Finding out what's going on inside engine control electronics can be frustrating and time consuming. Like any manager, the ECU needs a steady stream of clean data to make quality decisions.
“Garbage in, garbage out”. It’s an old expression dating to the early days of computer programming, and put simply, it means that the machine’s output is only as good as the data input. While that may be a self-evident truth for your home or office personal computer, it’s not so widely acknowledged under the hood. All automotive computers need a steady stream of clean, fresh data to make good choices about everything from ignition timing to the cabin air temperature. And if the system breaks down, it’s important to avoid blaming the brain; most often it’s on the sensor side of the equation.
The reflex that point technicians towards the ECU is an understandable one. It’s not uncommon for a computer to use 5000 pages of software code to manage engine operation, and with that kind of complexity, they would be expected to fail more frequently than they do. Like the real brain, however, the system doesn’t use all of it at any one time, a point that’s often forgotten where problems seem insurmountable.
Systems vary, but they typically operate is seven different modes:
1. Engine crank
4. Closed loop
These modes aren’t news to most technicians, but the often-overlooked part of troubleshooting is that there are two generic ways that the system can break down. One is to fail in a way that cripples one or more of the modes. A classic case is the crankshaft position sensor failure, resulting in an engine cranking mode failure and a no-start. The other generic way that systems can misbehave is by confusing the ECU into using a mode inappropriate to the operating conditions. An example is an RPM sensor that can’t recognize that the engine has started, and keeps the fuel trim in the heavily enriched cranking regime. Coolant temperature sensors are another common cause, either through sensor failure or the lack of a working fluid to sense, i.e. a leaking cooling system.
Ever find a code, correct the problem, then find a new code immediately set? Or find a bad component that’s not what the code points to? It could be a “confused mode” issue. One strategy that works in many cases is to consider which sensors and actuators operate across multiple modes, and which are unique to a single mode. An IAC motor, for example, isn’t likely to be the problem in the acceleration mode. At the other extreme, a MAF problem can have consequences across several modes. Diagnostic guides may or may not take this into account when trouble shooting, so it may make sense to improvise if common sense says to invert the order of, or substitute a test.
Many experienced techs diverge from the diagnostic “tree” significantly during sensor/actuator testing. Why? Because diagnostics “by the book” describe the components that are most likely to cause the problem first, followed by lower probability targets. Depending on the model, however, the testing methodology doesn’t follow what’s the easiest to test. If the TPS is number four on the list, is right in front of you and you have a multimeter in your hand, it may be logical to test it first. Statistically, the fastest average diagnostic time will happen by the book, but if you’re digging past the MAF to get to a coolant sensor or EGR valve, it may make sense to take a look on the way by.
Another source of possible confusion comes from the codes themselves. OBD II codes are defined as:
OBD II Code Format
P 0 1 0 1
P = Powertrain
B = Body
C = Chassis
0 = Standard
1 = Manufacturer
1 = Emissions
2 = Injector Circuit
3 = Ignition
4 = Auxiliary Emissions
5 = Vehicle Speed/Idle Control
6 = Computer/Output Circuits
7 = Transmission
This format is well known, but when a technician digs into the tables, terminology becomes very important. Four key concepts are “Circuit Malfunction”, Circuit Range/Performance, “Low Input” and “High Input”. “Circuit Malfunction” implies a major fault like no output or stuck sensor giving a fixed output where it should be variable. “Range Performance” is tougher to spot. It’s the code that describes a functional circuit that reports erratically, incorrect levels or values, or intermittent output. The difference can mean minutes or hours in a frustrating diagnosis. “Circuit Malfunction” means dead or fixed value. “Range Performance” means that the circuit is active, but not doing what it’s supposed to do. “Low Input”/”High Input” mean just that, but it’s important to remember that the voltage, frequency or signal that the code refers to is measured at the control module so the rest of the harness can’t be ignored when that P0405 tempts you to rip out the EGR sensor.
The other timesaver built into OBD II terminology is the use of numbering. Cylinder number one is in bank one, and sensors numbered one, two, three etc. are in order by the distance from the cylinder in question, so sensor two, bank one is further away from cylinder one than sensor one. It’s simple stuff, but a small error here can create chaos, if the technician goes after the wrong sensor.
Then there are the failures that come out of “left field”. An example is the OBD I vehicle that arrives with the “Check Engine” light illuminated, then proceeds to deliver a random set of trouble codes. “Something could be confusing the computer”, says Don Josefik, BWD manager of training support operations, adding, ” That’s a circumstance where you want to look at the possibility of spark plug wires deteriorating and causing enough electrical interference to affect the vehicle’s computer.” It turns out that deteriorating plug wires can cause a dramatic spike in voltage levels, sometimes up to 30,000 volts, enough to cause other wires in the harness to pick up currents inductively. “The driver doesn’t feel a miss”, says Josefik, “but the voltage in the bad plug wires increases to astronomical levels, and the EMI (electromagnetic interference) caused by the high voltage may interfere with the computer’s input and output signals.”
Driveability issues usually force the repair issue, but what about those codes that don’t appreciably change the “feel” of the car to the driver? They’re normally emissions related, and in mandatory I/M jurisdictions like British Columbia and Ontario, techs find that O2 sensors are often the solution.
How oxygen sensors feed information to the ECU is about to undergo a significant change, one that is more important that the switch to heated sensors. The new technology is called “wide band” and it will result in waveforms with a different look.
Figure 1 shows why. As oxygen content moves from rich to lean and back again, the voltage response is different, a characteristic called “hysterisis” (Note that voltage is plotted against stoichiometry, not time, so this isn’t a waveform seen in a typical test). Lots of systems in everything from stereo systems to plastics show hysterisis, but in this case, it limits the way that engine control electronics can use the information coming from the sensor. The easiest technique for the computer is to measure the familiar cross-counts and maybe amplitude when comparing pre-and post converter sensors to trigger codes like the dreaded P0420 “dead converter” message.
The better solution is to use a new type of sensor that responds linearly with oxygen concentration. That’s what wide band sensors do, and they promise better fuel/air ratio control. According to Chris Harrison, Product Manager, NGK Spark Plug Canada Ltd., “The ‘scoped output will look different. The wide band sensor does more than just react to the air-fuel ratio in an attempt to keep it at or near the ideal stoichiometric ratio of 14.7:1. It actually measures the air-fuel ratio to more efficiently control it. It used often in lean burn applications; the most popular one is 92-95 Honda Civic. It’s actually producing an
output signal that’s proportional to the oxygen level.”
Wide band O2 sensors are an outgrowth of another new technology called “planar” sensors. “The planar type O2 sensor heating element raises the sensor to operating temperature (625 – 650F) much faster — within a mere 10 seconds — with less electrical power than is required with previous heated sensors”, states David Pankonin, senior product manager for Robert Bosch Corporation. “This accelerated heat-time allows the planar sensor to obtain exhaust emissions and oxygen readings while the engine is still ‘cold’ (prior to the engine reaching operating temperature). The result is accurate and timely input information relayed to the engine management controller much faster than was previously possible, which allows the engine controller to respond faster to control fuel and air mixtures and emissions,” Pankonin adds. Figure 2 shows the difference in response to oxygen content.
The new wide-band O2 sensor features at least five wires in a spade-type connector, which also makes it easier to identify compared to older oxygen sensor designs with three or four wires. Planar sensors use four wires and wide-band uses five or more. Diagnostically, planars respond the same way as conventional heated sensors using the same scan tool technology. Wide band, however, requires new tools and the vehicle’s OBD for efficient diagnosis.
Will the new technology catch on? “We’re going to see them more and more. There’s a five-wire Volkswagen sensor and others too. We’re moving towards that trend,” says NGK’s Chris Harrison.
There are literally hundreds of issues involved in engine control electronics, and systems are getting more complex with each model year. It’s daunting, but to clean off the rust, you first have to scratch the surface. Watch future articles in SSGM’s CAT for more on this issue.
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