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. |
| • | None of the following DTCs are active: 21, 22, 26. |
| • | The MAP circuit voltage is less than 0.098 volt. |
| • | The TP sensor position is less than 4% at RPMs less than 1200. |
| OR |
| • | The TP sensor position is greater than 20% at RPMs greater than 1300. |
Action Taken When the DTC Sets
The ECU sets the DTC 34 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. Replace the sensor if the connector is OK.
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.
-
This step determines if DTC 34 is the result of a hard failure or an intermittent condition.
-
When the ECU connector is disconnected and voltage is still present, it eliminates the ECU as the possible cause of the fault.
-
By jumping the 5 volt reference to the signal circuit, this step determines if the sensor is at fault or if there is a problem with the circuit wiring.
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The scan tool may not display 5 volts. It is more important that the VCM recognized the voltage as more than 4 volts. This indicates that the VCM and the MAP signal are OK.
-
The VCM terminals CLR12 and CLR27 are spliced together inside the VCM. When checking the 5 volt reference for a short to ground, both the 5 volt reference circuits and all the components on the 5 volt reference circuits must be checked.
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 |
Does the scan tool display a MAP sensor voltage less than the specified value? | 0.5 V | Go to Step 4 | Go to Step 3 | |
3 | Backprobe the MAP sensor signal in the ECU connector using a DVOM. Does the DVOM display the MAP sensor signal less than the specified value? | 0.09 V | Go to Step 4 | Go to Step 10 |
Does the scan tool display a MAP sensor voltage less than the specified value? | 0.5 V | Go to Step 8 | Go to Step 5 | |
5 |
Is there continuity? | -- | Go to Step 6 | Go to Step 16 |
6 | Inspect the ECU connector for loose, corroded or damaged pins. Was a problem found? | -- | Go to Step 16 | Go to Step 7 |
7 | Replace the ECU. Refer to Engine Control Unit Replacement . Is the action complete? | -- | Go to Step 18 | -- |
Does the scan tool display a MAP sensor voltage greater than the specified value? | 4.7 V | Go to Step 11 | Go to Step 9 | |
Does the scan tool display a MAP sensor voltage greater than the specified value? | 4.7 V | Go to Step 14 | Go to Step 12 | |
10 | The DTC is intermittent. If no additional DTCs are stored, refer to Diagnostic Aids. If additional DTCs are stored, refer to those tables. Are any additional DTCs set? | -- | Go to the applicable DTC table | -- |
11 | Inspect for a faulty connection at the MAP sensor. Was a problem found? | -- | Go to Step 16 | Go to Step 15 |
12 | Inspect for an open MAP signal circuit. Was a problem found? | -- | Go to Step 16 | Go to Step 13 |
13 | Inspect the MAP sensor signal circuit for a short to ground. Was a problem found? | -- | Go to Step 16 | Go to Step 17 |
Inspect for an open in the MAP sensor 5 volt reference circuit. Was a problem found? | -- | Go to Step 16 | Go to Step 17 | |
15 | 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 18 | -- |
16 | Repair the circuit as necessary. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 18 | -- |
17 | 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 18 | -- |
18 |
Does the diagnostic run and pass? | -- | Go to Step 19 | Go to Step 2 |
19 | Are any ECU or VCM DTCs displayed that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
