784C, 785B, 785C, 785D, 789B, 789C, 789D, 793B, 793C, 793D, 797 and 797B Off-Highway Trucks VIMS ™ Caterpillar

MID 87 - Chassis ECM

CID 849 - System Air Pressure Sensor

FMI 8 - Abnormal Frequency, Pulse Width, or Period

Illustration 1	g06130073
System air pressure sensor circuit

This diagnostic code indicates that the VIMS ECM detects the system air pressure sensor signal is abnormal. The signal may have one of the following abnormal characteristics:

• Frequency

• Pulse width

• Period

The pressure sensor has the following characteristics:

• Pulse width modulation (PWM) type of sensor

• Operating duty cycle from 0% to 100%

• Operating pressure range from 0 kPa (0 psi) to 1135 kPa (164 psi)

• Pressure and duty cycle relationship is linear

The following can cause this code:

• Sensor signal circuit shorted to ground

• Sensor failure

• ECM failure

Note: The following test procedure may create other diagnostic codes. Ignore these created codes and clear these created codes after the original diagnostic code has been corrected. Ensure diagnostic code CID 849 FMI 8 is active before performing this procedure.

Note: Use a calibrated digital multimeter for this procedure.

Test Step 1. CHECK THE SENSOR

1. Turn the disconnect switch and the key switch to the on position.

2. Disconnect the sensor from the harness and observe the diagnostic code status.

Expected Result:

CID 849 FMI 8 remains active and the FMI does not change.

Results:

• OK - CID 849 FMI 8 remains active and the FMI does not change. The sensor is correct. Proceed to Test Step 2.

• NOT OK - CID 849 FMI 8 is not active and the FMI changes. The sensor has failed.

  Repair: Replace the sensor.

  STOP

Test Step 2. CHECK FOR A SHORT TO GROUND

1. Turn the key switch and the disconnect switch to the off position.

2. Disconnect the harness from the sensor.

3. Disconnect the harness from the ECM.

4. At the harness connector for the ECM, measure the resistance from contact J1-28 (wire M914-WH(White)) to all contacts of the harness connectors for the ECM.

Expected Result:

Each resistance is greater than 5000 ohms.

Results:

• OK - Each resistance is greater than 5000 ohms. Proceed to Test Step 3.

• NOT OK - A resistance is less than 5000 ohms. There is a short between contact J1-28 (wire M914-WH(White)) and the circuit with low resistance.

  Repair: Repair or replace the harness.

  STOP

Test Step 3. CHECK THE DIAGNOSTIC CODE

The key switch and the disconnect switch remain in the off position.

1. Inspect the harness.

   Clean the harness connector contacts.

   Inspect the harness connector contacts for corrosion or damage.

   Inspect the wire insulation for damage caused by heat, corrosion, or chafing.

2. Perform a 45 N (10 lb) pull test on each circuit wire.

3. Reconnect all harness connectors.

   Verify that the connectors are fully seated and that the clips are fastened.

4. Check for intermittent problems. Using Cat^® Electronic Technician (Cat^® ET), perform a wiggle test on the harness.

5. Turn the disconnect switch and the key switch to the on position.

6. Operate the machine.

7. Check CID849 FMI 8 status.

Expected Result:

CID 849 FMI 8 is not active.

Results:

• OK - CID 849 FMI 8 is not active.

  Repair: The initial diagnostic code was probably caused by a poor connection at one of the connectors that was checked.

  Resume normal operation.

  STOP

• NOT OK - CID 849 FMI 8 is active. The ECM may have failed.

  Repair: Exit this procedure and perform this procedure again. Contact the technical communicator at your dealership for possible consultation with Caterpillar. If the ECM requires replacement, see Testing and Adjusting, "Electronic Control Module (ECM) - Replace".

  STOP