GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The powertrain control module (PCM) uses dual crankshaft position (CKP A and CKP B) sensors in order to determine crankshaft position. The PCM supplies an ignition voltage and a ground for each sensor. During engine rotation, a slotted ring, machined into the crankshaft, causes the sensors to return a series of ON and OFF pulses to the PCM. The PCM uses these pulses to decode the position of the engine crankshaft.

The PCM uses two basic methods of decoding the engine position: Angle Based and Time Based (using either CKP A or CKP B sensor input). During normal operation, the PCM uses the angle based method. In order to operate in this mode, the PCM must receive signal pulses from both CKP sensors. The PCM uses the signal pulses to determine an initial crankshaft position, and to generate MEDRES (24X reference) and LORES (4X reference) signals. Once the initial crank position is determined, the PCM continuously monitors both sensors for valid signal inputs. As long as both signal inputs remain, the PCM will continue to use the angle based mode.

When either CKP signal is lost, the PCM will compare the MEDRES signal to the camshaft position (CMP) sensor signal. If the PCM detects a valid CMP signal, and the MEDRES to CMP signal correlation is correct, the PCM determines that CKP sensor A is at fault. However, if the MEDRES to CMP correlation is incorrect, the PCM determines that CKP sensor B is at fault. If the PCM determines that CKP sensor B is at fault, DTC P0385 will set. The PCM will switch from angle based mode to Time Based Mode A using CKP sensor A signal input.

Conditions for Running the DTC

The engine is cranking or running.

Conditions for Setting the DTC

The PCM detects a loss of CKP sensor B signal.

Action Taken When the DTC Sets

    • The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • 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 emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

If the condition is intermittent, refer to Intermittent Conditions .

Ignition system DTCs set with the ignition in the START position if the starter relay or the starter is inoperative. When the PCM enables starter operation, the PCM also initiates the diagnostic test routines for DTCs P0335, P0340, and P0385. If a condition exists which prevents the engine from cranking, the PCM will not receive signal input from the CKP and CMP sensors, and the DTCs will set.

If DTC P0615 is set, diagnose DTC P0615 first. If DTC P0335, P0340, and/or P0385 are set and no trouble is found, check for the following conditions:

    • Is there a no-crank condition or an intermittent no-crank condition?
    • Was an attempt made to crank the engine with the shift lever not in P/N?

Procedures for Selecting Crank Position Sensing Decode Mode

Diagnosing the crankshaft position sensors may require enabling a specific decode mode (Angle, Time A, or Time B). To enable a specific crank decode, using the scan tool, perform the following steps:

    • Turn the ignition to the RUN/ON position.
    • Select Engine Output Controls.
    • Select Crank Position Sensing Decode Mode.
    • Select the desired mode (Angle, Time A, or Time B) and press SELECT STATE.
    • Command the decode mode by pressing COMMAND STATE.

The commanded state remains valid for the current ignition cycle. A specific decode mode can not be commanded with the engine running, or after commanding a desired decode mode. To command a different decode mode, you must cycle the ignition OFF and ON.

Test Description

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

  1. This step determines if the 12-volt reference circuit to the CKP A, CKP B, and the CMP is shorted to a ground. If all 3 DTCs are set, this indicates the circuit is shorted to a ground or a sensor is internally shorted. All 3 DTCs set because the 12-volt reference circuit are internally connected within the PCM. The engine will not start with this condition.

  2. DTC P0336 indicates an intermittent loss of CKP sensor A signal while in Time A mode. DTC P0336 only sets after the PCM detects a loss of CKP sensor B signal and has switched to Time A mode.

    During normal operation, the PCM uses the angle mode in order to determine engine position. In order to operate in this mode, the PCM must receive valid input from both CKP sensors. If the PCM detects a loss of CKP sensor A signal, the PCM switches to a Time B mode using CKP sensor B input in order to determine engine position. If the PCM detects a loss of CKP sensor B, the PCM switches to a Time A mode using CKP sensor A input to determine engine position.

  3. During engine operation, if moving the harnesses and connectors related to CKP sensor B results in a switch to Time A mode, may also cause an engine stall condition, an intermittent condition in the harnesses or connectors is present. If the harnesses and connectors are OK, CKP sensor B may be the cause of the intermittent condition.

  4. In order to test the CKP sensor B signal circuit, the PCM must be commanded to Time B mode using the scan tool. This allows the PCM to monitor input signals from CKP sensor B. Refer to Diagnostic Aids for the procedures to selecting the Crank Decode Mode.

  5. This step determines if the CMP sensor is shorted internally. If the 12-volt reference parameter changes from Fault to OK, the condition is with the sensor.

  6. This step determines if the CKP A sensor is shorted internally. If the 12-volt reference parameter changes from Fault to OK, the condition is with the sensor.

  7. This step determines if the CKP B sensor is shorted internally. If the 12-volt reference parameter changes from Fault to OK, the condition is with the sensor.

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. Install a scan tool.
  2. Attempt to start the engine.
  3. Monitor the DTC information with a scan tool.

Are all three DTCs P0335, P0340, and P0385 set?

Go to Step 11

Go to Step 3

3

Important: If DTC P0336 is also set, continue with the diagnostic table for DTC P0385 before diagnosing DTC P0336.

  1. Start the engine.
  2. Monitor the CKP sensor status parameter with a scan tool.

Does the scan tool indicate the CKP sensor status as ANGLE?

Go to Step 4

Go to Step 5

4

  1. With the engine running, move the harnesses and connectors related to CKP sensor B (by hand only).
  2. If the CKP sensor status changes or an engine stall occurs, locate and repair the condition in the harnesses/connectors related to CKP sensor B. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Diagnostic Aids

5

  1. Turn ON the ignition, with the engine OFF.
  2. Disconnect the CKP sensor B. Refer to Crankshaft Position Sensor Replacement .
  3. Connect the test lamp between the 12 volt reference circuit of the CKP B sensor and a good ground.

Does the test lamp illuminate?

Go to Step 6

Go to Step 8

6

Connect the test lamp between the 12-volt reference circuit of the CKP B sensor and the low reference circuit of the CKP B sensor.

Does the test lamp illuminate?

Go to Step 7

Go to Step 9

7

  1. Turn ON the ignition, with the engine OFF.
  2. Clear the DTCs with a scan tool.
  3. With a scan tool, command the PCM to operate in time mode B.
  4. With the test lamp connected to battery positive voltage, repeatedly touch the CKP B sensor 2 signal circuit in the harness connector.
  5. Observe the 24X crank sensor data parameter with a scan tool.

Does the scan tool indicate an RPM while touching the test lamp to the signal circuit?

Go to Step 15

Go to Step 10

8

  1. Turn OFF the ignition.
  2. Disconnect the PCM harness connector. Refer to Powertrain Control Module Replacement .
  3. Test the 12-volt reference circuit of the CKP B sensor for an open. Refer to Testing for Continuity and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 16

9

  1. Turn OFF the ignition.
  2. Disconnect the PCM harness connector. Refer to Powertrain Control Module Replacement .
  3. Test the low reference circuit of the CKP B sensor for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 16

10

  1. Turn OFF the ignition.
  2. Disconnect the PCM harness connector. Refer to Powertrain Control Module Replacement .
  3. Turn ON the ignition, with the engine OFF.
  4. Test the CKP B signal circuit for an open, a short to ground or either CKP low reference, a short to voltage, or a short to the CKP A signal circuit. Refer to Testing for Continuity , Testing for Short to Ground , or Testing for a Short to Voltage and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 16

11

  1. Disconnect the CMP sensor harness connector. Refer to Camshaft Position Sensor Replacement .
  2. Monitor the 12-volt reference parameter with a scan tool.

Does the scan tool display OK?

Go to Step 17

Go to Step 12

12

  1. Raise the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  2. Disconnect CKP sensor A harness connector. Refer to Crankshaft Position Sensor Replacement .
  3. Monitor the 12-volt reference parameter with a scan tool.

Does the scan tool display OK?

Go to Step 18

Go to Step 13

13

  1. Disconnect the CKP B sensor harness connector. Refer to Crankshaft Position Sensor Replacement .
  2. Monitor the 12-volt reference parameter with a scan tool.

Does the scan tool display OK?

Go to Step 19

Go to Step 14

14

  1. Turn OFF the ignition.
  2. Disconnect the PCM harness connectors. Refer to Powertrain Control Module Replacement .
  3. Test the 12-volt reference circuit of the CKP A, the CKP B, and the CMP sensor for a grounded circuit. Refer to Testing for Short to Ground and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 20

15

Inspect for poor connections at the harness connector of the CKP B sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 19

16

Inspect for poor connections at the harness connector of the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 20

17

Replace the CMP sensor. Refer to Camshaft Position Sensor Replacement .

Did you complete the replacement?

Go to Step 21

--

18

Replace the CKP sensor A. Refer to Crankshaft Position Sensor Replacement .

Did you complete the replacement?

Go to Step 21

--

19

Replace the CKP sensor B. Refer to Crankshaft Position Sensor Replacement .

Did you complete the replacement?

Go to Step 21

--

20

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

Go to Step 21

--

21

  1. Clear the DTCs with a scan tool.
  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 22

Go to Step 2

22

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