Circuit Description
The Manifold Absolute Pressure (MAP) sensor responds to changes in the intake manifold pressure (vacuum). A 5 volt reference is applied to the sensor. A variable resistor moves in relation to manifold pressure. A voltage signal is then sent back to the ECU on the 5 volt reference circuit. This voltage signal varies from 1.0-1.5 volts at closed throttle (high vacuum) to 4.0-4.5 volts at wide open throttle (low vacuum).
A substantial change in throttle position and engine speed must precede any substantial change in MAP. If a substantial change in throttle position and engine speeds has occurred and a change in MAP did not occur, than a MAP malfunction has occurred.
This is a type B DTC.
Conditions for Setting the DTC
The following conditions will set the DTC:
| • | The vehicle is in the CNG operation mode. |
| • | The engine is running at greater than 500 RPM. |
| • | None of the following DTCs are active: 21, 22, 26. |
| • | The TP sensor position is less than 4%. |
| • | The MAP sensor voltage is greater than 4.4 volts. |
Action Taken When the DTC Sets
The ECU sets the DTC 33 after 2 consecutive driving trips with the fault active.
Conditions for Clearing the MIL/DTC
The ECU turns the MIL OFF after 3 consecutive driving trips without a fault condition present. A DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°C (72°F) from the start-up coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during that same ignition cycle) or the code clearing procedure has been used.
Diagnostic Aids
Always diagnose ANY VCM codes that are present before beginning this diagnostic procedure.
The manifold pressure is equal to the atmospheric pressure with the signal voltage high when the ignition is ON and the engine is OFF. The ECU uses this information as an indication of the altitude of the vehicle.
Comparison of this reading with a known good vehicle using the same sensor is a good way to check the accuracy of a suspect sensor. Both readings should be 0.4 volt.
Important: The electrical connector must remain securely fastened.
Important: Disconnect the sensor from the bracket. Twist the sensor by hand only in order to check for intermittent connections. If the output changes greater than 0.1 volt, it indicates a bad connector or connection. If the connector is OK, replace the sensor.
Important: Replace the MAP sensor-to-intake manifold seal after removing the MAP sensor from the intake manifold.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
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When the ECU connector is disconnected and voltage is still present, it eliminates the ECU as the possible cause of the fault.
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This step checks for a short to voltage on the 5 volt reference circuit.
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This step checks the integrity of the MAP sensor ground circuit.
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The VCM terminals CLR12 and CLR27 are spliced together inside the VCM. This step isolates the two pins from one another.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Alternative Fuels (AF) Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check |
2 |
Does the scan tool display a voltage less than the specified value? | 4.0 V | Go to Step 3 | Go to Step 6 |
3 | Backprobe the MAP sensor signal in the ECU connector using a DVOM. Does the DVOM display the MAP sensor signal greater than the specified value? | 4.4 V | Go to Step 4 | Go to Step 7 |
Does the DVOM display the MAP sensor signal greater than the specified value? | 4.4 V | Go to Step 6 | Go to Step 5 | |
5 | Replace the ECU. Refer to Engine Control Unit Replacement . Is the action complete? | -- | Go to Step 22 | -- |
6 |
Does the scan tool display a voltage less than the specified value? | 1.0 V | Go to Step 8 | Go to Step 12 |
7 | The DTC is intermittent. If no additional DTCs are stored, refer to the Diagnostic Aids. If additional DTCs are stored, refer to those tables first. Are any additional DTCs stored? | -- | Go to the applicable DTC table | -- |
Probe the 5 volt reference circuit at the MAP sensor harness connector with a DVOM connected to ground. Is the voltage greater than the specified value? | 5.2 V | Go to Step 13 | Go to Step 9 | |
Probe the MAP sensor ground circuit at the MAP sensor connector with a test light to B+. Is the test light ON? | -- | Go to Step 10 | Go to Step 17 | |
10 | Inspect the MAP sensor vacuum source for restrictions. Are any restrictions found? | -- | Go to Step 18 | Go to Step 11 |
11 | Replace the MAP sensor. Refer to MAP Sensor Replacement in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 22 | -- |
12 | Inspect for a short to the voltage in the MAP sensor signal circuit. Was a problem found? | -- | Go to Step 18 | Go to Step 21 |
Is the voltage greater than the specified value? | 5.2 V | Go to Step 14 | Go to Step 16 | |
14 |
Is the voltage greater than the specified value? | 5.2 V | Go to Step 15 | Go to Step 19 |
15 | Repair the short to voltage on the 5 volt reference circuit. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 22 | -- |
16 | Measure the voltage on the VCM connector terminal CLR12 with a DVOM connected to ground. Is the voltage greater than the specified value? | 5.2 V | Go to Step 20 | Go to Step 21 |
17 | Repair the MAP sensor ground circuit. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 22 | -- |
18 | Repair the circuit or vacuum source as necessary. Is the action complete? | -- | Go to Step 22 | -- |
19 | Replace the EGR valve. Refer to EGR Valve Replacement in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 22 | -- |
20 | Repair the short to voltage on the 5 volt reference circuit, terminal CLR12. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 22 | -- |
21 | Replace the VCM. Important:: Reprogram the VCM if it is faulty. Refer to VCM Replacement/Programming in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 22 | -- |
22 |
Does the diagnostic run and pass? | -- | Go to Step 23 | Go to Step 2 |
23 | Are any ECU or VCM DTCs displayed that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
