GM Service Manual Online
For 1990-2009 cars only

Circuit Description

Alcohol content information is supplied to the powertrain control module (PCM) from the fuel composition sensor. The fuel composition sensor (FCS) measures fuel temperature through an internal thermistor. The fuel conductivity and capacitance are electronically measured inside a small fuel cell. Internal circuitry combines the electrical measurement and fuel temperature information using a microprocessor to produce a Hertz frequency output.

Circuitry inside the sensor provides a default output of 170 Hz if the fuel conductivity is too high or an internal fault is detected. A test of the fuel must be made to determine the cause of the 170 Hz output.

Conditions for Running the DTC

    • The engine run time is more than 30 seconds.
    • The ignition voltage is more than 10.9 volts.

Conditions for Setting the DTC

The FCS frequency is more than 155 Hz for 12.5 seconds.

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

The Hz and ms parameters on the scan tool react immediately to any change of state in the ground or signal circuits. Hz and ms may show a change, but the fault may not occur long enough to set the DTC. Watch Hz and ms to identify intermittent problems in these two circuits.

The ignition positive voltage circuit needs a hard fault to set a code. An intermittent fault here may not show up by looking at Hz or ms. If the DTC will not reset and Hz and ms are steady, thoroughly inspect all FCS circuits while paying close attention to the ignition positive voltage circuit wire and or terminals for an intermittent open.

Important: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing/replacing the PCM. Verify that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

The following may cause an intermittent:

    • Mis-routed harness
    • Rubbed through wire insulation
    • Broken wire inside the insulation
    • Poor terminal contact

Test Description

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

  1. This step inspects for a frequency that is out of range or intermittent at the time of the test.

  2. This step determines if the intermittent high frequency is due to water droplets passing through the sensor.

  3. This step compares the reading from the FCS with an external test tool to determine the accuracy of the vehicle sensor and the compatibility of the fuel in the tank.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views or Powertrain Control Module Connector End Views

1

Did you perform the Diagnostic System Check-Engine Controls?

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Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

  1. Connect a scan tool.
  2. Start the engine and allow the engine to idle.
  3. Read the fuel composition frequency on the scan tool.

Is the reading steady, above the specification?

155 Hz

Go to Step 5

Go to Step 3

3

  1. Allow the engine to idle.
  2. Agitate the fuel in the fuel tank by pushing the rear of the vehicle from side to side for 5 seconds.
  3. Monitor the scan tool fuel composition frequency for 5 minutes.

Does the frequency rise above the specification at any time?

155 Hz

Go to Step 4

Go to Diagnostic Aids

4

  1. Turn OFF the engine.
  2. Relieve the fuel system pressure. Refer to Fuel Pressure Relief .
  3. Remove the flexible fuel line from the injector fuel rail.
  4. Place the flexible fuel line into a suitable container and activate the fuel pump. Collect about 1 liter (0.264 gal) of fuel and allow the fuel sample to settle.

Is any water present in the sample of fuel taken?

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Go to Step 9

Go to Step 5

5

Connect the flexible fuel line to the injector rail.

Perform an accuracy test of the fuel composition tester.

  1. Fill the tester with acetone.
  2. Connect a DMM to the tester terminals.
  3. Set the DMM to AC Hertz scale and take a reading.

Is the reading within 6 Hz of the specification?

136 Hz

Go to Step 6

Go to Step 11

6

Important: Drain the acetone completely from the tester. Clean the beaker with a clean dry paper towel before taking a fuel sample. Be careful not to allow fingerprints, sweat, water, or any other outside debris contaminate the fuel sample, beaker, tester or fuel gauge as this will cause an error in the test result, leading to a misdiagnosis.

  1. Perform a Fuel Test using the fuel composition tester.
  2. Install a fuel pressure gauge into the service port located on the rear of the injector rail.
  3. Start the engine and open the valve on the fuel pressure gauge. Allow enough fuel to pass through the gauge to flush any previous fuel from inside the hose.
  4. Draw fuel from the fuel pressure gauge into the beaker supplied with the test kit.
  5. Transfer the fuel from the beaker into the fuel tester.
  6. Turn ON the fuel tester and note the red and green LED.

Is the green LED ON?

--

Go to Step 8

Go to Step 7

7

Is the red LED ON?

--

Go to Step 9

Go to Step 11

8

  1. Connect a DMM to the fuel tester terminals.
  2. Set the DMM to the AC Hz scale and take a reading of the fuel sample.
  3. Start the engine and monitor the fuel composition frequency on the scan tool.

Is the reading from the scan tool and the DMM within the specified amount?

12 Hz

Go to Step 9

Go to Step 10

9

Connect the flexible fuel line to the injector rail.

  1. The fuel is contaminated.
  2. Drain the fuel tank. Refer to Fuel Tank Draining .
  3. Replace the fuel with E85 or less.

Did you complete the replacement?

--

Go to Step 13

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10

Replace the FCS. Refer to Fuel Composition Sensor Replacement .

Did you complete the replacement?

--

Go to Step 13

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11

Replace the 9-volt battery and refill with a fuel sample.

Is the RED or GREEN led ON?

--

Go to Step 5

Go to Step 12

12

The fuel composition tester has a malfunction. Send tester to a repair facility.

Refer to the user manual for shipping and address information.

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13

  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 14

Go to Step 2

14

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