Intermittent Diagnostic Techniques

Intermittent diagnostic techniques help find and isolate the root cause of intermittent faults associated with the 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 fault starts with recreating the fault symptom, accumulating and comparing 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 an alternative fuel system react abnormally.)
^ Technical Service Bulletins (TSBs) and OASIS messages, if available, have been reviewed.
^ Quick Test and associated pinpoint diagnosis have been completed without finding a fault, and the symptom is still occurring.
^ Customer Information Worksheet and the Bi-Fuel Drivability Diagnostic Form have been utilized to record customer test and data during diagnosis, or before contacting Hotline for technical assistance.

Recreating The Fault
Recreating the fault is the first step in isolating the cause of the intermittent symptom. Listed below are some of the conditions for recreating the fault.

CONDITIONS TO RECREATE FAULT

Conditions To Recreate Fault:

Accumulating Alternative Fuel Data
Alternative Fuel data can be accumulated in a number of ways. Gather as much data as possible when the fault is occurring to prevent misdiagnosis. Data should be accumulated during different operating conditions, and based on the customer description of the intermittent fault. Refer to the known good data values located at Reference Values in the Typical Diagnostic Reference Values. Typical Diagnostic Reference Values

This will require recording data in four conditions for comparison: 1) Key On Engine Off (KOEO), 2) HOT IDLE, 3) 48 km/h (30 MPH) and 4) 88 km/h (55 MPH). Acquisition of AFCM PID data using a diagnostic tool is one of the easiest ways to gather information. Refer to the following for instructions on gathering PID data using a diagnostic tool.


SELECTING AND VIEWING PIDs

NOTE: Always view PCM PIDs before AFCM PIDs.

1. Key on or start vehicle.
2. Select VEHICLE & ENGINE SELECTION and enter the proper year, vehicle and engine package from the menu.
3. Select DIAGNOSTIC DATA LINK.
4. Select AFCM-ALTERNATIVE FUEL CONTROL MODULE.
5. Select PID/DATA MONITOR AND RECORD.
6. 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.
7. Select the START key and access the PIDs.

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 diagnostic tool or Digital Multimeter (DMM). The diagnostic tool is capable of recording the value from the measurement jacks on the diagnostic tool 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.

Analyzing Alternative Fuel 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, as well as dome, trunk and hood lights are off. Use any of the following methods to diagnose a suspected alternative fuel wire circuit or device. Some methods are specific to a certain type of alternative fuel device.
^ Coil resistance (solenoids/relays)
^ Harness opens
^ Harness shorts

Change Input and Verify Output Response
The purpose is to verify how the compuvalve and actuator circuit respond to sensor input.
1. Select, view and record the appropriate sensor PID(s).
2. Create a condition to cause input condition to change.
3. Observe change in actuator PID or actuator signal circuit measured by use of a diagnostic tool.