Why Automotive Computers Fail By Ken Buie For those of you involved in on- car diagnostics, one of your more dif- ficult and time consuming diagnostics is ona vehicle with a dead computer. Some computers just go bad for no apparent reason. In most cases, though, computers fail due to a prob- lem external from the computer. Sometimes, a computer will inter- mittently “shut down.” Ifthe vehicle is carbureted, the car will usually con- tinue to run, but the check engine light will come on. Ifthe vehicle is fuel in- jected, the car will usually quit running and then restart after a period of time. In many cases, the computer does not set any trouble codes, making your diagnosis even tougher. It doesn’t matter if the car has an intermittent “shut down” problem, or comes in on the end of a tow truck. You must determine why the computer failed. In this article, we’re not concerned about drivability problems; only com- puter-related failures. When you be- come involved with any computer problem, follow a few rules: 1. Verify the customer complaint. 2. Get arecent history of repairs. 3. Road test the vehicle if possible. 4. Refer to any computer-related Technical Service Bulletins (TSBs). 5. Scan the computer for codes and data. In order to properly diagnose a computer system you need to under- stand how a computer operates and how outside factors affect its perfor- mance. The decision-making abilities of the computer rely on accurate sen- sor information. The wrong informa- tion can lead to wrong decisions made by the computer. The old computer saying of “garbage in equals garbage out,” still applies to automotive com- puters. A review of computer basics shows that all computers can be bro- ken down into four basic functions: 1. Inputs: sensors and power feeds; 2. Processing: decision-making and basic operation; 3. Storage: memory for diagnostics and operation; 4. Output: activation of computer- controlled circuits. Inputs. All inputs are in the form of voltage signals that the computer can understand. Typical sensor values vary from 0.5 to 4.5 volts with very low milliamps of current. An oxygen sen- sor operates at 100 to 900 millivolts. Off-on switches are also used as input signals. Some computer input signals use pulsed varying-frequency voltages. With input values like this, it’s easy to see how a computer can become con- fused or damaged by stray voltage spikes. Also included with inputs are ignition switch and direct battery feeds. The ground circuit is a very important input circuit for proper computer op- eration. Processing. All sensor informa- tion received by the computer is com- pared to the information stored in its internal program. If the sensor infor- mation agrees with the internal pro- gram, decisions are made by the com- puter to accurately control the engine’s fuel and ignition systems. If the sensor information does not agree with the internal program, the check engine light will turn on and set a code in the computer’s memory. Depending on which sensor fails, the computer will go into a “backup mode” and operate the engine’s systems based on what ac- AUGUST, 2001 curate information is available at the time. Storage. This is the electronic memory of the computer. As the car is being driven, sensor information is con- stantly being stored in a short-term memory area called RAM (random access memory). The basic operational program that makes the computer work is called ROM (read-only memory). Most computers use a re- placeable program chip that has all the vehicle information stored in it. This is called the PROM (programmable read-only memory) or calibration module. Output. After the sensor informa- tion has been processed, the computer decides which output devices will be activated. Output devices include: re- lays, solenoids, fuel injectors, ignition module, etc. This is the area of the computer that is most vulnerable to damage. Now back to why computers fail. Computer failures can be divided into four categories: * Mechanical failure; * Internal electronic component fail- ure; ¢ External component failure (caus- ing computer damage); ¢ Stupidity. Mechanical failure is the physi- cal attack on the computer by drill bits, water damage, collision damage, ham- mers, and welding equipment. Physi- cal damage to the computer is one of the easiest to find. Internal electronic failure is very rare and is usually caused by a manufacturing defect. This type of problem can be intermittent and very difficult to find. External component failure is the biggest cause of multiple computer failures on the same vehicle. Ifyou are Page 1 of 3