GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 GMC C-Series Medium Duty Truck C6/C7/C8 🢒 DTC 🢒 DTC P1659

Circuit Description

Ignition voltage is supplied directly to the engine overspeed lamp . The powertrain control module (PCM) controls the lamp by grounding the control circuit via an internal switch called a driver. The primary function of the driver is to supply the ground for the component being controlled. Each driver has a fault line which is monitored by the PCM. When the PCM is commanding a component ON, the voltage of the control circuit should be low, near 0 volts. When the PCM is commanding the control circuit to a component OFF, the voltage potential of the circuit should be high, near battery voltage. If the fault detection circuit senses a voltage other than what is expected, the fault line status will change causing the DTC to set.

Conditions for Running the DTC
    • The engine speed is more than 400 RPM.
    • The ignition voltage is between 6-18 volts.

Conditions for Setting the DTC

The PCM detects that the commanded state of the driver and the actual state of the control circuit do not match for a minimum of 5 seconds.

Action Taken When the DTC Sets
    • The control module stores the DTC information into memory when the diagnostic runs and fails.
    • The malfunction indicator lamp (MIL) will not illuminate.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records.
    • The driver information center, if equipped, may display a message.

Conditions for Clearing the DTC
    • A current DTC Last Test Failed clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.
    • Clear the DTC with a scan tool.

Diagnostic Aids
    • If the ignition feed circuit is suspected of being open, inspect for other lamps on that circuit not illuminating. An open fuse causes the entire cluster to be inoperative. Also, if the fuse is open, transmission DTCs set.
    • Using Freeze Frame/Failure Records data may aid in locating an intermittent condition. If the DTC cannot be duplicated, the information included in the Freeze Frame and Failure Records data can be useful in determining how many miles since the DTC set. The Fail Counter and Pass Counter can also be used to determine how many ignition cycles the diagnostic reported a pass or a fail. Operate vehicle within the same Freeze Frame conditions, such as RPM, load, vehicle speed, temperature etc., that were noted. This will isolate when the DTC failed. For an intermittent, refer to Intermittent Conditions .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Command both the ON and the OFF states. Repeat the commands as necessary.

    2.

  2. If you do not find trouble in the control circuit or the connection at the PCM, the PCM may be faulty. However, this is an extremely unlikely failure.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

  1. Verify whether the instrument panel is operational. If the instrument panel (IP) is completely inoperative, refer to Diagnostic System Check - Instrument Cluster in Instrument Panel, Gauges and Console.
  2. Verify whether the Engine Overspeed Lamp turns ON and OFF when commanded. Turn ON the ignition, with the engine OFF.
  3. Command the lamp ON and OFF using the scan tool. Repeat the commands as necessary.

Does the MIL turn ON and OFF when commanded?

Go to Diagnostic Aids

Go to Step 3

3
  1. Turn OFF the ignition.
  2. Disconnect the PCM connector C2 located on the opposite side of the manufacturer logo. Refer to Powertrain Control Module Replacement .
  3. Turn ON the ignition.

Is the lamp OFF?

Go to Step 4

Go to Step 5

4

Probe the lamp control circuit in the PCM harness connector with a fused jumper wire connected to ground. Refer to Using Fused Jumper Wires in Wiring Systems.

Is the lamp ON?

Go to Step 6

Go to Step 7

5

Repair the short to ground in the Engine Overspeed lamp control circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 9

--

6
  1. Inspect for poor connections at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
  2. If you find a poor connection, repair the terminal as necessary. Refer to Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 9

Go to Step 8

7

Test for the following conditions:

    • Faulty instrument panel cluster (IPC). Refer to Diagnostic System Check - Instrument Cluster in Instrument Panel, Gauges and Console.
    • Engine Overspeed Lamp battery positive voltage circuit open or shorted to ground. Refer to Wiring Repairs in Wiring Systems.
    • Control circuit open or shorted to B+. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 9

--

8

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

Go to Step 9

--

9
  1. Use the scan tool in order to clear the DTCs.
  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.

Does the DTC run and pass?

Go to Step 10

Go to Step 2

10

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