Ch. 26

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614 CHAPTER 26 CHECK THE SNORKELTUB€ HERT FOR CRACKS" Figure 26-26 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. 15?F I i What ls Meant by i a'5High-Authority Sensort't I A high-authority sensor is a sensor that has a major influj ence over the amount of fuel being delivered to the engine. For example, at engine start-up, the engine coolant temperature (ECT) sensor is a high-authority sensor and the oxygen sensor (O2S) is a low-authority senson However, as the engine reaches operating temperature, the oxygen sensor becomes a high-authority sensor and can greatly affect the amount offuel being supplied to the engine. See the chart. High-Authority Sensors ECT (especially when the engine starts and is warmins up) O2S (after the engine reaches closed-looo oDeration) MAP MAF TP Low-Authority Sensors IAT (intake air temperature) sensors modify and back uo the ECT TFT (transmission fluid temperature) PRNDL (shift position sensor) KS (knock sensor) Because this extra air is unmeasured, the computer does not provide enough fuel delivery and the engine operates too lean, especially at idle. A small hole in the air inlet hose would represent a fairly large percentage of false air at idle, but would represent a very small percentage of extra air at highway speeds. To diagnose for false air, look at long-term fuel trim numbers at idle and at 3000 RPM. Tap Test With the engine running at idle speed,gently tap the MAF sensor with the frngers of an open hand. If the engine stumbles or stalls, the MAF sensor is defective. This test is commonly called the tap test. Digital Meter Test of a MAF Sensor A digital multimeter can be used to measure the frequency (Hz) output of the sensor and compare the reading with specifications. The frequency output and engine speed in RPM can also be plotted on a graph to check to see ifthe frequency and RPM are proportional, resulting in a straight line on the graph.
Computer Sensor Operation, Diagnosis, and Service 6l5 ffi SENSOR TESTING USING DIAGNOSTIC TROUBTE CODES Many vehicles display diagnostic trouble codes (DTCs), yet do not display scan data. To check if the problem is the sensor itself or the electrical sensor circuit that is at fault, follow these steps. 1. Clear the DTC. 2. Create the opposite sensor condition. For example, if the DTC indicates an open engine coolant temperature (ECT) circuit, unplug the sensor and, using a jumper rvire, short the two terminals of the harness (not the sensor) together. -=1-===; If the ECT sensor wires are shorted togethea i the scan tool will display about 300' F (150' C) and i about -40' F (-40' C) ifthe sensorwires are open (dis- I connected). : 3. Start the engine and drive the vehicle. r If the computer set a code, the wiring is okay' Check the connectors and the sensor. r If the computer did not set a code, check the wiring for a break or an open in the circuit' When checking three-wire sensors' such as the throttle position (TP) sensor, MAP, or MA$ use a jumper wire to jump the 5-volt reference back into the signal return after disconnecting the connector from the sensor. Shorting the 5-volt reference to the signal should cause the vehicle computer to set a shorted sensor DTC. If a shorted sensor DTC is stored, simply clear the DTC and unplug the sensor. If the wiring is okay, the opposite (open) sensor DTC should be set. The Unplug lt Test lf a sensor is defective yet still produces a signal to the computer,the computer will often accePt the reading and make the required changes in fuel delivery and spark advance. lf, however, the sensor is not reading correctly, the computer will process this wrong information and perform an action assuming that information being supplied is accurate. For example, if a mass airflow (MAF) sensor is telling the computer that l2 grams of air per second is going into the englne,the comPuter will then pulse the iniector for 6.4 ms or whatever figure it is programmed to provide. However, if the air going into the engine is actually | 4 grams per second, the amount of fuel supplied by the injectors will not be enough to provide ProPer engine operation. lf the MAF sensor is unplugged,the comPuter knows that the sensor is not capable of supplying airflow information, so it defaults to a fixed amount of fuel based on the values of other sensors such as the TP and MAP sensors. lf the engine operates better with a sensor unplugged, then suspect that the sensor is defective. A sensor that is not supplying the correct information is said to be skewed. The computer will not see a diagnostic trouble code for this condition because the computer can often not detect that the sensor is supplying wrong information. :
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Ch. 26

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