DDEC III/IV Single ECM Troubleshooting – Section 77.3 Troubleshooting Flash Code 77

Section 77.3
Troubleshooting Flash Code 77

There is an established procedure to troubleshoot Flash Code 77, p 81/3 and p 81/4 only (exhaust back pressure failed high or low). Refer to "77.3.1 Exhaust Back Pressure Fault" . There is no established procedure to troubleshoot other variations of this code. ‪

• The code is used to determine if high fuel temperature may be a cause of reduced power levels.
• High fuel temperature will reduce available horsepower.
• Refer to the recommendations of the vehicle manufacturer regarding the possible need for additional fuel cooling.

Section 77.3.1
Exhaust Back Pressure Fault

Follow these steps to troubleshoot an exhaust back pressure fault:‪

1. If code 81/3 displays, refer to "77.3.2 Exhaust Back Pressure Failed High" .
2. If code 81/4 displays, refer to "77.3.6 Exhaust Back Pressure Sensor (BPS) Failed Low" .

Section 77.3.2
Exhaust Back Pressure Failed High

Follow these steps to troubleshoot exhaust back pressure failed high:‪

1. Disconnect exhaust back pressure sensor.
2. Start and run the engine.
3. Check codes displayed.
   1. If code 81/3 displays, refer to "77.3.3 Check for Short to +5 Line" .
   2. If code 81/4 displays, refer to "77.3.5 Check Connectors" .

Section 77.3.3
Check for Short to +5 Line

Follow these steps to check for a short to the +5 volt line:‪

1. Unplug the ECM vehicle connector.
2. Measure resistance between cavities B and C. See Figure "Engine Temperature and Back Pressure Sensor Wiring Schematic" .
   

   Figure 1. Engine Temperature and Back Pressure Sensor Wiring Schematic

   1. If the resistance measures greater than 5 Ω, refer to "77.3.4 Check for Open" .
   2. If the resistance measures less than 5 Ω, wire #916 is shorted to #510. Repair the short and test.

Section 77.3.4
Check for Open

Perform the following steps to check for open: ‪

1. Install jumper between cavities A3 and C3.
2. Measure resistance between cavities A and C of the BPS connector.
   • If measured resistance is less than 50 Ω, refer to "77.3.5 Check Connectors" .
   • If measured resistance is greater than 50 Ω, wire #952 is open. Repair the open and test.

Section 77.3.5
Check Connectors

Perform the following steps to check the connectors: ‪

1. If connectors are not damaged, replace the Backpressure Sensor. Test.
2. If the connectors are corroded, bent, or damaged, repair or replace them. Test.

Section 77.3.6
Exhaust Back Pressure Sensor (BPS) Failed Low

Perform the following steps to troubleshoot this condition: ‪

1. Unplug BPS.
2. Install a jumper cable between cavities A and B.
3. Start engine.
   • If code 81/3 displays, refer to "77.3.7 Check for Short" .
   • If code 81/4 displays, wire #510 is open. Repair the open and test.

Section 77.3.7
Check for Short

‪

1. Remove jumper cable.
2. Measure continuity between cavities B and C.
   • If measured continuity is greater than 50 Ω, refer to "77.3.8 Check Connectors" .
   • If measured continuity is less than 50 Ω, wire #510 is shorted to wire #952. Repair the short and test.

Section 77.3.8
Check Connectors

Perform the following steps to check the connectors: ‪

1. If connectors are not damaged, replace the Back Pressure Sensor. Test.
2. If the connectors are corroded, bent, or damaged, repair or replace them. Test.