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FAULT CODE 121

Engine Position Sensor (EPS) Circuit

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Overview

CODE REASON EFFECT
Fault Code: 121
PID: 190
SPN: 190
FMI: 10
LAMP: Yellow
SRT: 00-081		 No engine speed signal detected at one pair of pins, either pins 1 and 15 or pins 11 and 14 of the sensor harness. None on performance.

Engine Position Sensor (EPS) Circuit

Circuit Description

The engine position sensor monitors the engine position and the engine speed and passes this information to the electronic control module (ECM) through the sensor harness.

Component Location

  • M11 – The engine position sensor is located above the accessory drive.
  • N14 – The engine position sensor is located at the backside of the gear housing flange on the cylinder block, just above the lubrication pump.

Shop Talk

  • If the fault occurs
    only at a certain engine operating temperature, check the engine position sensor circuit while the engine is at that particular temperature.
  • Check the camshaft end play to make sure the camshaft gear is
    not moving too far away from the end of the engine position sensor.
  • Check for moisture in the sensor harness connector at the ECM.
  • If the engine runs rough, check that the engine position sensor is wired with proper polarity. Correct ECM-to-engine position sensor pin-to-pin wiring
    must be observed.

Cautions and Warnings

 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.
 CAUTION 

 

To ease installation and removal, use this special tool for the engine position sensor:
Part Number 3822747 – installation tool, engine position sensor.
 CAUTION 

 

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

Troubleshooting Steps

STEPS SPECIFICATIONS
STEP 1. Check the engine position sensor.
STEP 1A. Inspect the engine position sensor and the sensor harness connector pins. No damaged pins
STEP 1B. Read the fault codes. Fault Code 121 active
STEP 1C. Check for a short circuit to ground in the engine position sensor. More than 10M ohms
STEP 1D. Inspect the engine position sensor. No damaged engine position sensor
STEP 1E. Check the resistance of the engine position sensor. 1000 to 2000 ohms
STEP 1F. Check for a short circuit between the coils. More than 10M ohms
STEP 2. Check the sensor harness.
STEP 2A. Inspect the sensor harness and the ECM connector pins. No damaged pins
STEP 2B. Read the fault codes. Fault Code 121 active
STEP 2C. Check for proper pin-to-pin wiring between ECM and engine position sensor. Wired correctly
STEP 2D. Check for pin-to-pin continuity. Less than 10 ohms
STEP 2E. Check for a short circuit to the ground. More than 100k ohms
STEP 2F. Check for a short circuit from pin to pin. More than 100k ohms
STEP 2G. Check for an open circuit. 1000 to 2000 ohms
STEP 3. Clear the fault codes.
STEP 3A. Disable the fault code. Fault Code 121 inactive
STEP 3B. Clear the inactive fault codes. All fault codes cleared

Guided Step 1 – Check the engine position sensor.

Guided Step 1A – Inspect the engine position sensor and the sensor harness connector pins.

Conditions

  • Turn keyswitch OFF.
  • Disconnect the sensor connector from the engine position sensor.
  • Flush and clean the connector pins using electronic contact cleaner, Part Number 3824510.

Action

Inspect the engine postion circuit and the sensor harness connector pins for:

  • Loose connector
  • Corroded pins
  • Bent or broken pins
  • Pushed back or expanded pins
  • Moisture in or on the connector
  • Missing or damaged connector seals
  • Dirt or debris in or on the connector pins
  • Connector shell broken
  • Wire insulation damage
  • Damaged connector locking tab.
OK NOT OK
No damaged pins A damaged connection has been detected in the ECM connector or engine harness.

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

  • Flush the dirt, debris, or moisture from the connector pins, use 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 in Section 19.

Refer to Procedure 019-043 in Section 19.

  • Call for pre-authorization and replace the ECM.

Upon receipt of authorization, refer to Procedure 019-031 in Section 19.

Guided Step 1B – Read the fault codes.

Conditions

  • Turn keyswitch OFF.
  • Connect all components.

Action

Read the fault codes.

  • Start and idle the engine for one minute.
  • Read the fault codes using INSITE™ electronic service tool.
OK NOT OK
Fault Code 121 active Repair complete.

Guided Step 1C – Check for a short circuit to ground in the engine position sensor.

Conditions

  • Turn keyswitch OFF.
  • Disconnect the sensor harness from the engine position sensor.

Action

Check for a short circuit to the ground.

  • Measure the resistance from pin A of the engine position sensor to the engine block ground.
  • Measure the resistance from pin B of the engine position sensor to the engine block ground.
  • Measure the resistance from pin C of the engine position sensor to the engine block ground.
  • Measure the resistance from pin D of the engine position sensor to the engine block ground.

Note: Install the mating connector with short test leads to the sensor connector. While testing the sensor, electrical leads from the sensor to the sensor connector
must be gently flexed to check for damaged or partially broken wires under the insulation.
OK NOT OK
More than 10M ohms Replace the engine position sensor. Refer to Procedure
019-038 in Section 19.

 CAUTION 

 

To ease installation and removal, use this special tool for the engine position sensor:
Part Number 3822747 – installation tool, engine position sensor.
Guided Step 1D – Inspect the engine position sensor.

Conditions

  • Turn keyswitch OFF.
  • Disconnect the sensor connector from the engine position sensor.
  • Remove the engine position sensor using the engine position sensor installation tool, Part Number 3822747.

Action

Inspect the engine position circuit for:

  • Metal debris on end of sensor
  • Damage to end of sensor caused by camshaft gear
  • Oil leakage or insulation problems such as swelling
  • Damaged electrical potting in sensing end of sensor.
OK NOT OK
No damaged engine position sensor Replace the engine position sensor. Refer to Procedure
019-038 in Section 19.

Guided Step 1E – Check the resistance of the engine position sensor.

Conditions

  • Turn keyswitch OFF.
  • Disconnect the sensor connector from the engine position sensor.

Action

Check for an open circuit in the engine position circuit.

  • Measure the resistance from pin A to pin D in the sensor connector.
  • Measure the resistance from pin B to pin C in the sensor connector.

Note: Install the mating connector with short test leads to the sensor connector. While testing the sensor, electrical leads from the sensor to the sensor connector
must be gently flexed to check for damaged or partially broken wires under the insulation.
OK NOT OK
1000 to 2000 ohms Replace the engine position sensor. Refer to Procedure
019-038 in Section 19.

Guided Step 1F – Check for a short circuit between the coils.

Conditions

  • Turn keyswitch OFF.
  • Disconnect sensor connector from the engine position sensor.

Action

Check for a short circuit between the coils.

  • Measure the resistance from pin A to pin B of the engine position sensor.

Note: Install the mating connector with short test leads to the sensor connector. While testing the sensor, the electrical leads from the sensor to the sensor connector
must be gently flexed to check for damaged or partially broken wires under the insulation.

Note: A false low reading will occur if you are touching the test leads while taking the measurement.
OK NOT OK
More than 10M ohms Replace the engine position sensor. Refer to Procedure
019-038 in Section 19.

Guided Step 2 – Check the sensor harness.

 CAUTION 

 

To reduce the possibility damaging a new ECM, all other active fault codes must be investigated prior to replacing the ECM.
Guided Step 2A – Inspect the sensor harness and the ECM connector pins.

Conditions

  • Turn keyswitch OFF.
  • Disconnect the sensor harness connector from the ECM.
  • Flush and clean the connector pins using electronic contact cleaner, Part Number 3824510.

Action

Inspect the sensor harness and the ECM connector pins for:

  • Loose connector
  • Corroded pins
  • Bent or broken pins
  • Pushed back or expanded pins
  • Moisture in or on the connector
  • Missing or damaged connector seals
  • Dirt or debris in or on the connector pins
  • Connector shell broken
  • Wire insulation damage
  • Damaged connector locking tab.
OK NOT OK
No damaged pins A damaged connection has been detected in the ECM connector or engine harness.

Repair the damaged pins. Repair or replace the sensor harness or replace the ECM, whichever has the damaged pins.

  • Flush the dirt, debris, or moisture from the connector pins, use electronic contact cleaner, Part Number 3824510.
  • Install the appropriate connector seal if it is damaged or missing.
  • Repair the sensor harness.
    Refer to Procedure 019-204 in Section 19.

Refer to Procedure 019-043 in Section 19.

  • Call for pre-authorization and replace the ECM.

Upon receipt of authorization refer to Procedure 019-031 in Section 19.

Guided Step 2B – Read the fault codes.

Conditions

  • Turn keyswitch OFF.
  • Connect all components.

Action

Read the fault codes.

  • Start and idle the engine for 1 minute.
  • Read the fault codes, use INSITE™ electronic service tool.
OK NOT OK
Fault Code 121 active Repair complete.

Guided Step 2C – Check for proper pin-to-pin wiring between ECM and engine position sensor.

Conditions

  • Turn keyswitch OFF.
  • Disconnect sensor harness connector from ECM.
  • Disconnect engine position sensor from sensor connector.

Action

Check the engine postion circuit for proper pinout.

  • Engine position sensor red wire to pin D of sensor connector; D to pin 14 of sensor harness connector at ECM.
  • Engine position sensor black wire to pin A of sensor connector; A to pin 11 of sensor harness connector at ECM.
  • Engine position sensor white wire to pin B of sensor connector; B to pin 15 of sensor harness connector at ECM.
  • Engine position sensor green wire to pin C of sensor connector; C to pin 1 of sensor harness connector at ECM.
OK NOT OK
Wired correctly Correct the pinout.

Guided Step 2D – Check for pin-to-pin continuity.

Conditions

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

Action

Measure the resistance from pin to pin:

  • Measure from pin D of the sensor connector to pin 14 of the sensor harness connector of the ECM.
  • Measure from pin A of the sensor connector to pin 11 of the sensor harness connector of the ECM.
  • Measure from pin B of the sensor connector to pin 15 of the sensor harness connector of the ECM.
  • Measure from pin C of the sensor connector to pin 1 of the sensor harness connector of the ECM.
OK NOT OK
Less than 10 ohms Repair or replace sensor harness.

Guided Step 2E – Check for a short circuit to the ground.

Conditions

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

Action

Check for a short circuit to the ground.

  • Measure the resistance from pin 1 to the engine block ground.
  • Measure the resistance from pin 11 to the engine block ground.
  • Measure the resistance from pin 14 to the engine block ground.
  • Measure the resistance from pin 15 to the engine block ground.
OK NOT OK
More than 100k ohms Repair or replace the sensor harness.

Guided Step 2F – Check for a short circuit from pin to pin.

Conditions

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

Action

Check for a short circuit from pin to pin.

  • Measure the resistance from pin 1 to all other pins in the sensor harness connector.
  • Measure the resistance from pin 11 to all other pins in the sensor harness connector.
  • Measure the resistance from pin 14 to all other pins in the sensor harness connector.
  • Measure the resistance from pin 15 to all other pins in the sensor harness connector.
OK NOT OK
More than 100k ohms Repair or replace the sensor harness.

Guided Step 2G – Check for an open circuit.

Conditions

  • Turn keyswitch OFF.
  • Disconnect the sensor harness connector from the ECM.
  • Connect the engine position sensor to the sensor harness.

Action

Check for an open circuit.

  • Measure resistance from pin 11 to pin 14 at the sensor harness connector.
  • Measure resistance from pin 1 to pin 15 at the sensor harness connector.
OK NOT OK
1000 to 2000 ohms Repair or replace the sensor harness.

Guided Step 3 – Clear the fault codes.

Guided Step 3A – Disable the fault code.

Conditions

  • Connect all the components.

Action

Disable the fault code.

  • Start the engine and let it idle for 1 minute.
  • Verify Fault Code 121 is inactive.
OK NOT OK
Fault Code 121 inactive Return to the troubleshooting steps or contact a Cummins® Authorized Repair Location if all the steps have been completed and checked again.

Guided Step 3B – Clear the inactive fault codes.

Conditions

  • Connect all the components.

Action

Clear the inactive fault codes.

  • Use INSITE™ electronic service tool to clear the inactive fault codes.
OK NOT OK
All fault codes cleared? Troubleshoot any remaining active fault codes.
Repair complete
Appropriate troubleshooting charts
Last Modified:  22-Jan-2009
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