Circuit Description
The powertrain control module (PCM) uses the manifold absolute pressure (MAP) sensor output voltage in order to assist in controlling the fuel delivery and the ignition timing. The PCM supplies a 5 volt reference signal voltage and a ground circuit to the MAP sensor. As the manifold pressure changes, the output voltage of the sensor also changes. By monitoring the sensor output voltage, the PCM knows the manifold pressure. A lower pressure (low voltage) output voltage will read between 1.0 to 1.5 volts at idle. While higher pressure (high voltage) output voltage will read between 4.5 to 4.8 at wide open throttle (WOT). The PCM uses the MAP sensor to record the barometric pressure during an engine start up and at wide open throttle. This allows the PCM to make adjustments for different altitudes.
Conditions for Running the DTC
| • | DTCs P0122 or P0123 are not set. |
| • | The throttle position (TP) sensor is less than 12 percent. |
| • | The vehicle speed sensor is less than 2 km/h (1 mph). |
| • | The engine run time is more than 20 to 40 seconds. |
Conditions for Setting the DTC
| • | The MAP sensor is more than 82 kPa. |
| • | The above conditions are met for at least 10 seconds. |
Action Taken When the DTC Sets
| • | The malfunction indicator lamp (MIL) illuminates after 2 ignition cycles with the malfunction present. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The Freeze Frame and Failure Records buffers stores this information. |
| • | A history DTC is stored. |
| • | The PCM substitutes a fixed MAP value. The PCM uses the throttle position in order to control the fuel delivery. The scan tool will not show the defaulted value. |
| • | The vehicle will operate in Open Loop when the vehicle is under 5 mph. The vehicle will operate in Closed Loop when the vehicle is over 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. |
| • | A scan tool can clear the DTCs. |
Diagnostic Aids
Notice:
• After repairs use the scan tool Fuel Trim Reset function to reset
long term fuel trim to 128 (0 percent). • Whenever a misfire is present, repair the cause of the misfire
before using this table. The misfire counters may be used to determine which
cylinders are misfiring.
With the ignition ON and the engine OFF, the manifold pressure is equal to the atmospheric pressure and the signal voltage will be high. The PCM uses this information as an indication of the altitude of the vehicle. A 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 ± 0.4 volt.
Whenever a DTC P0108 is intermittent, refer to Driveability Symptoms or to Symptoms for further diagnosis.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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The Powertrain OBD System Check prompts you to complete some of the basic checks and to store the Freeze Frame and Failure Records data on the scan tool if applicable. This creates an electronic copy of the data captured when the malfunction occurred. The scan tool stores this data for later reference.
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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 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 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 the 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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Reprogram the replacement PCM and perform the Crankshaft Position System Variation Learn procedure. Refer to the latest Techline information for PCM programming.
Step | Action | Values | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | |||||||
Does the scan tool display a MAP voltage of the specified value or more? | 4.0 V | |||||||
Does the scan tool display a MAP voltage of the specified value or less? | 1.0 V | |||||||
4 |
Does the scan tool display a MAP voltage of the specified value or more? | 4.0 V | Go to Diagnostic Aids | |||||
Does the test lamp illuminate? | -- | |||||||
Was a repair necessary? | -- | |||||||
7 |
Does the DMM display near the specified value? | 5.0 V | ||||||
Was a condition found? | -- | |||||||
Was a repair necessary? | -- | |||||||
10 | Repair the vacuum source as necessary. Is the action complete? | -- | -- | |||||
11 |
Was a repair necessary? | -- | ||||||
|
Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | -- | ||||||
13 |
Is the action complete? | -- | -- | |||||
14 |
Does the scan tool indicate that this diagnostic Ran and Passed? | -- | ||||||
15 | Check to see if any additional DTCs are set. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to applicable DTC table | System OK |
