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 concern 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. 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 tests 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. Accumulate data during different operating conditions and based on the customer description of the intermittent concern. Reference the known good data values located in Reference Values, Control System Diagnostic Sheet Reference. 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
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. Sensors such as mass air flow (MAF) and RPM that change abruptly when the vehicle is traveling at a constant speed are clues to a possible concern area.

Look for an agreement in related signals. For example, if the APP1, or APP2, is changed during acceleration, a corresponding change should occur in RPM, fuel volume regulator (F_VCV) and fuel pulse width (FUELPW) PIDs.

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