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 at least 1.25 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 will default to 60°C for intake air temperature. The scan tool will not show the defaulted value. |
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 Symptoms 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 malfunction 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 malfunction 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 the 5 volt reference circuit and the sensor signal circuit are OK.
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This step looks for an open ground circuit. If the circuit was open, additional DTCs 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 the sensor signal circuit 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 .
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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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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.
-
The replacement PCM must be programmed and the crankshaft position variation procedure must be performed. Refer to the latest Techline procedure for PCM reprogramming and also refer to Crankshaft Position System Variation Learn for the Crankshaft Position Variation Procedure.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | |||
Does the scan tool display a MAP voltage of the specified value or more? | 4.0V | |||
Does the scan tool display a MAP voltage of the specified value or less? | 1.0V | |||
4 |
Does the scan tool display a MAP voltage of the specified value or more? | 4.0V | 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 and repair as necessary. Was a repair necessary? | -- | |||
7 | Connect a DMM to ground, probe the 5.0 volt reference circuit. Does the DMM display near the specified value or less? | 5.0V | ||
Check the MAP sensor vacuum source for being plugged or leaking. Was a problem found? | -- | |||
|
Important: The Powertrain Control Module (PCM) uses a single 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. Check the 5.0 volt reference circuit for a short to voltage and repair as necessary. Was a repair necessary? | -- | |||
10 | Repair the vacuum source as necessary. Is the action complete? | -- | -- | |
Check for an open, or poor connections in the MAP sensor ground circuit and repair as necessary Was a repair necessary? | -- | |||
12 | Replace the PCM. Refer to Powertrain Control Module Replacement Is the action complete? | -- | -- | |
13 | Replace the MAP sensor. Refer to Manifold Absolute Pressure Sensor Replacement Is the action complete? | -- | -- | |
14 |
Does the scan tool indicate that this diagnostic ran and passed? | -- | ||
15 | Check if any additional DTCs are set. Are any DTCs displayed that have not been diagnosed? | -- | Go to the Applicable DTC table | System OK |
