Engine Pressure Sensor Open or Short Circuit - Test
Engine Pressure Sensor Open or Short Circuit - Test
Engine Pressure Sensor Open or Short Circuit - Test
Troubleshooting
C15 On-highway Engines
Media Number -RENR5088-10 Publication Date -01/02/2009 Date Updated -25/02/2009
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Background Information
The Engine Control Module (ECM) continuously creates a pull-up voltage on the signal
wire for each sensor. The ECM uses this pull-up voltage in order to detect a problem in the
signal circuit. When the ECM detects voltage that is above a threshold on the signal wire,
the ECM activates a -3 diagnostic code. When the ECM detects voltage that is below a
threshold on the signal wire, the ECM activates a -4 diagnostic code.
Note: There may be a delay of 30 seconds in order for Caterpillar Electronic Technician
(ET) to display an active diagnostic code. When you check for a diagnostic code, be sure to
wait at least 30 seconds.
The most likely cause of a code is a problem with an electrical connector or wiring. The
least likely cause of a code is the ECM.
Some atmospheric pressure sensors that are located in the engine block actually sense
crankcase pressure rather than atmospheric pressure. With this type of installation,
crankcase pressure of approximately 14 kPa (2 psi) can cause the 108-3 diagnostic code to
be generated.
Before you proceed with this test, determine whether the sensor is sensing crankcase
pressure or atmospheric pressure.
If the sensor is sensing crankcase pressure, connect Caterpillar Electronic Technician (ET)
to the ECM and clear the diagnostic code. Do not start the engine. Maintain electrical
power to the ECM. Wait at least 30 seconds for generation of the diagnostic code. Slowly
wiggle the wiring for the sensor in an attempt to generate an intermittent code.
If the 108-3 code is not generated, there is either a problem with high crankcase pressure or
the code is intermittent, due to a loose connector or damaged wiring.
A plugged crankcase breather will cause excessive crankcase pressure. Leakage of the
crankshaft seal can occur. Sealing of the piston rings can be lost. This will enable crankcase
blowby to further increase the crankcase pressure.
If the piston rings do not seal, blowby will increase the crankcase pressure and the problem
will become worse. Performance will deteriorate. The engine will have poor combustion.
Deposits will build up on the pistons, on the valves, and in the cylinder heads. Downward
pumping of the pistons against the increased crankcase pressure will further reduce the
performance.
Sticking of the pistons will also increase the crankcase pressure. If the problem is not
resolved, piston seizure will result. If a problem with a piston is suspected, do not operate
the engine. Inspect the cylinder liner for scoring and for scuffing. Replace the piston rings
and the cylinder liner, if necessary.
To eliminate crankcase pressure as the cause of the 108-3 diagnostic code, inspect the
crankcase breather. If the breather is plugged, clean the breather or replace the breather.
Make sure that the air passages for the breather's system are clear.
To verify that the plugged breather was the problem, operate the engine at a high load for at
least 30 seconds. If the code does not recur, return the engine to service.
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Illustration 1
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Illustration 2
Locations of the engine pressure sensors (typical left side engine view)
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Illustration 3
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Illustration 4
Terminal locations at the P2 ECM connector for the engine pressure sensors
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Illustration 5
Terminal locations at the sensor connector for the engine pressure sensors
(Terminal C) Signal
C. Monitor the active diagnostic code and the logged diagnostic codes on Cat ET.
Note: Wait at least 30 seconds in order for diagnostic codes to become active.
Results:
Logged code -
Repair: Do not troubleshoot a logged code unless the code relates to an operator
complaint. If the code is logged and the code does not relate to an operator
complaint, clear the code.
If the code is logged and the code relates to an operator complaint, proceed to Test
Step 2.
Active code - Proceed to Test Step 3.
C. Check for an intermittent problem at the connector for the suspect sensor:
Note: Do not disconnect any harness connectors in order to perform this procedure.
a. Carefully install the spoons (multimeter probes) into the terminal locations
for the +5 volt sensor supply and the sensor return at the connector for the
suspect sensor.
b. While you observe the voltage reading on the multimeter, wiggle the wires
and pull on the wires at the connector for the suspect sensor.
The voltage reading will not vary more than 0.5 volts for a solid electrical
connection.
Note: Do not disconnect any harness connectors in order to perform this procedure.
a. Carefully install the spoons (multimeter probes) into the terminal locations
for the +5 volt sensor supply and the sensor return at the P2 ECM connector.
b. While you observe the voltage reading on the multimeter, wiggle the wires
and pull on the wires for the +5 volt sensor supply and the sensor return at
the ECM connector.
The voltage reading will not vary more than 0.5 volts for a solid electrical
connection.
Expected Result:
The voltage reading did not vary more than 0.5 volts during either test.
Results:
OK - The voltage reading did not vary more than 0.5 volts.
Repair: The connections are OK at the connectors. There does not appear to be an
intermittent problem in the circuit at this time. Return the engine to service.
STOP
Not OK - The voltage reading varied more than 0.5 volts during the test.
Repair: Disconnect the suspect connector and inspect the connector and the
terminals for moisture, damage, and corrosion. Repair the connectors and/or the
terminals. Return the engine to service.
STOP
Measure the voltage between terminal A (sensor supply) and terminal B (sensor return) at
the harness connector for the sensor that relates to the code.
Expected Result:
Results:
OK - The voltage is between 4.5 VDC and 5.5 VDC. The supply voltage is at the
sensor connector. Record the voltage measurement. If you are troubleshooting a -3
diagnostic code, proceed to Test Step 4. If you are troubleshooting a -4 diagnostic
code, proceed to Test Step 7.
Not OK - The voltage is less than 4.5 VDC or the voltage is greater than 5.5 VDC.
Repair: The voltage of the sensor supply is incorrect. Refer to Troubleshooting, "5
Volt Engine Pressure Sensor Supply Circuit - Test".
STOP
Measure the voltage between the signal terminal and the return terminal on the harness
connector for the sensor that relates to the diagnostic code.
Expected Result:
Results:
OK - The voltage measurement is less than the +Battery voltage. The signal wire is
not shorted to the +Battery. Proceed to Test Step 5.
Not OK - The voltage measurement is approximately equal to the +Battery voltage.
The signal wire is shorted to the +Battery.
Repair: Repair the wiring, when possible. Replace parts, if necessary. Verify that
the problem is resolved.
STOP
B. Connect a jumper wire between the signal terminal and the return terminal on the
harness connector for the sensor that relates to the diagnostic code. This will replace
the sensor with a short circuit.
Expected Result:
The -3 code does not change to a -4 code when the jumper wire is connected.
Results:
OK - The -3 code does not change to a -4 code when the jumper wire is connected.
Connect the sensor. There is a problem with the wiring harness or with the ECM.
Proceed to Test Step 6.
Not OK - The -3 code changes to a -4 code when the jumper wire is connected. The
wiring harness and the ECM are OK.
2. Connect a new sensor to the engine harness. Do not install the sensor into
the engine.
5. Clear all logged diagnostic codes and return the engine to service.
STOP
A. Determine the P2 signal terminal and the P2 return terminal that relates to the
diagnostic code.
B. Remove the signal terminal and the return terminal from the P2 connector.
C. Install a jumper wire into the P2 connector between the signal terminal and the
return terminal. This will replace the engine wiring with a short circuit.
Expected Result:
Results:
OK - A -4 code is active when the jumper wire is installed. The ECM detected the
jumper wire at the P2 connector. However, the ECM did not detect the jumper wire
at the harness connector for the sensor. There is an open circuit in the wiring.
Repair: Repair the wiring, when possible. Replace parts, if necessary. Verify that
the problem is resolved.
STOP
Not OK - A -4 code is not active when the jumper wire is installed. The ECM does
not detect the jumper wire at the P2 connector. There is a problem with the ECM.
4. Install the original ECM and verify that the original problem returns.
5. If the problem returns with the original ECM, replace the original ECM.
Refer to Troubleshooting, "Replacing the ECM".
STOP
Expected Result:
Results:
1. Connect a new sensor to the engine harness. Do not install the sensor into
the engine.
4. Clear all logged diagnostic codes and return the engine to service.
STOP
Expected Result:
A -3 code is active when the signal wire is disconnected from the P2 connector.
Results:
Repair: Repair the wiring or the connector, when possible. Replace parts, if
necessary. Verify that the problem is resolved.
STOP
NOt OK - A -3 code is not active when the signal wire is disconnected from the P2
connector. The ECM did not detect the open circuit at the P2 connector. There is a
problem with the ECM.
4. Install the original ECM and verify that the original problem returns.
5. If the problem returns with the original ECM, replace the original ECM.
Refer to Troubleshooting, "Replacing the ECM".
STOP
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