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 results from engine load, or intake manifold vacuum, and the RPM changes, and converts these into voltage outputs. The Powertrain Control Module (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 knows the manifold pressure. A lower pressure, or low voltage, output voltage will be approximately 1.0 to 1.5 volts at idle. Higher pressures, or higher voltage, output voltage will be approximately 4.5 to 4.8 volts at wide open throttle (WOT). The MAP sensor is also used to measure barometric pressure, allowing the PCM to make adjustments for different altitudes.
Conditions for Setting the DTC
| • | DTC(s) P0122 or P0123 not set. |
| • | Engine has run for at least 20-40 seconds. |
| • | Throttle Position (TP) sensor is less than 12 %. |
| • | Vehicle speed is less than 1 mph (2 km/h). |
| • | MAP is greater than 82 kPa. |
| • | Above conditions met for 10 seconds. |
Action Taken When the DTC Sets
| • | The Malfunction Indicator Lamp (MIL) will illuminate. |
| • | The Powertrain Control Module (PCM) 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. |
| • | Coolant fan turns ON. |
| • | The PCM will substitute a fixed MAP value and use Throttle Position (TP) to control the fuel delivery (scan tool will not show defaulted value.) |
| • | The vehicle will operate in Open Loop when the vehicle is under 8 km/h (5 mph). The vehicle will operate in Closed Loop when the vehicle is over 8 km/h (5 mph). |
Conditions for Clearing the MIL/DTC
| • | The MIL will turn OFF after three 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. |
| • | The MIL/DTCs can be cleared by using the scan tool. |
Diagnostic Aids
Important:
• After repairs use scan tool Fuel Trim Reset function to reset
long term fuel trim to 128 (0 %). • DTC P0108 may set as a result of misfire. If misfire is present,
repair the cause of misfire before using this table. The misfire counters
may be used to determine which cylinder(s) is misfiring.
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 PCM 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 ± .4 volt.
If a DTC P0108 is intermittent, refer to Symptoms or to Manifold Absolute Pressure Sensor Output Diagnosis for further diagnosis.
Test Description
Number(s) below refer to the step number(s) 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 and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in 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 PCM recognizes the change, the PCM and CKT 416 and CKT 432 are OK.
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This step looks for an open CKT 469. If the circuit was open, additional DTC's will also be set. If no other DTC(s) are set and the circuit is found to be open, then the open must be between the MAP sensor and the electrical connector sensor ground splice.
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When CKT 432 is shorted to battery voltage, the TP will be displayed above 0 % at all times and A/C High Side will be displayed high. The vehicle will also remain in Open Loop .
-
The MAP sensor vacuum source should only supply vacuum to the MAP sensor. Check the vacuum port for a restriction caused by casting flash.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | |||
With the engine idling, install a scan tool. 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 | |
Probe the MAP sensor signal ground circuit with a test light connected to battery voltage. Does the test light illuminate? | -- | |||
Check the MAP sensor signal circuit for a short to voltage or a short to the 5 volt reference circuit. Was a problem found? | -- | |||
Check the MAP sensor vacuum source for being plugged or leaking. 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. Is the action complete? | -- | -- | |
10 | Repair the MAP sensor signal circuit. Is the action complete? | -- | -- | |
11 | Replace the PCM. Is the action complete? | -- | -- | |
12 | Replace the MAP sensor. Is the action complete? | -- | -- | |
13 |
Does the scan tool indicate that this diagnostic ran and passed? | -- | ||
14 | Check if any additional DTC's are set. Are any DTC's displayed that have not been diagnosed? | -- | Go to applicable DTC table | System OK |
