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Electronic Control System Gas Plus Engines – FAULT CODE 256

Compressor Inlet Temperature Sensor Circuit – Shorted Low

Overview

Compressor Inlet Temperature Sensor Circuit – Shorted Low

CODE REASON EFFECT
Fault Code: 256
PID: P171
SPN: 171
FMI: 4
LAMP: Amber
SRT: 

Compressor Inlet Temperature Sensor Circuit – Voltage below normal, or Shorted to low source. Low signal voltage detected on the compressor inlet circuit at the ECM.

Possible reduced engine performance. Fan will stay ON if controlled by ECM. No engine protection for compressor inlet temperature.

Compressor Inlet Temperature Sensor Circuit
SMALL | MEDIUM | LARGE

Circuit Description

The electronic control module (ECM) uses the compressor inlet temperature sensor input to determine the temperature of the air entering the turbocharger.

Component Location

The compressor inlet temperature sensor is located in the intake piping to the turbocharger (combination sensor with humidity).

Shop Talk

  • The compressor inlet temperature sensor is a combination sensor with the humidity sensor.

All temperature sensors:

  • The resistance of the sensor varies with temperature. The reading observed must compare to the following table if the sensor is functioning properly.
Temperature (°C) Temperature (°F) Resistance (ohms)
0 32 30k to 36k
25 77 9k to 11k
50 122 3k to 4k
75 167 1350 to 1500
100 212 600 to 675

Cautions and Warnings

 CAUTION 

To reduce the possibility of damaging a new ECM, all other active fault codes must be investigated prior to replacing the ECM.
 CAUTION 

To reduce the possibility of pin and harness damage, use the following test leads when taking a measurement:
Part Number 3822758 – male Deutsch/AMP/Metri-Pack test lead
Part Number 3823996 – female Weather-Pack test lead.

Troubleshooting Steps

STEPS SPECIFICATIONS
STEP 1. Check the fault codes.  
  STEP 1A. Check for an inactive fault code. Fault Code 256 inactive?
STEP 2. Check the compressor inlet temperature sensor and circuit.  
  STEP 2A. Inspect the compressor inlet temperature sensor and connector pins. Dirty or damaged pins?
  STEP 2B. Check the circuit response. Fault Code 249 active and Fault Code 256 inactive?
  STEP 2C. Check for circuit response Fault Code 256 active?
STEP 3. Check the ECM and engine harness.  
  STEP 3A. Inspect the ECM and engine harness connector pins. Dirty or damaged pins?
  STEP 3B. Check for a pin to pin short circuit in the engine harness. Greater than 100k ohms?
  STEP 3C. Check for a pin short to ground. Greater than 100k ohms?
  STEP 3D. Check for an inactive fault code. Fault Code 256 inactive?
STEP 4. Clear the fault codes.  
  STEP 4A. Disable the fault code. Fault Code 249 inactive?
  STEP 4B. Clear the inactive fault codes. All fault codes cleared?

Guided Step 1 – Check the fault codes.

Guided Step 1A – Check for an inactive fault code.

Conditions

  • Turn keyswitch ON
  • Connect the INSITE™ electronic service tool.

Action

Check for an inactive fault code.

Use INSITE™ electronic service tool to read the fault codes.

Fault Code 256 inactive?
YES NO
No Repair No Repair

Guided Step 2 – Check the compressor inlet temperature sensor and circuit.

Guided Step 2A – Inspect the compressor inlet temperature sensor and connector pins.

Conditions

  • Turn keyswitch OFF
  • Disconnect the compressor inlet temperature sensor from the engine harness.

Action

Inspect the engine harness and compressor inlet temperature sensor connector pins for the following:

  • Corroded pins
  • Bent or broken pins
  • Pushed back or expanded pins
  • Wire insulation damaged
  • Moisture in or on the connector
  • Missing or damaged connector seals
  • Connector shell broken
  • Dirt or debris in or on the connector pins.

For general inspection techniques, refer to Component Connector and Pin Inspection, Procedure 019-361.

Dirty damaged pins?
YES NO

A defective connection has been detected in the sensor or harness connector.

Repair the damaged pins. Repair or replace the engine harness, whichever has the damaged pins.

  • Flush the dirt, debris, or moisture from the connector pins using electronic contact cleaner, Part Number 3824510.
  • Install the appropriate connector seal if it is damaged or missing.
  • Repair the engine harness. Refer to Procedure 019-204.
  • Replace the engine harness. Refer to Procedure 019-043.
No Repair

Guided Step 2B – Check the circuit response.

Conditions

  • Turn keyswitch OFF
  • Disconnect the compressor inlet temperature sensor from the engine harness
  • Turn keyswitch ON
  • Connect INSITE™ electronic service tool.

Action

Check for the appropriate circuit response after 30 seconds.

  • Use INSITE™ electronic service tool to read the fault codes.

Fault Code 249 active and Fault Code 256 inactive?
YES NO
No Repair No Repair

Guided Step 2C – Check the fault codes and verify sensor condition.

Conditions

  • Turn keyswitch OFF
  • Disconnect the compressor inlet temperature sensor from the engine harness
  • Turn keyswitch ON
  • Connect INSITE™ electronic service tool.

Action

Check for the appropriate circuit response after 30 seconds.

  • Use INSITE™ electronic service tool to read the fault codes.

Fault Code 256 active?
YES NO

A defective sensor has been detected.

Replace the humidity/temperature sensor. Refer to Procedure 019-356.

 No Repair

Guided Step 3 – Check the ECM and engine harness.

Guided Step 3A – Inspect the engine harness and ECM connector pins.

Conditions

  • Turn keyswitch OFF
  • Disconnect engine harness connector 1 from the ECM.

Action

Inspect the engine harness and ECM connector 1 pins for the following:

  • Corroded pins
  • Bent or broken pins
  • Pushed back or expanded pins
  • Wire insulation damaged
  • Moisture in or on the connector
  • Missing or damaged connector seals
  • Connector shell broken
  • Dirt or debris in or on the connector pins.

For general inspection techniques, refer to Component Connector and Pin Inspection, Procedure 019-361.

Dirty damaged pins?
YES NO

A defective connection has been detected in the ECM connector or engine harness connector.

Repair the damaged pins. Repair or replace the engine harness, whichever has the damaged pins.

  • Flush the dirt, debris, or moisture from the connector pins using electronic contact cleaner, Part Number 3824510.
  • Install the appropriate connector seal if it is damaged or missing.
  • Repair the engine harness. Refer to Procedure 019-204.
  • Replace the engine harness. Refer to Procedure 019-043.
No Repair

Guided Step 3B – Check for a pin to pin short circuit in the engine harness.

Conditions

  • Turn keyswitch OFF
  • Disconnect the engine harness connector 1 and connector 2 from the ECM
  • Disconnect the compressor inlet temperature sensor from the harness.

Action

Check for a pin to pin short.

  • Measure the resistance between the compressor inlet temperature SIGNAL pin in the engine harness ECM connector 1 and all other pins in the engine connector 1.
  • Measure the resistance between the compressor inlet temperature SIGNAL pin in the engine harness ECM connector 1 and all other pins in the engine connector 2.

Refer to the circuit diagram or wiring diagram for component pin identification.

For general inspection techniques, refer to Component Connector and Pin Inspection, Procedure 019-360.

Greater than 100k ohms?
YES NO
No Repair

A pin to pin short circuit on the SIGNAL line has been detected in the engine harness

Repair or replace the engine harness, whichever has the damaged pins.

  • Flush the dirt, debris, or moisture from the connector pins using electronic contact cleaner, Part Number 3824510.
  • Install the appropriate connector seal if it is damaged or missing.
  • Repair the engine harness. Refer to Procedure 019-204.
  • Replace the engine harness. Refer to Procedure 019-043.

Guided Step 3C – Check for a pin short circuit to ground.

Conditions

  • Turn keyswitch OFF
  • Disconnect the engine harness connector 1 from the ECM
  • Disconnect the compressor inlet temperature sensor from the engine harness.

Action

Check for a pin to ground short.

  • Measure the resistance between the compressor inlet temperature SIGNAL pin in the engine harness ECM connector 1 and ground.

Refer to the circuit diagram or wiring diagram for connector pin identification.

For general resistance measurement techniques, refer to Measurements Using a Multimeter and Wiring Diagram, Procedure 019-360.

Greater than 100k ohms?
YES NO
No Repair

A pin to pin to ground short circuit on the SIGNAL line has been detected in the engine harness.

Repair or replace the engine harness.

  • Flush the dirt, debris, or moisture from the connector pins using electronic contact cleaner, Part Number 3824510.
  • Install the appropriate connector seal if it is damaged or missing.
  • Repair the engine harness. Refer to Procedure 019-204.
  • Replace the engine harness. Refer to Procedure 019-043.

Guided Step 3D – Check for an inactive fault code.

Conditions

  • Connect all components
  • Turn keyswitch ON
  • Connect the INSITE™ electronic service tool.

Action

Check for the appropriate circuit response after 30 seconds.

Use INSITE™ electronic service tool to read the fault codes.

Fault Code 256 inactive?
YES NO
No Repair

Replace the ECM. Call for pre-authorization. Refer to Procedure 019-031.

Guided Step 4 – Clear the fault codes.

Guided Step 4A – Disable the fault code.

Conditions

  • Connect all components
  • Turn keyswitch ON
  • Connect the INSITE™ electronic service tool.

Action

Disable the fault code.

  • Start the engine and let it idle for one minute.
  • Use INSITE™ electronic service tool to verify that the fault code is inactive.

Fault Code 256 inactive?
YES NO
No Repair No Repair

Guided Step 4B – Clear the inactive fault codes.

Conditions

  • Connect all components
  • Turn keyswitch ON
  • Connect the INSITE™ electronic service tool.

Action

Clear the inactive fault codes.

  • Use INSITE™ electronic service tool to erase the inactive fault codes.

All fault codes cleared?
YES NO
No Repair No Repair
Repair complete
Appropriate troubleshooting charts
Last Modified:  02-May-2005
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