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Standardizing On-board DiagnosticsPosted 6/11/1997By Mark Warren, AAM
Eighty percent or more of the vehicles that go through an emission tailpipe test pass. This means that these vehicle owners wasted their time and effort so that we can identify the 20 percent that fail. A car could pass an emission test and have a failure the next day, then emit excess emissions for the entire year until it is retested. There are two possible ways to prevent these problems. The first is an on-board diagnostic system (OBD II). The second method is a roadside sensing station with a camera to record license plate numbers of failing cars. The OBD II system is mandatory on all cars since model year 1996. This system is supposed to turn on the Malfunction Indicator Light (MIL) when any emission gas exceeds 1.5 times the Federal Test Procedure (FTP) standards. While this system is far from perfect, it is a good step forward. So, will OBD II replace tailpipe testing? Not in the short term. However, the discussions are well under way. The proposals I have seen involve a transponder on the car that alerts a roadside sensing station that your vehicle is failing and gives the station your vehicle identification number (VIN). This is a delicate personal-rights issue; do you want big brother tracking your whereabouts? I have a solution: Rather than have drivers' cars narc on them, invoke limited operating strategies that will drive the vehicle owners into the repair shop. I propose that when the MIL is lit, the car should be restricted to 60 mph top speed. This should not cause a safety problem. For each drive cycle after the MIL is lit, the top speed will be shaved by 5 mph. Surely when the top speed is 5 mph, people will seek service. Let's look at OBD II: the good, the bad and the ugly. First, the good stuff. Standard terminology - Society of Automotive Engineers (SAE) document J1930 designates standard industry terminology for emission system parts. This is cool. Now we can leave the tower of babble and all speak the same language across all platforms. Yeah, some of it gets kind of goofy - for example, we got used to DIS meaning "distributorless ignition system," and now a distributor system is DI (distributor ignition) and a distributorless system is EI (electronic ignition). Oh well, it can't all be perfect. Standardized codes are a good idea. While I have never been big on memorizing code numbers, a standard system should help. Of course, original equipment manufacturers (OEMs) always interpret standards differently, and I hear some codes don't follow the system. Harry Truman said that if he asked all of his economic advisors to line up in a straight line, they would all point in different directions. Once again, the execution isn't perfect, but it's a good step. A standard diagnostic connector - now this one seems to have panned out pretty well. Sure we need keys to customize the communication on the enhanced pins, but that's no big deal. Also, the connector location is standard. Cool! OBD II requires all cars to have a standard protocol for the required scan data. This is semi-cool. What was required was a minimum standard. This falls under the category "Be careful what you ask for; you just might get it." The OBD II required scan data is so slow that it makes watching snails look like a NASCAR race. On the bright side, this forces manufacturers that previously had no scan data to now at least have some. Also, most manufacturers have their enhanced scan data on proprietary pins. The law of information availability "guarantees" the independents' access to any and all emission-related information released to new car dealers. I believe the terminology used was within "reasonable time and at reasonable cost." Some manufacturers have been very cooperative, and others have been downright stubborn. This information includes access to the enhanced scan data on the factory tools. I believe the central problem here was the lack of a specific mechanism for the release of this information. We need a specific time line and a specific location to confirm OEM compliance on this issue. OBD II has 10 mandatory monitors. Engine misfire is a new function. Engine misfire has the greatest potential to destroy the catalytic converter and therefore cause increased emissions. A high resolution crankshaft sensor, generally an optical sensor with 360 slots, is used to detect changes in crankshaft speed. Some manufacturers are experimenting with a spark ionization detector that would detect misfire from the spark plug. The misfire monitor has the potential to save a lot of diagnostic time, assuming that the filters are set right. Every engine misfires intermittently, especially when cold. Rough roads mimic a misfire by transmitting the road roughness to the crankshaft. Filters must be in place to make sure that only significant misfires set a code. Some manufacturers will use software filters to recognize the signature of rough roads, while others will use wheel speed differences from the anti-lock braking system (ABS) sensors, and still others will use low-cost accelerometers. The issue of when to turn on the monitor is a hotly debated one. The EPA wants it on at start up; most manufacturers want it at 30 to 60 seconds after initial warm-up misfires can be controlled. This is a primary reason for having a reprogrammable prom. Many of these new monitors will have to be tested by the consumer for acceptance and program filters will have to be modified to suit customer taste. No one wants their MIL light on all the time. Remember the seat belt interlock fiasco in the early 1970s? The government mandated a seat belt connection detection system connected to the starting system. If the seat contact was closed and the belt wasn't buckled, then the starter wouldn't engage. The problem was that if Fido or some other heavy object was on the seat, that seat's belt had to be buckled. The public hated the system and it was abandoned. The government allowed bypass systems to be installed. Public acceptance may be sufficient cause for reprogramming. Another issue is "true/false" misfire. When the tank is low on fuel, misfires will occur and the MIL light may come on. The customer refuels and brings the car to you. Of course they fail to yield the critical bit of information you need, so off to diagnostic land we go. Sure enough, a random misfire code is affecting all cylinders. Well, you'll never fix this one! Some manufacturers will use the fuel level as an input and turn off the misfire monitor when the gas is low. Others will use the Evap system pressure test to extrapolate the amount of fuel in the tank by checking how long it takes the tank to reach pressure. They will disengage the misfire monitor when the fuel is low. These two scenarios point out the critical need for information on precisely how these monitors work and are set. The cylinder-specific misfire monitors will be used to shut off fuel to the affected cylinders. This means all OBD II systems with this function will have to be sequential fuel injected. Because of the advanced nature of OBD II adaptive strategies, all diagnostics need to start with a scanner. Testing for an injector pulse on a missing cylinder and finding none, and thereby assuming the injector driver is bad on one of these systems would be a costly mistake.
Next month, more on OBD II monitors.
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