GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Savana 🢒 DTC 🢒 DTC P0370

Circuit Description

The optical sensor provides a high resolution signal to the powertrain control module (PCM) by counting the pulses on the sensor disk which is located in the injection pump. The high resolution is one of the most important inputs by the PCM for the fuel control and the timing. This test monitors the number of high resolution pulses which have been missed (not detected). This test is based on a comparison between the number of pulses that were detected since the last pump cam pulse and the number of the pulses that should have occurred.

Conditions for Running the DTC

The engine is operating.

Conditions for Setting the DTC

A number of high resolution pulses missing (internal to the PCM) per every 8 cam reference pulse.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/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
    • Intermittent DTCs P0251, P0370 and P1216 may be caused by air entering the fuel system when fuel levels get below 1/8 of a tank while performing hard acceleration or turning maneuvers. DTC P0251, P0370 and P1216 may set if the vehicle has run out of fuel. Customer driving habits should be checked to determine if vehicle has been performing in these manners. If the vehicle has been performing under these conditions, bleed fuel system of all air and test drive vehicle.
    • When the PCM is in backup fuel, fast idle and poor performance problems will exist. If DTC P0251 is also stored, the Snapshot mode on the scan tool should be used to in order to properly identify a malfunction. DTC P0335, P1216, and P1217 may set along with this DTC.
    • The least likely cause of failure is the PCM.

Test Description

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

  1. This step determines if the 5-volt reference is present.

    2.

  2. This step checks the ground circuit.

    3.

  3. This step determines if the problem is currently active by the scan tool displaying Last Test Failed. Do not proceed any further with this table if the scan tool does not display the term Last Test Failed. Duplicating the conditions in Freeze Frame/Failure Records can help create an active fault.

    4.

  4. This step determines if a High Res signal is being sent to the PCM. Refer to the RPM vs Hertz table to compare the Hertz readings at different RPM. Use the Freeze Frame/Failure Records to help identify the RPM when the problem occurs.

    5.

  5. The PCM supplies 5 volts on the signal circuit. This step determines if that voltage is present, not present, or too much voltage is present.

    6.

  6. This step determines if the signal circuit is shorted to 5 volts. A normal high res signal circuit will have 3-5 mA. Any reading over 25 mA indicates a short to 5 volts.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Important: Before clearing the DTCs, use the scan tool Capture Info to save the Freeze Frame/Failure Records for reference. The data in the control module is deleted once the Clear Info function is used.

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

  1. Turn OFF the ignition.
  2. Disconnect the optical/fuel temperature sensor electrical connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Use a DMM in order to measure the voltage between the optical sensor 5-volt reference circuit at the harness connector and chassis ground.

Does the voltage measure at the specified value?

4.8-5.2 V

Go to Step 3

Go to Step 6

3

  1. Verify the optical/fuel temperature sensor electrical connector is still disconnected.
  2. Probe the sensor ground circuit with an J 35616-200 Unpowered Test Light connected to B+ at the harness connector.

Does the test lamp illuminate?

--

Go to Step 4

Go to Step 7

4

  1. Reconnect the optical/fuel temperature sensor electrical connector.
  2. Start the engine.
  3. Operate the engine close to the conditions saved in the Freeze Frame/Failure Records.
  4. Use the scan tool in order to select DTC and the Specific DTC function.
  5. Enter the DTC number that was set.

Does the scan tool display the term Last Test Failed?

--

Go to Step 5

Go to Diagnostic Aids

5

    Important: The scan tool must display Last Test Failed (under DTC, Specific). This ensures the fault is active while performing this test.

  1. Maintain the engine speed at the point of failure, per the conditions saved in the Freeze Frame/Failure Records.
  2. With the DMM on the Hertz (Hz) scale, backprobe the high resolution signal circuit at the PCM.

Referring to the Engine Speed Versus Hertz (Hz) table, does the Hertz reading in the table correspond (plus or minus 100 Hz) with the Hertz reading on the DMM at the designated RPM?

--

Go to Step 16

Go to Step 9

6
  1. Turn OFF the ignition.
  2. Disconnect the PCM, and check the optical sensor 5-volt reference circuit for the following:
    3. • An open
    • A short to ground
    • A short to the sensor ground circuit
  3. If the optical sensor 5-volt reference circuit is open or shorted to ground, repair the circuit as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you perform a repair?

--

Go to Step 19

Go to Step 8

7
  1. Check for an open or a poor sensor ground terminal connection at the PCM.
  2. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you perform a repair?

--

Go to Step 19

Go to Step 18

8

Check the optical sensor 5-volt reference circuit for a poor connection at the PCM and replace the terminal if necessary. Refer to Wiring Repairs in Wiring Systems.

Did you perform a repair?

--

Go to Step 19

Go to Step 18

9

  1. Disconnect the optical/fuel temperature sensor electrical connector.
  2. Turn ON the ignition, with the engine OFF.
  3. With a DMM connected to ground, probe the high resolution signal circuit at the harness connector.

Does the voltage measure within the specified value?

4.8-5.2 V

Go to Step 13

Go to Step 10

10

Does the voltage measure more than the specified value?

4.8-5.2 V

Go to Step 11

Go to Step 12

11

Repair the short to battery/ignition voltage on the high resolution signal circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 19

--

12
  1. Turn OFF the ignition.
  2. Disconnect the PCM, and check the high resolution signal circuit for any of the following conditions:
    3. • An open
    • A short to ground
    • A short to the sensor ground circuit
  3. If a problem is found, repair the problem as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you perform a repair?

--

Go to Step 19

Go to Step 16

13

With a DMM set to the mA scale, measure the current between the high resolution signal circuit and the ground circuit at the optical/fuel temperature sensor electrical connector.

Does the current measure less than the specified value?

25 mA

Go to Step 15

Go to Step 14

14

Repair the short to reference voltage on the high resolution signal circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 19

--

15
  1. Check for a poor connection at the injection pump.
  2. If a problem is found, repair the problem as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you perform a repair?

--

Go to Step 19

Go to Step 17

16
  1. Turn OFF the ignition.
  2. Disconnect the PCM, and check the high resolution signal circuit for a poor connection at the PCM.
  3. If a problem is found, repair the problem as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you perform a repair?

--

Go to Step 19

Go to Step 18

17

Important: The new injection pump must be timed.

Replace the fuel injection pump. Refer to Fuel Injection Pump Replacement and to Fuel Injection Pump Timing Adjustment .

Did you complete the replacement?

--

Go to Step 19

--

18

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 19

--

19
  1. Use the scan tool in order to clear the DTCs.
  2. Start the engine.
  3. Allow the engine to idle until the engine reaches the normal operating temperature.
  4. Select DTC and the Specific DTC function.
  5. Enter the DTC number that was set.
  6. Operate the vehicle within the Conditions for Setting this DTC until the scan tool indicates the diagnostic ran.

Does the scan tool indicate the diagnostic passed?

--

Go to Step 20

Go to Step 2

20

Does the scan tool display any additional undiagnosed DTCs?

--

Go to Diagnostic Trouble Code (DTC) List

System OK