Intermittent Diagnostic Techniques

INTERMITTENT DIAGNOSTIC TECHNIQUES

Intermittent diagnostic techniques help find and isolate the root cause of intermittent concerns associated with the powertrain control module (PCM). The material is organized to help find the concern and carry out the repair. There are examples that illustrate the diagnostic techniques. The process of finding and isolating an intermittent starts with recreating a concern symptom, accumulating PCM data, comparing that data to typical values, and analyzing the results.

Before proceeding, make sure that:
- Customary mechanical system tests and inspections do not reveal a problem. (Remember, mechanical component problems can make a PCM system react abnormally.)
- Review the Technical Service Bulletins (TSBs) and OASIS messages, if available.
- The Quick Test and associated pinpoint diagnoses have been completed without finding a concern, and the symptom is still occurring.

Recreating the Fault
Recreating the fault is the first step in isolating the cause of the intermittent symptom. If freeze frame data is available, it may help in recreating the conditions at the time of a malfunction indicator lamp (MIL) diagnostic trouble code (DTC). Listed below are some of the conditions for recreating the fault.

Conditions To Recreate The Fault:

Accumulating Powertrain Control Module (PCM) Data
PCM data can be accumulated in a number of ways. Gather as much data as possible when the concern is occurring to prevent misdiagnosis. Data should be accumulated during different operating conditions and based on the customer description of the intermittent concern. Reference the known good data values located in Reference Values Typical Diagnostic Reference Values. This requires recording data in 4 conditions for comparison: 1) Key On Engine Off (KOEO), 2) Hot Idle, 3) 48 km/h (30 mph), and 4) 89 km/h (55 mph). Acquisition of PCM PID data using a scan tool is one of the easiest ways to gather information.

For information on the functions of your scan tool, refer to the manufacturer's manual.

Below is a list of the functions available:
- selecting and viewing PIDs
- storing PIDs
- recording measurements along with PIDs
- playback of stored PIDs
- peripheral inputs

Analyzing PCM Data
Once the concern area is identified, the circuit must be checked to determine if the wiring or component is damaged. When making circuit and component measurements, make sure all accessories and dome and hood lamps are off. Use any of the following methods to diagnose a suspected PCM wire circuit or device. Some methods are particular to a certain type of PCM device.

- Change Condition to Cause Response by Input
- Change Input and Verify Output Response
- Click Testing/Output State Control (Solenoids/Relays)
- Coil Resistance (Solenoids/Relays)
- Harness Opens
- Harness Shorts

Change Conditions to Cause Response by Input
The purpose is to verify the sensor receives and responds to changes.

1. Access, monitor, and record the appropriate sensor PID(s).
2. Create the condition or cause the condition to change.
3. If the value changes appropriately, then it should be operating OK.
Examples:
- The engine oil temperature (EOT) should change from a higher voltage (2.6 volts) for a cold engine, to a lower voltage as the engine warms (0.6 volts).
- Apply and release the accelerator pedal while monitoring the accelerator pedal position (APP) PID for a change of state.
- Apply and release the brake pedal while monitoring the brake pedal position (BPP) PID for a change of state.

Change Input and Verify Output Response
The purpose is to verify how the PCM and actuator circuit respond to sensor input.

1. Access, monitor, and record the appropriate sensor PID(s).
2. Create the condition or cause the input condition to change.
3. Monitor any change (response) in the actuator PID or actuator signal circuit with a measuring device.
Example:
- increase the accelerator pedal position under no load, and observe the RPM DSD PID and circuit change

Coil Resistance (Solenoids/Relays)
The purpose is to measure the correct resistance value of a device.

1. Ignition OFF.
2. Disconnect the component from the vehicle harness.
3. Using an ohmmeter, measure across the component terminals.

Harness Opens
The purpose is to check the harness for open circuits.

1. Ignition OFF.
2. Disconnect the component and the PCM from the vehicle harness.
3. Disconnect the appropriate PCM harness.
4. Using an ohmmeter, isolate the circuit in question from the harness pin to the component connector pin.
5. The resistance value should be less than 5 ohms.

Harness Shorts
The purpose is to check the harness for short circuits (to ground or voltage).

1. Ignition OFF.
2. Disconnect the component and the PCM from the vehicle harness.
3. Using a Digital Multimeter (DMM) FLU77-4 or equivalent, measure the resistance between the signal circuit and signal return circuit, power ground circuit, VREF circuit, or vehicle power.
4. If the resistance value is less than 10,000 ohms, the 2 circuits may be shorted.

Glow Plug Lamp Cycles On/Glow Plugs Recycle/Engine Stumble/Stalls/No Accelerator Pedal Authority Until Return to Idle Position
A PCM reset occurs when the PCM momentarily reboots or is turned off and on while the engine is operating. If the condition occurs a single time, the engine momentarily stumbles and the PCM goes through a normal ignition on cycle, including turning the glow plug lamp and glow plugs on, and also attempts to validate the accelerator pedal position. If the pedal is not at the idle position when this concern occurs, pedal authority is not allowed by the PCM until the accelerator pedal is released and the engine returns to idle.

Probable Causes
- Momentary loss of power to the PCM or fuel injection control module (FICM): power relays, shorted or open harness, intermittent connectors, grounds.
- Momentary short to ground of VREF: shorted harness or connector, or sensors that use VREF including exhaust pressure (EP) sensor, injection control pressure (ICP) sensor, manifold absolute pressure (MAP) sensor, barometric pressure (BARO) sensor, accelerator pedal position (APP) sensor, and exhaust gas recirculation (EGR) actuator.

Procedures
- Check all power and ground connections for the PCM and FICM.
- Inspect the camshaft position (CMP) sensor and crankshaft position (CKP) sensor harness connectors and the harness for a signal short to ground condition.
- Remove and inspect the CMP sensor and CKP sensor for possible damage.
- If the PCM reset condition is repeatable, disconnect the following sensors one at a time and operate the engine to determine if the reset reoccurs: EP sensor, ICP sensor, MAP sensor, APP sensor, EGR sensor. Inspect each harness, including the connector, upon removal.