Possibly the most frustrating thing to deal with on an electronic fuel-injection car is the dreaded check engine light. Things were simpler with carburetors and distributors--you could hold the offending part in your hand and physically see the problem. Not so with computerized cars, though. Mysterious sensors and controllers contain circuit boards, resistors, diodes, and relays that can all fail, and you'd never know by looking. Still, EFI cars operate on the same principles as carbureted ones, and therefore you can diagnose any problems that arise. You just need a couple of extra tools in your arsenal and a thorough understanding of the affected system. To get a handle on trouble codes and data streams, we talked to our friends at Galpin Auto Sports, Galpin Ford, and Westech Performance Group. Fear not--it isn't as intimidating as you might think.
ECM--Electronic Control Module. This is the computer that controls everything. It's also r
Definitions
OBD--Stands for Onboard Diagnostics. Currently, there are two versions: OBD-I, which refers to the system found on early fuel-injected and electronic carbureted cars, and the later OBD-II, which is a standardized interface that was phased in sometime between 1994 and 1996.
Background
Contrary to what some people believe, onboard diagnostic systems were not created by the manufacturers to make it impossible for people to work on their own cars. Rather, they were developed in response to government regulations for tighter emissions controls. It all began with the requirement of catalytic converters in the exhaust. The cats clean up the exhaust gases by converting the carbon monoxide to carbon dioxide and breaking up the oxides of nitrogen, but they need to work within a certain air/fuel range, just like an engine. If the exhaust is too rich or too lean, the converters can overheat and fail. So to make the cats live, the manufacturers needed to abandon carburetors and distributors for electronically controlled fuel injections and distributorless ignitions.
In the early systems, the computers that controlled the electronic carburetors and throttle-body fuel-injection systems were pretty advanced for their time, though they would be considered stone-age technology today. They used input from oxygen sensors in the exhaust stream to determine the proper air/fuel ratio the engine should run at to produce the least possible emissions and were capable of warning the car owner, via the check engine light, if the engine was generating exhaust emissions outside the range of what the catalytic converters could handle. Of the three domestic manufacturers, GM was the early pioneer in electronic controls, and its system was the most comprehensive and best organized--it used the same data link connector and trouble codes across its model line. GM also provided a scan tool (the Tech 1) to its dealership technicians, used to read the codes and diagnose the faults.
ALDL--Assembly Line Data Link. This is the plug used to connect with the ECM; it's also re
That's not to say the system was without problems. The number of trouble codes was limited and the descriptions were rather vague, often leaving technicians to guess at what the real cause of a trouble code was. Things were even more difficult for Ford and Chrysler technicians. The locations and shapes of the data connectors varied from car line to car line, as did the procedures for retrieving the trouble codes. Of course, their codes were different from GM's codes. Imagine what havoc that played on guys at independent repair shops. They had to have myriad scanners, connectors, cables, and adapters, plus the expensive software for each manufacturer. You can see how difficult and expensive diagnostic repairs are.
OBD-II
By the '90s, OBD-I systems had become much more sophisticated as they were put to use controlling multiport fuel-injection systems. But the systems still lacked uniformity among the manufacturers and accessibility to information to the aftermarket and independent repair shops. To remedy this situation, the government mandated a standardized system of trouble codes and required the manufacturers to give access to the codes and data stream to independent shops. The resulting system was dubbed OBD-II. It still operates much the same way as OBD-I, utilizing oxygen sensor readings to adjust fuel trim and deliver the most efficient air/fuel ratios, but it is much more sophisticated and features numerous redundant checks and monitoring systems. Simply put, the ECM has several ways to check most of the sensors to verify that they are working correctly. As a result, the trouble code system was revised to four-digit codes instead of the two- (and in some cases three-) digit codes of the previous system.