GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC C Pickup - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 DTC 37 MAF Sensor Circuit High Voltage

Object Number: 322571 Size: LF
Handling Electrostatic Discharge Sensitive Parts Notice
Handling Electrostatic Discharge Sensitive Parts Notice
Handling Electrostatic Discharge Sensitive Parts Notice
Handling Electrostatic Discharge Sensitive Parts Notice
Handling Electrostatic Discharge Sensitive Parts Notice
Engine Controls Components
EVAP and EGR Controls
OBDII Symbol Description Notice

Circuit Description

The Mass Air Flow (MAF) sensor measures the amount of air entering the engine during a given time. The ECU uses the MAF information for fuel delivery calculations. A large quantity of air entering the engine indicates an acceleration or high load situation. A small quantity of air indicates deceleration or idle.

The MAF sensor produces a signal which can be monitored using a scan tool. The signal varies within a range of around 5 to 7 g/s at idle to near 125 g/s at maximum engine load. This DTC sets if the signal from the MAF sensor does not match a predicted value based on the MAP pressure, coolant temperature, throttle position and engine speed.

This is a type A DTC.

Conditions for Setting the DTC

Cranking Test

The following conditions will set the DTC:

    • The vehicle is in the CNG operation mode.
    • The ignition is ON and the engine is OFF.
    • The battery voltage is greater than 8 volts.
    • The MAF frequency is greater than 10.39 kHz (> 331 g/s) for 0.5 second.

Running Low Frequency Test

The following conditions will set the DTC:

    • The vehicle is in the CNG operation mode.
    • The engine run time is greater than 10 seconds.
    • The battery voltage is greater than 10 volts.
    • The engine is running at greater than 500 RPM.
    • The MAF frequency is greater than 10.39 kHz (> 331 g/s) for 0.5 seconds.

Action Taken When the DTC Sets

The ECU will set a DTC 37 and illuminate the MIL lamp after one fault has occurred if a MAF high input fault is detected. The ECU will simultaneously initiate a fuel mode switch-over to gasoline. The vehicle will not attempt to operate on CNG again until the next key cycle.

Conditions for Clearing the MIL/DTC

The ECU turns the MIL OFF after 3 consecutive driving trips without a fault conditions present. A DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°C (72°F) from the start-up coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during that same ignition cycle) or the code clearing procedure has been used.

Diagnostic Aids

Always diagnose ANY VCM codes that are present before beginning this diagnostic procedure.

Check for the following conditions:

    • A poor connection at the ECU or MAF sensor. Inspect the harness connectors for the following conditions:
       - Backed-out terminals
       - Improper mating
       - Broken locks
       - Improperly formed or damaged terminals
       - Poor terminal to wire connection
    • A mis-routed harness. Inspect the MAF sensor harness in order to ensure it is not routed too close to high voltage wires, such as spark plug leads.
    • A damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the scan tool while moving the connectors and the wiring harnesses related to the MAF sensor. A change in the display indicates the location of the fault.

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

  1. This step verifies the problem is present at idle.

    2.

  2. A frequency with the MAF sensor connector disconnected indicates an Electromagnetic Interference (EMI)-related fault or a poor connection.

Step

Action

Value(s)

Yes

No

1

Was the Alternative Fuels (AF) Powertrain On-Board Diagnostic (OBD) System Check performed?

--

Go to Step 2

Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check

2

  1. Start the engine.
  2. Monitor the MAF display on the scan tool with the engine idling.

Is the MAF above the specified value?

20 g/s

Go to Step 7

Go to Step 3

3

  1. Backprobe the MAF circuit in the ECU connector using a DVOM set to read frequency.
  2. Monitor the MAF circuit frequency on the DVOM with the engine idling.

Is the MAF frequency above the specified value?

10.39 kHz

Go to Step 7

Go to Step 4

4
  1. Operate the vehicle within the conditions for setting this DTC as specified in the supporting text.
  2. Verify that the MIL illuminates.

Does the MIL illuminate?

--

Go to Step 5

Go to Diagnostic Aids

5
  1. Turn OFF the engine.
  2. Disconnect ECU connector C003.
  3. Start the engine.
  4. Monitor the MAF frequency display on the DVOM with the engine idling.

Is the MAF frequency above the specified value?

10.39 kHz

Go to Step 7

Go to Step 6

6
  1. Inspect for a poor connection at the ECU connector.
  2. Replace the faulty terminal if a poor connection is found. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual.

Did you find a poor connection?

--

Go to Step 14

Go to Step 12

7
  1. Turn OFF the ignition.
  2. Disconnect the MAF sensor connector.
  3. Start the engine and allow it to idle.
  4. Monitor the MAF frequency using a scan tool.

Does the scan tool indicate an MAF frequency at the specified value?

0.0 g/s

Go to Step 8

Go to Step 10

8
  1. Inspect for a poor connection at the MAF sensor.
  2. Replace the faulty terminal if a poor connection is found. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual.

Did you find a poor connection?

--

Go to Step 14

Go to Step 9

9

Replace the MAF sensor. Refer to MAF Sensor Replacement in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual.

Is the action complete?

--

Go to Step 14

--

10
  1. Perform the following MAF sensor harness inspections for incorrect routing:
    2. • Near the secondary ignition wires or coils
    • Near the other high voltage components (solenoids, relays, motors)
  2. Correct any incorrect harness routing.

Was a problem found?

--

Go to Step 14

Go to Step 11

11
  1. Inspect the MAF signal circuit terminal connections at the VCM.
  2. Repair any poor connections that you find. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual.

Was a poor connection found?

--

Go to Step 14

Go to Step 13

12

Replace the ECU. Refer to Engine Control Unit Replacement .

Is the action complete?

--

Go to Step 14

--

13

Replace the VCM.

Important:: Reprogram the VCM if it is faulty. Refer to VCM Replacement/Programming in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual.

Is the action complete?

--

Go to Step 14

--

14
  1. Clear the DTC.
  2. Operate the vehicle within the conditions for setting this DTC as specified in the supporting text.
  3. Verify that the MIL does not illuminate.

Does the diagnostic run and pass?

--

Go to Step 15

Go to Step 2

15

Are any ECU or VCM DTCs displayed that have not been diagnosed?

--

Go to applicable DTC table

System OK