GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC K Sierra - 4WD (LPG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 DTC P1635

Circuit Description

Certain engine sensors require a 5 volt reference voltage source. The PCM supplies this voltage. The individual sensor 5 volt reference voltage circuits are shared within the PCM. The alternative fuels ECM monitors the 5 volt reference voltage the PCM supplies.

Conditions for Running the DTC

The ignition voltage is between 6-18 volts.

Conditions for Setting the DTC
    • The alternative fuels ECM monitors a 5 volt reference voltage that is less than 4 volts or greater than 6 volts.
    • The condition exists for at least 1 second.

Action Taken When the DTC Sets
    • The alternative fuels ECM will illuminate the MIL.
    • The alternative fuels ECM will store this DTC as History during one drive trip in which the diagnostic runs and fails.
    • The alternative fuels ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

Conditions for Clearing the DTC
    • The Last Test Failed status will clear when the DTC runs and passes.
    • The alternative fuels ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
    • The History status will clear after 40 consecutive warm-up cycles with no failures of any DTC.
    • All DTC information can be cleared using a scan tool.

Step

Action

 

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Alternative Fuels Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

2
  1. Observe the Freeze Frame/Failure Records data for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC, as specified in the supporting text, or as close as possible to the Freeze Frame/Failure Records data that you observed.

Does the DTC fail this ignition?

--

Go to Step 3

Go to Intermittent Conditions

3

Are both DTC P1635 and P1639 also set in the PCM?

--

Go to Step 7

Go to Step 4

4

Is DTC P1639 set in the PCM?

--

Go to Step 8

Go to Step 5

5
  1. Turn OFF the ignition.
  2. Disconnect the alternative fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .
  3. Turn ON the ignition, with the engine OFF.
  4. Measure the voltage on the 5 volt reference signal circuit with a J 39200 DMM.

Does the DMM display a voltage within the specified range?

4.8 V-5.2 V

Go to Step 11

Go to Step 6

6

Test the 5 volt reference circuit between the alternative fuels ECM and the PCM for an open condition. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 12

7
  1. Turn OFF the ignition.
  2. Disconnect the alternative fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .
  3. Turn ON the ignition, with the engine OFF.
  4. Measure the voltage on the 5 volt reference signal circuit with a J 39200 DMM.

Does the DMM display a voltage above the specified value?

5.2 V

Go to Step 9

Go to Step 13

8
  1. Turn OFF the ignition.
  2. Disconnect the alternative fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .
  3. Turn ON the ignition, with the engine OFF.
  4. Measure the voltage on the 5 volt reference signal circuit with a J 39200 DMM.

Does the DMM display a voltage within the specified range?

4.8 V-5.2 V

Go to Step 13

Go to Step 10

9

Test the following 5 volt reference circuits for a short to battery voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

    • alternative fuels ECM 5 volt reference signal
    • gasoline fuel tank pressure (FTP) sensor
    • TP sensor
    • gasoline fuel level signal
    • MAP sensor
    • EGR sensor

Did you find and correct the condition?

--

Go to Step 15

Go to Step 14

10

Test the following 5 volt reference circuits for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

    • alternative fuels ECM 5 volt reference signal
    • gasoline fuel tank pressure (FTP) sensor
    • throttle position (TP) sensor

Did you find and correct the condition?

--

Go to Step 15

Go to Step 12

11

Inspect for poor connections at the harness connectors of the alternative fuels ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 13

12

Inspect for poor connections at the harness connectors of the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 14

13

Replace the alternative fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 15

--

14

Important: KL6 models require a switchover to gasoline with a scan tool in order to perform the Crankshaft Position Variation Learn Procedure.

KL8 models do not require the Crankshaft Position Variation Learn Procedure.

Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls in the C/K Pickup Service Manual.

Did you complete the replacement?

--

Go to Step 15

--

15
  1. Use the scan tool in order to clear the DTCs from the following control modules:
    2. • Alternative Fuels ECM
    • PCM
  2. Turn OFF the ignition for 30 seconds.
  3. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 16

Go to Step 2

16

With a scan tool, observe the stored information, Capture Info.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK