Circuit Description
While the engine is running on either gasoline or CNG, the ECU monitors the Natural Gas Operation (NGO) enable circuit, Malfunction Indicator Lamp (MIL) circuit and fuel pump relay control circuit for a short to ground or for an open or a short to power. A failure of the NGO enable circuit may cause driveability complaints caused by the VCM delivering gasoline spark timing while operating on CNG. A failure of the MIL circuit may not result in loss of MIL operation but only a loss of the ability of the Alternative Fuel Engine Control Unit (AF ECU) to control the MIL. A failure of the fuel pump relay control circuit may result in either no gasoline operation or premature fuel pump failure depending on the nature of the fault. This is a type D DTC.
Conditions for Setting the DTC
The following conditions will set DTC 27:
| • | The ignition is ON. |
| • | The ECU MIL output circuit is open, shorted to ground or shorted to power. |
| • | The NGO enable circuit is open, shorted to ground or shorted to power. |
| • | The AF fuel pump relay control circuit is open, shorted to ground or shorted to power. |
Action Taken When the DTC Sets
The ECU will set DTC 27 after one driving trip with the fault active.
Conditions for Clearing the MIL/DTC
The ECU turns the MIL OFF after three consecutive driving trips without a fault condition present. A DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°C (72°F) from the start-up coolant temperature, and the engine coolant temperature exceeds 70°C (160°F) during that same ignition cycle) or the code clearing procedure has been used.
Diagnostic Aids
This diagnostic will not detect a fault in a circuit shorted to ground if the active state of the circuit should be grounded. This also applies to circuits which are open or shorted to power.
| • | Diagnose all other VCM and ECU diagnostic trouble codes before beginning this diagnostic procedure. |
| • | Verify that the vehicle has a sufficient quantity of fuel to start on CNG. |
| • | Verify that the vehicle starts and runs on CNG: |
| - | Monitor the fuel mode indicator lamp. |
| - | Depress the fuel gauge selector switch and observe the fuel gauge movement. |
Inspect any circuitry that is suspected of causing an intermittent complaint for the following conditions:
| • | Backed out terminals |
| • | Improper mating |
| • | Broken locks |
| • | Improperly formed or damaged terminals |
| • | Poor terminal wiring connections |
| • | Physical damage to the wiring harness |
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
-
This step determines if the MIL circuit is shorted to ground.
-
This step determines if the NGO enable circuit is shorted to ground.
-
This step determines if the fuel pump relay control circuit is open.
-
This step determines if the fuel pump relay control circuit is shorted to ground.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Alternative Fuels (AF) Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check |
2 |
Is the MIL ON? | -- | Go to Step 3 | |
Does the MIL turn OFF when commanded? | -- | Go to Step 4 | ||
4 |
Is the fuel indicator lamp OFF? | -- | Go to Step 5 | Go to Diagnostic Aids |
5 | Inspect the status of the natural gas operation (NGO) circuit using a scan tool. Does the NGO circuit display ENABLED? | -- | Go to Step 6 | Go to Step 7 |
Does the NGO circuit display DISABLED? | -- | Go to Step 10 | Go to Step 9 | |
7 |
Is there continuity? | -- | Go to Step 8 | Go to Step 21 |
8 | Inspect the NGO enable circuit for a short to power using a DVOM. Is a problem found? | -- | Go to Step 21 | Go to Step 17 |
9 |
Is there continuity? | -- | Go to Step 21 | Go to Step 19 |
Is the test lamp illuminated? | -- | Go to Step 14 | Go to Step 11 | |
Is the test lamp illuminated? | -- | Go to Step 12 | Go to Step 15 | |
12 |
Is the test lamp illuminated? | -- | Go to Step 13 | Go to Ignition Relay Diagnosis |
13 | Inspect for an open in the AF fuel pump relay between pin 85 and pin 86. Is a problem found? | -- | Go to Step 16 | Go to Intermittent Conditions |
14 |
Is there continuity to ground? | -- | Go to Step 21 | Go to Step 22 |
15 |
Is there continuity? | -- | Go to Step 19 | Go to Step 21 |
16 | Replace the AF fuel pump relay. Is the action complete? | -- | Go to Step 23 | -- |
17 | Inspect for faulty VCM connections. Is a problem found? | -- | Go to Step 21 | Go to Step 18 |
18 |
Important:: Reprogram the VCM if it is faulty. Refer to VCM Replacement/Programming in Engine Controls - 5.0, 5.7L in the 1998 C/K Truck Service Manual. Is the action complete? | -- | Go to Step 23 | -- |
19 | Inspect for faulty ECU connections. Is a problem found? | -- | Go to Step 21 | Go to Step 20 |
20 | Inspect for faulty connections at connector C007. Is a problem found? | -- | Go to Step 21 | Go to Step 22 |
21 | Repair the circuit as necessary. Refer to Repair Procedures in Electrical Diagnosis in the 1998 C/K Truck Service Manual. Is the action complete? | -- | Go to Step 23 | -- |
22 | Replace the ECU. Refer to Engine Control Unit Replacement . Is the action complete? | -- | Go to Step 23 | -- |
23 |
Are any DTCs displayed that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
