PCM - IDM Communications

Signal Functions

Cylinder Identification

Cylinder Identification (CI) signal is a 0 to 12 volt wave form signal that communicates from the PCM to the IDM the position of cylinders 1 & 4. This signal is used by IDM to synchronize the injector firing sequence. This signal is calculated from the signal generated from the Camshaft Position Sensor (CMP). The CI signal is generated by the PCM by "pulling down" (switching to grd.) a 12 volt communication circuit in the IDM.

Fuel Demand Command Signal

Fuel Demand Command signal is a 0 to 12 volt wave form signal that communicates from the PCM to the IDM the required engine timing and duration of injector firing. The timing and duration of the signal is determined by the PCM calibration and the signals of various sensor inputs. The FDCS signal is generated by the PCM by "pulling down" (switching to ground) a volt communication circuit in the IDM.

Injector Driver Module Feedback

Injector Driver Module Feedback signal is a 0 to 12 volt wave form signal that communicates from the IDM to the PCM a mirror image of the FDCS signal. Extensions of the EF (Electronic Feedback) signal can indicate to the PCM possible problems with the injectors by the IDM as the engine is running. An EF toggle (83 hz. Signal) is generated when the key is first turned "ON" and before the engine starts, communicates to the PCM that the IDM is powered up. In an engine off or engine running diagnostic mode, the EF signal is also used to communicate diagnostic information from the IDM to the PCM. The EF signal is generated by the IDM by "pulling down" (switching to gnd) a 12 volt communications circuit in the PCM.

Fault Detection/Management

Cylinder Identification

Intermittent open or short to ground conditions can be detected by the IDM through IDM stuck high or low codes (1218 & 1219). Active faults can be detected by an on demand output circuit check performed during engine off tests.

NOTE: The engine will not operate without a functioning CI circuit.

Fuel Demand Command Signal

Active faults can be detected by an on demand output circuit check performed during engine off tests.

NOTE: The engine will not operate without a functioning FDCS circuit.

Injector Driver Module Feedback

Active faults can be detected by a toggle sequence that the PCM looks for on start up. The WARN lamp will be illuminated if this is detected.

NOTE: IDM diagnostic fault codes will not be transmitted if the EF line is not functioning, however, the engine will operate normally.

Key ON Engine Off - Voltage Checks (Check with breakout box installed with ignition key "ON" engine off)

Test Points	Spec.	Signal	Comments
#96 to #91	0.6 ± 0.1 v	CI signal	Less than .5v indicates an open between the PCM and the IDM. 12 volts indicates an open in the PCM or breakout box
#95 to #91	0.6 ± 0.1 v	FDCS signal
#48 to #91	1 to 4 volts	EF feedback	With the key on, EF has a 83 hz. digital signal that will measure 1-4 volts w/DVOM

Connector Checks to Ground (B-) (Check with breakout box installed, ignition key should be in the OFF position)

Test Points	Spec.	Signal	Comments
#96 to #91	> 1000 ohms	CI signal	Less than 1000 ohms indicates a short to ground either through the harness or internal in the PCM or IDM. Disconnect the IDM and measure to ground. If short still present, disconnect the PCM and measure to ground. If short still present, repair harness
#95 to #91	> 1000 ohms	FDCS signal
#48 to #91	> 1000 ohms	EF feedback

Harness Resistance Checks (Check with breakout box installed, ignition key should be in the OFF position)

Test Points	Spec.	Signals	Comments
#96 to #16	< 5 ohms	CI signal	Resistance from PCM connector to IDM connector
#95 to #17	< 5 ohms	FDCS signal	Resistance from PCM connector to IDM connector
#48 to #4	< 5 ohms	EF feedback	Resistance from PCM connector to IDM connector
#91 to #2	< 5 ohms	SIG Grd	Resistance from PCM connector to IDM connector

Key On Engine Running (at low idle) - Voltage Checks - Intermittent Faults (Check with breakout box installed and engine at low idle) (Monitor suspect lines with voltmeter and wiggle appropriate wires and connectors while engine is idling)

Test Points	Spec.	Comments
#96 to #91	5 - 8 volts	CI signal (0 to 12 volt digital signal), monitor engine RPM w/Fluke 88 in TACH mode
#95 to #91	1 - 1.5 volts	FDCS signal, 40 - 50 hz. - 0 to 12 volt signal
#48 to #91	.5 - 1.5 volts	EF signal, 40 hz. - 0 to 12 volt signal

Diagnostic Trouble Code Descriptions

1663 = FDCS high or low resistance in the circuit detected during engine off test, OCC* test (REPAIR THIS FAULT FIRST).
1667 = CI high or low resistance in the circuit detected during engine off test. OCC* test (REPAIR THIS FAULT FIRST).
1219 = CI signal stuck low, harness or internal circuits of IDM shorted to ground (intermittent problem, historic fault only).
1218 = CI signal stuck high, harness or internal circuits of IDM shorted to voltage source (intermittent problem, historic fault only).
1668 = IDM feedback (EF) (83 hz. frequency transmitted during key ON, engine OFF) not detected.