Circuit Description
The 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 Setting the DTC
| • | The MAP sensor is greater than 82 kPa. |
| • | The Throttle Position (TP) sensor is less than 12%. |
| • | The vehicle speed sensor is less than 1 mph (2 km/h). |
| • | The above conditions are met for 10 seconds. |
| • | The engine run time is greater than 20 to 40 seconds. |
| • | DTCs P0122 or P0123 is not set. |
Action Taken When the DTC Sets
| • | The Malfunction Indicator Lamp (MIL) illuminates. |
| • | 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 opeerate 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 malfunction indicator lamp (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
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 Manifold Absolute Pressure Sensor Output Diagnosis 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 the technician to complete some basic checks and store the Freeze Frame Data and the Failure records on the scan tool if applicable. This creates an electronic copy of data taken when the fault occurred. This information on the scan tool can be referred to later.
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This will determine if DTC P0108 is the result of a hard failure or an intermittent condition.
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Changes recognized by the PCM indicate that PCM, the MAP sensor 5V reference circuit and the MAP sensor signal circuit are OK.
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An open MAP sensor ground circuit can cause additional DTCs to set. When no other DTCs are set and the circuit is found to be open, then the open is between the MAP sensor and the electrical connector.
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A shorted MAP sensor signal circuit that is shorted to battery voltage will display TP sensor voltage above 0 % at all times. The A/C high side will be displayed high. The vehicle will remain in an Open Loop.
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Before replacing the PCM, check terminals for improper mating, broken locks, or physical damage to the wiring harness. Reprogram the replacement PCM. Refer to the latest Techline Information for programing procedures.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | |||
Does the scan tool display a MAP voltage at or greater than the specified value? | 4 V | |||
Does the scan tool display the MAP voltage at or less than the specified value? | 1 V | |||
4 |
Does the scan tool display a MAP voltage of the specified value or more? | 4 V | Go to the Diagnostic Aids | |
Probe the MAP sensor ground circuit with a test lamp connected to battery voltage. Does the test lamp turn on? | -- | |||
Check the MAP signal circuit for a short to voltage or a short to the 5 volt reference circuit. Was a problem found? | -- | |||
7 | Check the MAP sensor vacuum hose for a restriction or leakage. Was a problem found? | -- | ||
8 | Repair the vacuum hose as necessary. Is the action complete? | -- | -- | |
9 | Repair the open in the MAP sensor ground circuit. Refer to Wiring Repairs in Engine Electrical. Is the action complete? | -- | -- | |
10 | Repair the MAP sensor signal circuit. Refer to Wiring Repairs in Engine Electrical. Is the action complete? | -- | -- | |
Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | -- | ||
12 | Replace the MAP sensor. Refer to Manifold Absolute Pressure Sensor Replacement . Is the action complete? | -- | -- | |
13 |
Does the scan tool indicate that this diagnostic ran and passed? | 90°-95° C | ||
14 | Check if any additional DTCs are set. Are any DTCs displayed that have not been diagnosed? | -- | Go to the Applicable DTC Table | System OK |
