Intermittent Symptom Faults (Includes No Start)

Intermittent diagnostic techniques help find and isolate the root cause of intermittent faults associated with the OBD II system. 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 Powertrain Control Module (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), if available.
- Quick Test and associated Pinpoint diagnosis have been completed without finding a fault and the symptom is still occurring.

RECREATING THE FAULT
Recreating the fault is the first step in isolating the cause of the intermittent symptom. A thorough investigation should start with the Customer Information Worksheet. 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.

CONDITIONS TO RECREATE FAULT

Engine Type Conditions
- Engine Temperature
- Engine RPM
- Engine Load
- Engine idle/accel/decel

Non-Engine Type Conditions
- Ambient Temperature
- Moisture Conditions
- Road Conditions (smooth-bumpy)

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 in the Diagnostic Reference Value Charts. This will require recording data in four conditions for comparison:

- Key ON, Engine OFF (KOEO)
- Hot Idle
- 30 MPH
- 55 MPH

Acquisition of PCM Parameter Identification (PID) data using a scan tool is one of the easiest ways to gather information. Listed below are instructions for gathering PID data using a New Generation STAR (NGS) scan tool. If another type of scan tool is used, follow instructions supplied by the manufacturer of the scan tool.

SELECTING AND VIEWING PIDs:

NGS Main Menu:

1. Select Vehicle and Engine Selection and enter the proper year, vehicle and engine package from the menu.
2. Select DIAGNOSTIC DATA LINK.
3. Select POWERTRAIN CONTROL MODULE (PCM).
4. Select PID/DATA MONITOR AND RECORD.
5. Select the PIDs from the intermittent symptom chart or the PCM Pinpoint Test. The star symbol next to each PID on the screen indicates that PID has been selected.
6. Turn ignition key ON or START vehicle.
7. Select the START key and access the PIDs.

STORING PIDs
1. When ready to capture and store the selected PIDs, press the trigger button.
2. Press trigger again when ready to save information.
3. The information is now located in the main recording area. Store to a viewing area before starting another recording or the data will be overwritten.

RECORDING MEASUREMENTS WITH PIDs
1. Select DIGITAL MEASUREMENT SYSTEM.
2. Select a meter (i.e., VOLT METER).
3. Select LINK and LINK MONITOR.
4. Select the PIDs and START recording.
5. Press REC to save DVOM function and PID data.

PLAYBACK OF STORED PIDs
Look for abnormal behavior or values that are clearly incorrect. Inspect the signals for abrupt or unexpected changes. For example, during a steady cruise most of the sensor values should be relatively stable. Sensors such as Throttle Position (TP), Mass Airflow Sensor (MAF), and RPM that change abruptly when the vehicle is traveling at a constant speed are clues to a possible fault area.

Look for agreement in related signals. For example, if TP is changed during acceleration, a corresponding change should occur in Idle Air Control (IAC), RPM, and SPARK DEGREES.

Make sure the signals act in proper sequence. An increase in RPM after the TP is increased is expected. However, if RPM increases without a TP change, then a problem may exist.

1. Select VIEW RECORDER AREAS.
2. Select a viewing area.
3. Select up to four PIDs to review in the table format or two PIDs to review in the graph mode.

Playback Of Stored PIDs:

4. Table Format: Scroll through the PID data while analyzing the information. Look for sudden drops or spikes in the values. Notice the major jump in the TP voltage while scrolling through the information. This example would require a smooth and progressive throttle pedal travel during a key ON and engine OFF mode.

Playback Of Stored PIDs:

5. Graph Format: Scroll through the PID data while analyzing the information. Look for sudden drops or spikes in the linear lines showing the transformation of values to the line graph. This example would require a smooth and progressive throttle pedal with the key ON and the engine off.

PERIPHERAL INPUTS
Some signals may require certain peripherals or auxiliary tools to aid in diagnostics. In some cases, these devices can be inserted into the measurement jacks of the scan tool or DVOM. The NGS is capable of recording the value from the measurement jacks on the NGS while storing the value from other PIDs. For example, connecting an electronic fuel pressure gauge to monitor and record the fuel pressure voltage reading, and capturing the data, would help find the fault. Listed are two types of peripheral devices available.

- Multi-Point Auxiliary Adapter - A/C, Fuel Pump (Rotunda 007-00023 or equivalent)
- Electronic Fuel Pressure/Vacuum Adapter - Fuel Pressure, Vacuum (Rotunda 007-00022 or equivalent)

30 MPH Readings (Typical Example):

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 the Diagnostic Reference Value Charts. Diagnostic Reference Values

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, trunk and hood lights are OFF. Use any of the following methods to diagnose a suspected PCM wire circuit or device:

- Change Condition to Cause Response by Input
- Change Input and Verify Output Response
- Click Testing/Output Test Mode (Solenoids)
- 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:

- View ECT PID while engine warms up.
- It should change from a higher voltage (2.6v) for a cold engine, to a lower voltage as the engine warms up (O.6v).
- Move TP, observe TP PID change.
- Press brake pedal, watch STP SW 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.

Example:

- Increase TP under load, observe SPARK PID and circuit change.

Click Testing (Solenoids/Relays)

The purposes to activate a solenoid or relay from the PCM by entering Output Test Mode.
1. Key ON.
2. Enter Output Test Mode.
3. Turn outputs ON and then OFF.
4. Listen for relays to click ON and OFF. If a Breakout Box (BOB) is connected to the PCM, measure the control circuit while turning the outputs ON and OFF. A voltage change of greater than 4 volts should occur during the ON and OFF transition.

Examples:

- EGR CHK, EPO, and IAC.

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 and referencing the Static Resistance Value Chart, measure across the component terminals. Static Resistance Values

Harness Opens

The purpose is to check the harness for open circuits.
1. Key OFF.
2. DLC disconnected from any diagnostic tools.
3. Disconnect component from vehicle harness.
4. Install Breakout Box (BOB).
5. Using an ohmmeter, isolate the circuit in question from the BOB 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 diagnostic tools.
3. Disconnect component from vehicle harness.
4. Using an ohmmeter, measure between the signal circuit and signal return circuit or power ground circuit or vehicle power.
5. If reading is less than 10K ohms, then the two circuits may be shorted.