OBD (On-Board Diagnostic) systems

Location and pinouts for the Diagnostics Connector



OBD systems are found in most vehicles in use today. OBD is an electronic system designed to control engine functions and diagnose engine problems or component failures, as well as minimize emissions. OBD-II, a new standard introduced in cars built after January 1, 1996, provides almost complete engine control and also monitors parts of the chassis, body and accessory components, as well as the diagnostic control network of the vehicle.

OBD-II equipped vehicles have a connector located in the passenger compartment, usually accessible from the driver's seat (under the dash above the hood release lever in the Grand Cherokee WJ). A cable is plugged into the OBD-II connector and connected to a scanning tool. This can range from a simple hand-held meter that provides a coded read-out of the various diagnostic functions, up to a large console computer-based unit costing thousands of dollars.

Fault codes usually appear after the "Check Engine" light has come on. The data provided by the OBD-II system can sometimes pinpoint the specific component that has malfunctioned, saving substantial service time and cost. Scanning OBD-II signals can also provide important information when purchasing a used vehicle. Recent introductions of less costly scanning devices have allowed home mechanics and small shops accessibility to OBD-II signals to use them for their own testing and repairs.







Malfunction Indicator Lamp (MIL)

In the Grand Cherokee WJ, the MIL gives an indication to the vehicle operator when the Powertrain Control Module (PCM) has recorded a Diagnostic Trouble Code (DTC) for an On-Board Diagnostics II (OBDII) emissions-related circuit or component malfunction. The MIL is controlled by a transistor on the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster from the PCM over the Programmable Communications Interface (PCI) data bus.

The MIL bulb is completely controlled by the instrument cluster logic circuit, and that logic will only allow this indicator to operate when the instrument cluster receives a battery current input on the fused ignition switch output (run-start) circuit. Therefore, the bulb will always be off when the ignition switch is in any position except On or Start. The bulb only illuminates when it is provided a path to ground by the instrument cluster transistor.

The instrument cluster will turn on the MIL for the following reasons:

BULB TEST - Each time the ignition switch is turned to the On position the MIL is illuminated for about three seconds as a bulb test.

MIL LAMP-ON MESSAGE - Each time the cluster receives a MIL lamp-on message from the PCM, the indicator will be illuminated. The indicator can be flashed on and off, or illuminated solid, as dictated by the PCM message. For some DTC’s, if a problem does not recur, the PCM will send a lamp-off message automatically. Other DTC’s may require that a fault be repaired and the PCM be reset before a lamp-off message will be sent.

COMMUNICATIONS ERROR - If the cluster receives no lamp-on or lamp-off messages from the PCM for twenty consecutive seconds, the MIL is illuminated by the instrument cluster. The indicator remains controlled and illuminated by the cluster until a valid lamp-on or lamp-off message is received from the PCM.