilude Chart atmospheric) Allitude (feet) Volts (V) 0 1.59 ffi BAROMETRIC PRESSURE sEt{soR The barometric pressure (BP or BARO) sensor is used by the engine computer to sense the barometric pressure. This input not only allows the computer to adjust for changes in atmospheric pressure due to weather, but also is the primary sensor used to determine altitude. 1,000 1.56 2,000 t_cJ 3,000 t.cu 4,000 1.47 5,000 1.44 6,000 141 7,000 1.39
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Ch. 26

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