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Nissan

Nissan Driveability Solutions: Repairing the Computer by "Thinking Like the Computer"

Edited from an article by Dave Darr, ImportCar, July 2000

 

Well, you knew it could happen and today is the day. You just sent your scanner out for a repair and the shop scope needs to be updated. Even though diagnostic equipment like that shown in the photo makes our job easier, we’ve all been in this situation and we’ve all struggled through it. I like to keep focused on the problem and try to remember how we all had to do things before computers and scanners.

We will use a common problem. You scheduled a tuneup for a good customer and when he brings in his Nissan he tells you that it’s idling poorly and using too much fuel. Our senses begin working as soon as we start the car to take it on a road test. We notice a strong fuel smell, we see black smoke at the tailpipe and the engine doesn’t idle very well. Just what the customer told us. After the road test and the car is back in the service bay, we notice that there isn’t a "check engine" light on.

The first thing the Nissan repair manual wants us to do is to place the computer in its diagnostic mode. This is an easy task. Locate the Electronic Control Unit (ECU) and on the side of it you will find a small hole with a screwdriver slot inside it. By placing a screwdriver inside the hole and turning it you can select five different operating modes.

Turn the ignition switch on, then turn the diagnostic mode selector on the control unit fully clockwise and wait for the inspection (LED) lamps to flash. Count the number of flashes until the inspection lamps have flashed the number of the mode you desire, then immediately turn the selector fully counterclockwise.

Mode 1
This mode allows us to monitor the oxygen sensor for proper switching. The green LED located on the side of the ECU will flash on and off. In a closed-loop condition, the LED is on when a lean condition exists and off when it senses a rich condition.

Mode 2
While in this mode the ECU checks optimum control of the mixture ratio. The red LED flashes according to the different conditions of the mixture. If the LED remains either on or off (doesn’t flash) during closed-loop operations, the mixture ratio isn’t being properly controlled. Inspect the idle, ignition timing and idle mixture for misadjustment.

Mode 3
The ECU is constantly checking all sensors and actuators. When a fault occurs, the ECU stores it in memory. Codes are retrieved by watching the red and green LED lights flash. First the red LED flashes, then the green. The red LED corresponds to units of 10 and the green LED to units of 1.

Mode 4
In this mode, the ECU monitors the on/off condition of the idle switch, starter switch and the output (above and below 12 mph) of the vehicle speed sensor.

Mode 5
While in the test-drive mode the ECU monitors the crank angle sensor, ignition signal, air flow signal and fuel pump, and these systems will set codes.

Now that we know how to use the ECU to help us, let’s start by placing the ECU in Mode 1 and checking the oxygen sensor signal.

After entering closed-loop, we can see that the oxygen sensor isn’t changing very well. At this point, some technicians would replace the sensor, but we know better. Let’s place the ECU in Mode 2 and check it. The red LED is blinking on and off in closed-loop. The blinking LED tells us that the ECU is trying to control the fuel mixture. Placing the mode selector in the number 3 position will certainly tell us which sensor has failed, but the LEDs tell us nothing of the kind. Then it must be Mode 5 — remember crank angle, air flow, ignition signal and fuel pump. The test showed nothing! Call 911.

Now we must do things the hard way. First, let’s stop and think of what we have — an oxygen sensor that is not switching very well, a rich condition (black smoke from the exhaust) and poor gas mileage.

Plugs? Remember that if a plug will not ignite the air-fuel charge in the cylinder, the fuel will not burn. But the air in the cylinder did not burn either, so the exhaust will actually go lean during a misfire. Fuel pump? We already have too much fuel and if it had low pressure we wouldn’t be running rich. Fuel pressure regulator? Could be. It’s easy enough to check. Pull off the vacuum hose and look for fuel. No fuel. EGR? Another easy check. Remove the vacuum hose and apply a vacuum. It’s not the EGR. Vacuum leak? The engine idle will be increased.

This is starting to get difficult. What could cause the engine to run rich? So rich that the computer can’t control the system and it didn’t store any codes. It didn’t store any codes. That seems strange that the computer didn’t see a fault. Or maybe the computer doesn’t have any reason to see a fault.

Let’s try another angle. Think about what could send the computer data that would cause it to increase the fuel mixture and not be considered a fault? Throttle Position Sensor? That will set a code. Coolant sensor? Easy to check. Unplug it and jump the wires to show a hot engine. Nope!

What type of load device does the computer use? Mass Air Flow. What if the computer is seeing a load on the engine. Would it increase the fuel mixture? YES! I like the word yes.

Nissan uses a two-wire Mass Air Flow sensor to measure the amount of air and its temperature. The sensor uses the "hot-wire" technique to operate, which means that the computer 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. This sensor will cause the computer to set a fault code if it fails, not if it is degraded.

How can we check this sensor without a scanner? First, you must remove it from the air filter housing. Although Nissan uses tamperproof screws, torx head with a pin in the center, tool manufacturers make the sockets in quarter-inch drive so you can remove them, or you can fabricate your own. After carefully removing it, unplug it and hold it up to a good light. Then check the hot wires for any debris or coating. The manufacturers of these sensors tell us not to service them, but we can.

Holding the sensor and spraying the hot wires with a good carburetor spray cleaner will clean anything from the wires. The most common occurrence is the presence of air filter fibers on the wires. The fiber acts like an insulator causing the computer to apply more voltage and enrich the mixture. Low-quality air filters, especially, will release fibers into the air filter housing. These fibers can easily find their way to the sensor and cause any number of driveability problems — stalling, throttle tip-in hesitation, surging, poor idle, loss of gas mileage and even engine ping. Cleaning this sensor should become part of your preventive maintenance program.

So there you have it. You can still repair the high-tech vehicle even without the use of high-tech equipment. Sometimes we need to think like the computer to repair the computer.

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