Intermittent Fault Conditions

Intermittent Diagnostic Techniques 

Intermittent diagnostic techniques help find and isolate the root cause of intermittent concerns associated with the PCM. The material is organized to help find the concern and carry out the repair. There are examples illustrating the diagnostic techniques. The process of finding and isolating an intermittent concern starts with recreating a concern symptom, accumulating PCM data, 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 concern. Mechanical component conditions can make a PCM system react abnormally.
• TSB and OASIS messages, if available, are reviewed.
• Quick Test and associated diagnostic subroutines have been completed without finding a concern, and the symptom is still present.

Recreating the Fault 

Recreating the concern 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 MILDTC. Listed below are some of the conditions for recreating the concern:

CONDITIONS TO RECREATE FAULT 

Accumulating Powertrain Control Module (PCM) Data 

PCM data can be accumulated in a number of ways. This includes circuit measurements with a DMM or scan tool PID data. Acquisition of PCMPID data using a scan tool is one of the easiest ways to gather information. Gather as much data as possible when the concern is occurring to prevent improper diagnosis. Data should be accumulated during different operating conditions and based on the customer description of the intermittent concern.

For information on the functions of your scan tool, refer to the scan tool manufacturers manual for 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

Analyzing PCM Data 

Look for abnormal events 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. Sensor values such as MAF and RPM that change abruptly when the vehicle is traveling at a constant speed are clues to a possible concern.

Look for an agreement in related signals. For example, if the APP1, or APP2, changes during acceleration, a corresponding change should occur in RPM, F_VCV and FUELRATE PIDs.

Scroll through the PID data while analyzing the information. Look for sudden drops or spikes in the values.