Intermittent Diagnostic Techniques

INTERMITTENT DIAGNOSTIC TECHNIQUES

Intermittent diagnostic techniques help find and isolate the root cause of intermittent faults associated with the PCM. The material is organized to help find the fault and perform the repair. There are examples that illustrate the diagnostic techniques. The process of finding and isolating an intermittent, starts with recreating a fault symptom, accumulating PCM data and comparing that data to typical values and analyzing the results.

Before proceeding, be 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 Technical Service Bulletins (TSBs) and OASIS messages, if available.
- Quick Test and associated Pinpoint diagnosis have been completed without finding a fault, and the symptom is still occurring.

Recreating the Fault

Conditions To Recreate 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 Diagnostic Trouble Code (MIL-type DTC). Listed below are some of the conditions for recreating the fault.

Accumulating PCM Data
PCM data can be accumulated in a number of ways. Gather as much data as possible when the malfunction is occurring to prevent misdiagnosis. Data should be accumulated during different operating conditions and based on the customer description of the intermittent fault. Reference the known good data values located in Reference Values. This will require recording data in four conditions for comparison: 1) KOEO, 2) HOT IDLE, 3) 30 mph and 4) 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 manual for scan tool instructions.

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

Comparing PCM Data
After the PCM values have been acquired, it is necessary to determine the fault area. Typically, it will require the comparison of the actual values from the vehicle to the typical values from Reference Value Charts.

Analyzing PCM Data
Once the fault area is identified, the circuit must be checked to determine if the wiring or component is at fault. When making circuit and component measurements, make sure all accessories and dome and hood lights 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. Select, view and record the appropriate sensor PID(s).
2. Create condition or cause condition to change.
3. If reading changes appropriately, then it should be operating OK.
Examples:
- EOT should change from a higher voltage (2.6V) for a cold engine, to a lower voltage as the engine warms up (0.6V).
- Move accelerator pedal, observe APP PID change.
- Press brake pedal, watch BOO PID change states.

Change Input and Verify Output Response
The purpose is to verify how the PCM and actuator circuit responds to sensor input.
1. Select, view and record the appropriate sensor PID(s).
2. Create condition to cause input condition to change.
3. Observe change (response) in actuator PID or actuator signal circuit measured by a measuring device.

Coil Resistance (Solenoids/Relays)
The purpose is to measure the correct resistance value of a device.
1. Key off.
2. Data Link Connector (DLC) disconnected from any diagnostic tools.
3. Disconnect component from vehicle harness.
4. Using an ohmmeter, measure across the component terminals.

Harness Opens
The purpose is to check harness for open circuits.
1. Key off.
2. DLC disconnected from any scan tools.
3. Disconnect the component and PCM from vehicle harness.
4. Disconnect the appropriate PCM harness.
5. Using an ohmmeter, isolate the circuit in question from the harness pin to the component connector pin.
6. Reading should be less than 5 ohms.

Harness Shorts
The purpose is to check the harness for short circuits (to ground or power).
1. Key off only.
2. DLC disconnected from any scan tools.
3. Disconnect the component and PCM from vehicle harness.
4. Using a digital multimeter, measure the resistance between the signal circuit and signal return circuit, power ground circuit, VREF circuit or vehicle power.
5. If reading is less than 5 ohms, then the two circuits may be shorted.

PCM Graphs and Charts

Glow Plug Lamp Cycles On/Glow Plugs Recycle/Engine Stumble/Stalls/No Accelerator Pedal Authority until Return to Idle Position
A powertrain control module (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 will momentarily stumble and the PCM will go through a normal key on cycle, including turning the glow plug lamp and glow plugs on and will also attempt to validate the accelerator pedal position. If the pedal is not at the idle position when this fault occurs, pedal authority will not be 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 FICM: Defective power relays, shorted or open harness, intermittent connectors, poor grounds.
- Momentary short to ground of VREF: Shorted harness or connector, defective sensor (sensors that use VREF include - ICP injection control pressure, MAP manifold absolute pressure sensor, BARO barometric pressure sensor, APP accelerator pedal position sensor), Exhaust Gas Recirculation (EGR), Mass Air Flow Sensor (MAF).

Procedures
- Check all power and ground connections for the PCM and FICM.
- Inspect the CMP sensor and the CKP sensor harness connectors and the harness for a signal short to ground condition.
- Remove and inspect the CMP camshaft position sensor and CKP crankshaft position 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 will reoccur. (ICP injection control pressure, MAP manifold absolute pressure sensor, APP accelerator pedal position sensor, EGR exhaust gas recirculation sensor.) Inspect each harness, including the connector upon removal.
- Remove the valve covers and inspect the under valve cover wiring for possible rub through against the valve train.