ONBOARD AUTOMOTIVE COMPUTER – EEC
by Lance Wright

Modern cars and trucks are lightyears from even vehicles of just 20 years ago. Just about every vehicle system is constantly monitored in real time by a computer which is typically located behind the dashboard, but sometimes in the engine compartment. A myriad of sensors constantly measure key components and send these readings (data) to the onboard computer. The onboard computer (also known as EEC – Electronic Engine Control) then compares these readings to specs and determines if they are within an acceptable range. If not, the computer generates a trouble code (OBD-II code for 1996 and later, OBD-I for 1995 and earlier) and then illuminates the “Check Engine” or “Service Engine Soon” light which is on the instrument panel. When you see this light on, it doesn’t necessarily indidate a serious problem requiring an expensive auto repair procedure.
The onboard computer control module is a microprocessor that is used to manage fuel delivery, operate controlled components, process sensor information, and perform system diagnostics. Contained in the control module are logic and memory circuits, voltage buffers, transistors and driver modules. Constant fused battery voltage and switched ignition voltage are connected to the control module as a power supply. Most control modules will use two or more redundant ground circuits to ensure a good ground connection. The control module will output a buffered voltage signal to operate and read information sensors. Most control module systems use a 5 volt reference signal to operate information sensors. A dedicated sensor ground may be supplied by the control module as well.

The control module is calibrated to tailor system operation to a specific vehicle powertrain and emission configuration. Calibration of the control module is achieved either using ROM (read only memory) or a replaceable PROM (programmed read only memory) chip. Some control modules use an erasable PROM that can be recalibrated by downloading a calibration file from a computer. Some control modules use an adaptive learning strategy to tailor engine control to vehicle operating conditions. Idle speed, fuel delivery and is some cases, transmission shift points, can be modified to adapt to wear, malfunction or driving habits. Most adaptive learning strategy is contained in volatile memory circuits and is erased when battery power is removed from the vehicle such as during an auto repair project. This may affect vehicle operation until the strategy is relearned.

Most control modules operate system components by providing a ground path for the controlled component electrical circuit. The ground circuit is usually provided by transistors or internal driver modules contained inside the control module. Current handling capabilities of the transistors or driver modules is of moderate capacity, so system components are designed with a high enough resistance to limit amperage flow. Control module damage can result from high current flow from a shorted components. Prior to replacing a faulty control module, the circuit resistance of controlled components should be checked.

(Lance owned his own auto repair shop for 30 years before retiring in 2006.)