Alternative Fuels (AF) Powertrain On Board (OBD) System Check. GMC Truck GMC C Pickup

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC C Pickup - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Alternative Fuels (AF) Powertrain On Board (OBD) System Check

Circuit Description

The Alternative Fuels (AF) Powertrain On-Board Diagnostic (OBD) System Check is an organized approach to identifying a problem created by a control module system malfunction. The OBD is the starting point for any driveability concern diagnosis. The OBD directs the service technician to the next logical step in diagnosing the concern. This system check will address possible faults in two separate control module systems.

Important: Follow the system check carefully in order to avoid unnecessary diagnostic time.

Perform this check ONLY if there is a driveability complaint or if you have been sent to this system check from another section in the service information.

Understanding the table properly reduces the diagnostic time. Proper use of the OBD helps prevent the unnecessary replacement of good parts.

Diagnostic Aids

Important: Do not clear DTCs unless directed by a diagnostic procedure. Clearing the DTCs will also clear valuable Freeze Frame and Failure Records data.

Inspect all related wiring and connections including the connections at the VCM. These may cause an intermittent malfunction.

Inspect any circuitry that is suspected of causing an intermittent condition for the following conditions:

•	Backed out terminals

•	Improper mating

•	Broken locks

•	Improperly formed or damaged terminals

•	Poor terminal-to-wire connections

•	Physical damage to the wiring harness

•

Corrosion

Code Types

•	Type A: Emissions related. Once cycle to set the MIL and the DTC.

•	Type B: Emissions related. Two consecutive driving cycles to set the MIL and the DTC.

•	Type C: NOT USED: Non-emissions related. No MIL, but the SES light will turn on.

•	Type D: Non-emissions related. No MIL or SES light set.

AF OBD Trip

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

This step determines if the customer complaint is an MIL problem or a driveability problem with no MIL.

The scan tool should display serial data when the ignition switch is turned to ON. This step determines if there is power to the VCM.

Refer to DTC Identification for a list of valid DTCs. A faulty scan tool, VCM Programming or VCM may result in an invalid DTC.

This step verifies fuel indicator light operation.

This step enables the AF ECU DTC display mode

DTCs are displayed in ascending order. Each DTC flashes three (3) times prior to displaying the next available DTC. After all stored DTCs have been displayed, the AF ECU will begin the cycle again at DTC 12 until the AF ECU returns to normal operation.

This step clears all AF ECU DTCs and resets the fuel trim to its initial value of 1.0.

This test determines if the VCM can control the MIL.

This step verifies that the engine will start on CNG. If the vehicle does not start on CNG after an eight second crank, the vehicle will start on gasoline.

This step verifies CNG operation.

Comparison of an actual control system data with the Typical Scan Tool Data Values is a quick check to determine if any parameter is not within limits. A base engine problem (i.e., advanced cam timing) may substantially alter sensor values.

Alternative Fuel (AF) Powertrain OBD System Check

Step

Action

Value(s)

Yes

Important: Inspect for applicable service bulletins before proceeding with this test.

Important: Perform this test only if there is a driveability complaint or if you have been sent to this table from another section in the service information.

Important: Do not clear DTCs unless instructed by this diagnostic.

Turn the ignition OFF.
Connect the scan tool.
Turn the ignition ON with the engine OFF.

Is the MIL on?

Go to Step 2

Go to Malfunction Indicator Lamp Circuit Check

Turn the ignition ON with the engine OFF.

Does the scan tool display the VCM data?

Go to Step 3

Go to No Serial Data (1997 models) in the 1997 C/K Truck Service Manual or Data Link Connector Diagnosis (1998 models) in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual

Inspect for the following DTC information:

•	DTC Status Last Test Failed

•	Test Failed Since Code Cleared

•	Test Failed This Ignition

•	MIL Request

•	History DTC

Save the DTC Freeze Frame and Failure Records information using the scan tool if any of the above are set.

Were any VCM DTCs set?

Go to the applicable DTC table in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual

Go to Step 4

Turn the ignition OFF for ten seconds.
Turn the ignition ON with the engine OFF.
Observe the fuel indicator lamp in the fuel gauge selector switch.

Does the fuel gauge indicator lamp illuminate for the specified number of seconds and then turn off?

2 seconds

Go to Step 5

Go to Fuel Gauge Selector Switch/Fuel Indicator Lamp Diagnosis

Press the fuel gauge selector switch three times within ten seconds.

Does the fuel indicator lamp begin to flash?

Go to Step 6

Go to Fuel Gauge Selector Switch/Fuel Indicator Lamp Diagnosis

Important:: All DTCs will flash three times before proceeding to the next DTC. Codes 12 and 13 are normally present.

Observe and record any AF DTCs.

Are any DTCs set?

Go to Step 7

Go to Step 8

Press and hold the fuel gauge selector switch for ten seconds before going to the applicable DTC table. This will clear all the AF ECU DTCs and reset the fuel trim.

Important:: If the ignition switch is not cycled OFF for ten seconds after clearing any DTCs and the vehicle is started, the AF ECU OBD system will be disabled for that key cycle.

Is the action complete?

Go to the applicable DTC table