Fig. 18 Engine ControllerÐAC Body

Fig. 19 Engine ControllerÐAG and AJ Bodies

ON BOARD DIAGNOSTICS

The engine controller has been programmed to monitor many different circuits of the fuel injection system. If a problem is sensed with a monitored circuit often enough to indicate an actual problem, the controller stores a fault. If the problem is repaired or ceases to exist, the engine controller cancels the Fault Code after 51 vehicle key on/off cycles.

Certain criteria must be met for a fault code to be entered into engine controller memory. The criteria may be a specific range of engine RPM, engine temperature, and/or input voltage to the engine controller.

It is possible a fault code for a monitored circuit may not be entered into memory even though a malfunction has occurred. This may happen because one of the fault code criteria for the circuit has not been met. For example, assume one of the fault code criteria for the MAP sensor circuit is the engine must be operating between 750 and 2000 RPM. If the MAP sensor output circuit shorts to ground when engine RPM is above 2400 RPM (resulting in a 0 volt input to the engine controller) a fault code will not be

Fig. 20 Relay IdentificationÐAA Body

Fig. 21 Relay IdentificationÐAC Body

entered into memory. This is because the condition does not occur within the specified RPM range.

There are several operating conditions that the engine controller does not monitor and set fault codes for. Refer to Monitored Circuits and Non-Monitored Circuits in this section.

Stored fault codes can be displayed either by cycling the ignition key On - Off - On - Off - On, or through use of the Diagnostic Readout Box II (DRB II). The DRB II connects to the diagnostic connector in the vehicle (Fig. 17, Fig. 18 or Fig. 19).

MONITORED CIRCUITS

The engine controller can detect certain fault conditions in the fuel injection system.