Nissan's roots can be traced back to the founding of the Kwaishinsha Motor Car Works in 1911, and its cars began arriving in the U.S. in 1958. But it wasn't until about 25 years ago that we wrote our first service article on a Nissan product. That was the Datsun B-210, a strange-looking car with ungainly proportions and odd honeycomb covers for its tiny wheels. Its horsepower and handling weren't exactly breathtaking, either. It was, however, as simple, dependable, and serviceable as anything we'd ever seen, so we couldn't help liking it. To say that Nissans have become fabulously more sophisticated since then is a world-class understatement. Instead of styling that looked like it came out of a '50s space comic, the recent crop is eminently smooth and handsome. Performance is great, too. But, as with all other carmakers who do business in this environmentally-enlightened country, that simplicity we loved so well is gone, supplanted by amazingly complex technology.

Fortunately, Nissan's engineers have managed to keep random changes in their electronic engine control system to a minimum, and have built as necessary on a relatively stable and understandable basis instead. So, we can give you some truly useful info that applies to many models within the compass of one section.

*The same but different

For years, Nissan has called its computerized engine management set-up "ECCS," which stands for "Electronic Concentrated Control System." The basic concept is the same as that of all the other electronic engine controls out there -- a computer issuing operating parameter commands on the basis of input from sensors and information stored in ROM -- but we should mention a few interesting items specific to typical late-model Nissans:

-There are two injection modes. During normal operating conditions, the injectors are energized sequentially for max efficiency. Simultaneous injection is used when the engine is being started or when the system falls back on its fail-safe programming.

-Three strategies for handling malfunctions are available. If the computer receives a coolant temperature or detonation sensor signal that's out of range, the "Fail-Safe" mode is engaged. Stored data is substituted for the faulty signal, which is ignored. In the "Backup" mode, information from a bad sensor circuit's replaced by that from a good one. For instance, if the air flow meter is in trouble, the ECU relies on the throttle position sensor signal instead. Finally, if the brain itself has some kind of electronic hemorrhage, there's the "Limp-Home" mode wherein the car remains "minimally driveable," as the factory training literature so honestly states.

-To avoid uneven idle in closed loop, a "clamping" function has been included. That is, injector pulse width is held at a constant value, which is the mean of a 5-10 second reading of dynamic adjustments. -Quicker and more accurate mixture control is provided by the ECU's self-learning capabilities -- it catalogs and memorizes input "sets" to revise ROM data.

SELF CRITICISM

Nissan has made a commendable effort to help us in the service business by incorporating comprehensive self-diagnostics into its ECCS. While not exactly the be all and end all of troubleshooting, they can be a big help in getting you started on the right track by indicating the specific component or circuit that's the most likely cause of a driveability or performance problem.

Originally, Nissans carried two-mode self-diagnostics, but by '87 all models were endowed with a five-mode system. That's what we'll explore here, specifically the '91 federal version found on the popular Maxima.

To reiterate what we've said in every electronic engine control article we've done over the last couple of decades (and that's a lot), don't assume that the ECCS is the source of the complaint just because it's present and complicated. Look for traditional, basic troubles first: fouled plugs and faulty ignition components, weak or uneven compression, vacuum leaks (as with other computerized systems, unmetered intake air will reduce idle speed), restricted intake, a cam drive that's out of synch, improper fuel supply pressure and volume, low voltage or poor connections in the electrical system, and even such things as transmission and A/C operation. Also, do a thorough visual exam of the computer system's wiring and connectors, including those extremely important grounds. You may want to check oxygen sensor activity with a DMM or dedicated tester, too.

But if all that is okay, and/or the Check Engine light is on (Nissans didn't get this warning device until '88 for California and '89 federally), the next logical step is to engage the self-diagnostics. Unlike most other carmakers, Nissan has you go right to the ECU for this. Remove it from the passenger's side kick panel, then find the mode selector screw and the port in the housing through which a red and a green LED are visible.

Switch on the ignition, turn the selector screw clockwise all the way, and watch the LEDs. They'll flash once, then pause, twice then pause, and on up to five times to indicate the five diagnostic modes. When you see the number of the mode you want, turn the screw counter-clockwise.

*Modes & codes

Mode 1 is called the "Exhaust Gas Sensor Monitor" because it informs you about oxygen sensor activity. With the engine running and fully warmed up, the green LED should blink, going on when the sensor sends a lean signal and off when it sends a rich signal. You should see 5-10 flashes every 10 seconds. If the LED is on more than it's off, there's a lean condition, and vice versa. Sluggish blinking should make you suspect a fouled sensor.

Mode 2, the Mixture Ratio Feedback Control Monitor, lets you know whether or not the air/fuel mixture is being controlled within the proper range. If the red and green LED's flash pretty much simultaneously with the engine running, mix control is okay. If the red LED is off more often than the green one, richness is indicated. If the red is on more than its mate, think lean.

Mode 3 has the name "Self-Diagnostic," and it's more what you're used to on other vehicles because it yields fault codes, both hard and intermittent, which are communicated to you by means of the flashing of both LED's. The red one gives the first digit, and the green one the second digit. For instance, if the red flashes twice, then the green flashes once, you've got a Code 21.

Get the engine up to normal temp, preferably by driving the car for 10 minutes, then pull the codes. In the case of a no-start, crank the engine for at least two seconds. If you see 55, all is well (in older models, 44 meant the same thing). Be sure to write down any other codes because they'll be erased when you go on to Mode 4, which means you will have lost any help on intermittents. Normally, codes are retained in memory for fifty starts.

Mode 4, called the "Switches On/Off Diagnostic Mode," checks the function of the switches that serve as inputs to the ECU, specifically those for ignition key start position, idle, and vehicle speed. For the first two, the red LED will go on or off when switch status is changed. In other words, with the ignition on, the red light should illuminate both when you step on the gas pedal and when you turn the key to start. If not, check the appropriate circuit. The vehicle speed sensor lights the green LED when you exceed 12 mph (get the drive wheels off the floor to do this in the shop, or have a helper drive while you keep your eyes on the ECU).

Mode 5 ("Real-Time") gives instantaneous trouble codes to let you know what's going on right now in four monitored circuits. Codes are flashed out just once when a malfunction is detected, then they're immediately forgotten. And the way you read them is different -- you observe the flashing of one LED or the other, the red one reporting on the crank angle sensor and the fuel pump circuits, and the green on the air flow meter and ignition signal circuits.

If the red LED pulses out a series of long flashes separated by equally long pauses, a malfunction in the crank angle sensor or its circuit's indicated. Groups of three short flashes of the red LED point to the fuel pump or its circuit. With the green LED, two medium flashes followed by a pause, then two again, and so on, should cause you to suspect the air flow meter and its related wiring. Groups of four flashes mean there's a problem with the ignition signal.

This is an extremely sensitive and very useful mode. You can wiggle wires and connectors, rap on components, and have somebody else drive the car while you watch for those fleeting indications of trouble. You'll know everything's okay if you see no flashing in five minutes of revving and idling.

*Help!

Self-diagnostics are helpful, but they're no substitute for real-world experience. So, we went to a likely source for this type of information: Paul Kujawa, a Nissan specialist with Autoline Telediagnosis who has fielded thousands of "help-me!" calls from techs in the trenches and keeps every fragment of factory data close at hand. During a long conversation (he's a very patient man), he gave us some invaluable troubleshooting tips for situations you're bound to encounter sooner or later.

"If you get an '86 or older VG70 V6 that will barely idle and has no power, suspect the air flow meter," he said. "Go to the air flow meter connector, roll the boot back, leave the connector plugged in, and check voltage at the output wire, which may be white or black. It'll be the second wire in from one side. The second wire in from the other side is battery voltage, so you'll know if you've got the wrong one. With the key on, you need 1.6 volts. The book says plus or minus .5 volt, but if it's just a little low it'll go very lean even though it's not low enough for the ECU to shift to fail safe. If you don't get 1.6 volts, you can do voltage drop tests on the feed wire and the ground wires, which are black and in the center of the connector. And if you unplug the air flow meter, the computer will go into fail-safe and the engine will start and idle smoothly, but won't accelerate well.

"There's a service bulletin about the TPS on '86-1/2 and '87 trucks, and the '87 Pathfinder, van, Sentra Sport Coupe and 4WD wagon, and the Pulsar NX (not the twin-cam) for an intermittent hesitation or surge under load," Kujawa told us. "It'll feel like the ignition is shut off for less than a second, then turned back on. Go to Mode 3 and read the codes. In those years, Code 44 means everything's okay, but check the TPS anyway because it's probably bad but isn't setting a code. Unplug each wire from the three-wire connector that's molded into the TPS (the two-wire connector is for the idle switch), and check for continuity to ground at the throttle body. You should have infinite resistance. Otherwise, the TPS is shorting out.

"If you get a car with fast, hard surges that won't let rpm exceed 2,400, the air flow meter circuit has a problem and the ECU is shutting off the injectors," Kujawa continued. "On trucks, the rpm limit will be 2,800, and the cause will either be the air flow meter circuit or the injector electrical circuit. Check the codes in Mode 3 to find out which.

"There have been some cases of the injectors failing mechanically on trucks with the V6 and dual throttle body," he said. "You'll have to keep working the throttle to keep it idling, and there may be a low power condition. What's happening basically is that it's running on one injector. Since it's a mechanical problem, the computer won't recognize it. It's sort of hard to see down there, but look for spray from both injectors. Or, you can isolate the bad one by disabling one injector at a time. The computer will recognize this electrical problem and go to fail-safe. It'll start and run on one injector, but not at all or very poorly on the other."

Kujawa also mentioned that when the ECU detects a problem in the cylinder head temperature sensor (also known as the engine temperature or water temperature sensor) circuit, it'll fall back on a 68 deg. F. setting while cranking, then switch to 176 deg. F. once the engine is running. This can cause hard starting when the temperature's low and poor cold driveability, but there should be no problem after things have warmed up. The temperature sensor circuit fault code should show up in Mode 3 because the computer has to have seen trouble or it wouldn't have switched to this fail-safe strategy.

Finally, Kujawa told us that carbon deposits on the backs of the intake valves have been a frequent and major problem, especially on the V6, and that they not only affect driveability when the engine's cold (they absorb gasoline like little sponges, so they cause lean meaness until they're saturated), but also when it's warm because they disrupt air flow. Walnut shell blasting takes care of the problem, but you can do pretty well with a heavy-duty injector cleaning procedure, too.

MODE 5 TROUBLE INDICATORS