Circuit Description
The Powertrain Control Module (PCM) 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 result from the engine load (intake manifold vacuum) and the RPM changes and converts these measurements into voltage outputs. The PCM sends a 5 volt reference voltage to the MAP sensor. As the manifold pressure changes, the output voltage of the MAP sensor also changes. By monitoring the MAP sensor output voltage, the PCM determines the manifold pressure. A lower pressure (low voltage) output voltage will be about 1.0 to 1.5 volts at idle. A higher pressure (high voltage) output voltage will be about 4.5 to 4.8 volts at wide open throttle (WOT). The MAP sensor is also used under certain conditions to measure the barometric pressure, allowing the PCM to make adjustments for different altitudes.
Conditions for Running the DTC
| • | A DTCs P0122 or a DTC P0123 is not set. |
| • | The throttle position (TP) is less than 12 percent. |
| • | The vehicle speed is less than 2 km/h (1 mph). |
| • | The engine run time is more than 20 seconds. |
Conditions for Setting the DTC
| • | The MAP is more than 82 kPa. |
| • | The above conditions are met for at least 1.25 seconds. |
Action Taken When the DTC Sets
| • | The Malfunction Indicator Lamp (MIL) will illuminate. |
| • | The PCM will record the 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 coolant fan turns on. |
| • | The PCM will substitute a fixed MAP value and use the TP to control the fuel delivery. The scan tool will not show the defaulted value. |
| • | The vehicle will operate in Open Loop when the vehicle speed is less than 8 km/h (5 mph). The vehicle will operate in Closed Loop when the vehicle speed is more than 8 km/h (5 mph). |
Conditions for Clearing the MIL/DTC
| • | The MIL will turn off after 3 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. |
| • | Use a scan tool to clear the DTCs. |
Diagnostic Aids
Important: After completing any repairs, use the scan tool Fuel Trim Reset function to reset the long term fuel trim to 128 (0 percent).
With the ignition switch on and the engine stopped, the manifold pressure is equal to the atmospheric pressure and the signal voltage will be high.
This information is used by the PCM as an indication of the vehicle's 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 within 0.4 volt.
If a DTC P0108 is intermittent, refer to Symptoms . or to Manifold Absolute Pressure Sensor Output Diagnosis for further diagnosis.
A DTC P0108 may set as a result of misfire. If misfire is present, repair the cause of the misfire before using this table. The misfire counters may be used to determine which cylinder is misfiring.
If a DTC P0172 is also set, check the 5.0 volt reference circuit for a short to voltage.
Test Description
The number(s) below refer(s) to the step number(s) on the Diagnostic Table.
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This step simulates the conditions for a DTC P0107. If the PCM recognizes the change, the PCM, the 5.0 volt reference and the MAP sensor signal circuits are OK.
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This step will determine if the DTC P0108 is the result of a hard malfunction or an intermittent condition.
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This step looks for an open in the MAP sensor ground circuit.
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When the MAP sensor signal circuit is shorted to battery voltage, the TP will display above 0 percent at all times and the A/C High Side will display 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. Check the vacuum port for a restriction caused by casting flash.
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Important: The Powertrain Control Module (PCM) uses a single internal 5.0 volt power supply to supply power to all the engine control components. If one component shorts its 5.0 volt reference circuit to ground or battery voltage, the other 5.0 volt reference circuits will also have a low or high voltage.
Disconnect all of the sensors that use a 5.0 volt reference one at a time while monitoring the short on the 5.0 volt reference circuit. Replace any sensor that may have caused the short on the 5.0 volt reference circuit.
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The replacement PCM must be programmed and the Crankshaft Position System Variation Learn procedure must be performed.
Step | Action | Value(s) | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | |||||
2 | Start and idle the engine. Does the MAP voltage read more or equal to the specified value? | 4.0 V | Go to Step 3 | Go to Step 4 | ||||
Does the MAP voltage read the specified value or less? | 1.0 V | Go to Step 5 | Go to Step 6 | |||||
Operate the vehicle within the Freeze Frame conditions and Conditions for Running the DTC as specified in the supporting text. Does the MAP voltage read the specified value or more? | 4.0 V | Go to Step 3 | Go to Diagnostic Aids | |||||
Does the test lamp illuminate? | -- | Go to Step 7 | Go to Step 11 | |||||
Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 12 | |||||
7 |
Does the DMM display read near the specified value? | 5.0 V | Go to Step 8 | Go to Step 9 | ||||
Did you find a problem? | -- | Go to Step 10 | Go to Step 12 | |||||
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Important:: The Powertrain Control Module (PCM) uses a single internal 5.0 volt power supply to supply power to all the engine control components. If one component shorts its 5.0 volt reference circuit to ground or battery voltage, the other 5.0 volt reference circuits will also have a low or high voltage.
Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 13 | |||||
10 | Repair the vacuum source as necessary. Did you complete the repair? | -- | Go to Step 15 | -- | ||||
11 |
Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 13 | ||||
12 |
Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 14 | ||||
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Important:: The replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 15 | -- | |||||
14 | Replace the MAP sensor. Refer to Manifold Absolute Pressure Sensor Replacement . Did you complete the replacement? | -- | Go to Step 15 | -- | ||||
15 |
Does the DTC reset? | -- | Go to Step 2 | System OK |
