Underhood: ABS Diagnostics
Adapted from Dave Darr’s article in Import Car.
Taking A Look Under The Hood
In case you’re not aware, Nissan controls basic engine functions with the use of an Electronic Control Module (ECM), as is the case with other vehicle manufacturers. The ECM controls the length of time that the injectors stay open. (This is known as injector pulse duration.) The amount of fuel injected is a program value in the ECM memory. The program value is preset by engine operating conditions. These conditions are determined by input signals from both the camshaft position sensor and the mass air flow sensor.
The amount of fuel that is injected is compensated for in order to improve engine performance. This is done under various operating conditions as indicated in the following chart.
Mixture Ratio Feedback Control
The Mixture Ratio Feedback system is used for precise control of the air/fuel ratio to the stoichiometric point. The three-way catalyst can better reduce CO, HC and NOx emissions. This system uses a front heated oxygen sensor in the exhaust manifold to monitor the mixture ratio. The ECM adjusts the injector pulse according to sensor voltage signals. This maintains the mixture ratio within the range of stoichiometric.
Mixture Ratio Self-Learning Control System
This system monitors the mixture ratio signal transmitted from the front heated oxygen sensor. This signal is then sent to the ECM, which controls the basic mixture ratio as close to the theoretical mixture ratio as possible. However, the basic mixture ratio is not necessarily controlled as originally designed. Both manufacturing differences and characteristic changes during operation directly affect the mixture ratio. Accordingly, the difference between the basic and theoretical mixture ratio is monitored in this system, which is then computed in terms of injector pulse duration to automatically compensate for the difference between the two ratios.
As you know, injector clogging and/or incorrect throttle position voltage, engine coolant voltage or mass air flow voltage could result in a driveability problems.
Mass Air Flow (MAF) Sensor
Nissan uses a two-wire Mass Air Flow sensor to measure the amount of air entering the engine and its temperature. The sensor operates under the hot-wire technique, in which the ECM sends voltage to the sensor to keep the wires at a given temperature. As the air moves past the wire, it cools and requires more voltage to keep it hot. This increased voltage is then calculated into the grams per second of air entering the engine and its temperature.
Coolant Temperature Sensor
The engine coolant temperature sensor is used to detect the engine coolant’s temperature. The sensor modifies a voltage signal from the ECM. The modified signal returns to the ECM as the engine coolant temperature input. The sensor uses a thermistor, which is sensitive to the change in temperature. The electrical resistance of the thermistor decreases as the temperature increases.
The below chart should be helpful in plotting the coolant sensor against a correct set of specs, when diagnosing a no-start condition.
Throttle Position Sensor
The throttle position sensor (TPS) responds to the accelerator pedal movement. This sensor is a potentiometer that transforms the throttle position into output voltage and emits the voltage signal to the ECM. In addition, the sensor detects the opening and closing speed of the throttle valve and sends the voltage to the ECM.
Idle position of the throttle valve is determined by the ECM receiving the signal from the throttle position sensor. This sensor controls engine operations such as fuel cutoff. In addition, for models other than non-California manual transmission models, the wide open and closed throttle position switches are built into the throttle position sensor unit.
O2 Sensor – The ECM’s Scorecard
If you think of the oxygen sensor as a scorecard for the computer, you can understand why most import auto manufacturers require the replacement of this sensor at 40,000 and 80,000 miles. The oxygen sensor lives in a harsh environment where exhaust temperatures can reach more than 1,000 F. It’s not that we will see a complete failure of this sensor under such conditions, but more of a slowing response, or slow switching characteristics.
When the oxygen sensor is cold (under 600 F), it will not produce any voltage. The Electronic Control Module (ECM) will run the engine from an internal program using coolant temperature, throttle position voltage and mass air flow values, all the while keeping the car within the federal emissions levels. This is also referred to as “open loop.” The oxygen sensor is so critical to the operation of the engine, that engineers have now installed a heating element around the sensor to help it reach the 600-degree operating temperature faster.
Replacement of the oxygen sensor will provide a faster switching voltage. Any time the ECM increases the fuel injector “on” time (rich condition), the oxygen sensor will produce more than 500 millivolts, and under 500 millivolts any time the engine runs lean. This function happens often – five to 10 times a second – so if we allow this sensor to slow its response rate, we also will slow the response of the ECM.
This sensor will not cause a no-start or hard-start condition, or even an engine misfire, it only tells the ECM how well of a job it’s doing.
Here is a great little test for this sensor:
Unplug the wiring harness to the oxygen sensor.
Place a paper clip into the ECM end of the wiring plug.
Hold the paper clip between your thumb and forefinger, and using the forefinger of your other hand, touch the negative battery terminal. This will send low voltage to the ECM, causing it to see a lean engine and respond by increasing the injector pulse and running the engine rich.
By placing your finger on the positive battery post you will, in effect, send a high-voltage signal to the ECM, which will see this as a rich condition and respond by decreasing the fuel injector “on” time, sometimes even stalling the engine. You can perform this test with confidence because your body will absorb all of the amperage from the battery.