Circuit Description
The engine control module (ECM) uses the manifold absolute pressure (MAP) sensor to control the fuel delivery and the ignition timing. The MAP sensor measures the changes in the intake manifold pressure which results from engine load, intake manifold vacuum, and the RPM changes, and it converts these into voltage outputs. The ECM sends a 5-volt reference voltage to the MAP sensor. As the manifold pressure changes, the output of MAP sensor also changes. By monitoring the MAP sensor output voltage, the ECM knows the manifold pressure. A low pressure, low voltage, output voltage will be about 1-1.5 volts while the higher pressure, high voltage, output voltage will be about 4.5-4.8 volts at wide open throttle (WOT). The MAP sensor is also used, under certain conditions to measure barometric pressure (BARO), allowing the ECM to make adjustments for different altitude.
Conditions for Setting the DTC
| • | The MAP is greater than 103 kPa (15 psi). |
| • | No throttle position (TP) sensor fail conditions are present. |
| • | The engine is running more than 10 seconds. |
| • | The TP sensor is less than 20 percent if the RPM is less than 3,000. |
| • | The TP sensor is less than 30 percent if the RPM is greater than 3,000. |
Action Taken When the DTC Sets
| • | The malfunction indicator lamp (MIL) will illuminate. |
| • | The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers. |
| • | A history DTC is stored. |
| • | The ECM will substitute a fixed MAP value and use TP sensor to control the fuel delivery. The scan tool will not show defaulted value. |
Conditions for Clearing the MIL/DTC
| • | The MIL will turn OFF after 4 consecutive ignition cycles in which the diagnostic runs without a fault. |
| • | A history DTC will clear after 40 consecutive warm-up cycles without a fault. |
| • | DTCs can be cleared by using the scan tool. |
| • | Disconnect the ECM battery feed for more than 10 seconds. |
Diagnostic Aids
With the ignition ON and the engine stopped, the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the ECM as an indication of vehicle altitude. Comparison of this reading with a known good vehicle with the same sensor is a good way to check the accuracy of a suspect sensor. Readings should be the same 12 kPa (1.7 psi).
If a DTC P0108 is intermittent, refer to Manifold Absolute Pressure Sensor Diagnosis for further diagnosis.
Important: After repairs, use the scan tool FUEL TRIM RESET function to reset long-term fuel trim to 128 (0 percent).
DTC P0108 may set as result of a misfire. If misfire is present, repair the cause of misfire before using this table. The misfire counters may be used to determine which cylinders is misfiring.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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The On-Board Diagnostic System Check prompts the technician to complete some basic checks and store the Freeze Frame and Failure Records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
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This step will determine if DTC P0108 is the result of a hard failure or an intermittent condition.
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This step simulates conditions for a DTC P0107. If the ECM recognizes the change, the ECM, the 5-volt reference and the sensor signal circuits are OK.
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This step also looks for an open in the sensor ground circuit. If the circuit was open, additional DTCs will also be set. If no other DTCs are set and the circuit is found to be open, then the open must be between the MAP sensor and the electrical connector ground splice.
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When the sensor signal circuit is shorted to battery voltage, the TP will be displayed above 0 percent at all times and A/C high side will be displayed high. The vehicle will also remain in open loop.
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The MAP sensor vacuum source should only supply vacuum to the MAP sensor. Inspect the vacuum port for a restriction caused by casting flash.
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Disconnect all sensors that use a 5-volt reference one at a time while monitoring the short on the 5-volt reference circuit. Replace any sensor that may have caused the short on the 5-volt reference circuit.
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The replacement ECM must be programmed. to the latest Techline procedure for the ECM reprogramming.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Perform an On-Board Diagnostic System Check. Was the check performed? | -- | Go to Step 2 | ||
Does the scan tool display a MAP equal to or greater than the specified value? | 85 kPa (12.3 psi) | Go to Step 3 | Go to Step 4 | |
Does the scan tool display MAP less than the specified value? | 28 kPa (4 psi) | Go to Step 5 | Go to Step 6 | |
4 |
Does the scan tool display MAP equal to or greater than the specified value? | 85 kPa (12.3 psi) | Go to Step 3 | Go to Diagnostic Aids |
Probe the MAP sensor signal ground circuit at terminal A with a test light connected to battery +. Does the test light illuminate? | -- | Go to Step 7 | Go to Step 11 | |
Inspect the MAP sensor signal circuit at terminal A24 of the engine control module (ECM) for a short to voltage and repair as needed. Is a repair necessary? | -- | Go to Step 14 | Go to Step 12 | |
7 | With a ohmmeter connected to the ground, probe the 5-volt reference circuit terminal A31. Is the resistance near the specified value? | 5 V | Go to Step 8 | Go to Step 9 |
Inspect the MAP sensor vacuum source for being plugged or leaking. Is a repair necessary? | -- | Go to Step 10 | Go to Step 13 | |
Inspect the 5-volt reference circuit at terminal A31 for a short to voltage and repair as needed. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 12 | |
10 | Repair the vacuum source as needed. Did you complete the repair? | -- | Go to Step 14 | -- |
Inspect for an open in the MAP sensor ground circuit at terminal 1 and repair as needed. Did you complete the repair? | -- | Go to Step 14 | Go to Step 12 | |
12 |
Did you complete the replacement? | -- | Go to Step 14 | -- |
13 | Replace the MAP sensor. Refer to Manifold Absolute Pressure Sensor Replacement Did you complete the replacement? | -- | Go to Step 14 | -- |
14 |
Does the scan tool indicate that this diagnostic has run and passed? | -- | Go to Step 15 | Go to Step 2 |
15 | Are any DTCs displayed that have not been diagnosed? | -- | Go to applicable DTC table | System OK |
