Powertrain Controls - ECM/PCM

Code 33 Diagnostic Chart:

Manifold Absolute Pressure (MAP) Sensor Wiring Schematic:

CIRCUIT DESCRIPTION
The Manifold Absolute Pressure (MAP) sensor measures the changes in the intake manifold pressure which results from engine load (intake manifold vacuum) and RPM changes; and converts these into a voltage output. 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 sensor also changes. By monitoring the sensor output voltage, the PCM can calculate the manifold pressure. A lower pressure (low voltage) output voltage will be about 1.0 to 1.5 volts at idle. While higher pressure (high voltage) output voltage will be about 4.5 to 4.8 at Wide Open Throttle (WOT).

When the engine is not running, the manifold absolute pressure is equal to the barometric pressure. When the ignition is turned "ON", and before the engine is started, the PCM "reads" the MAP sensor signal and uses this to calculate barometric pressure, and make base adjustments for different altitudes. The PCM uses the map sensor signal to control fuel delivery and ignition timing.

NOTE: Manifold Absolute Pressure and manifold vacuum are inversely proportional. "HIGH" manifold pressure is "LOW" vacuum, and "LOW" manifold pressure is "HIGH" vacuum.

DTC 33 WILL SET WHEN
^ Engine running.
^ MAP greater than approximately 82 kPa.
^ TP sensor less than 12%.
^ VSS less than 1 mph.
^ DTC 21 or 22 not present.
^ Above conditions met for 5 seconds.

ACTION TAKEN (PCM WILL DEFAULT TO)
Malfunction Indicator Lamp (MIL) illuminates. The PCM will substitute a fixed MAP value and use the Throttle Position (TP) sensor to control fuel delivery.

NOTE: Tech 1 will not show defaulted value.

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. This step will determine if DTC 33 is the resultofa hard failure or an intermittent condition
2. This step simulates conditions for a DTC 34. If the PCM recognizes the change, the PCM and CKT 416 and CKT 432 are OK. If CKT 469 is open, there may also be a stored DTC 23.

DIAGNOSTIC AIDS
^ 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 accuracy of a "suspect" sensor. Headings should be the same +/-0.4 volt.
^ A DTC 33 will result if CKT 469 is open or if CKT 432 is shorted to voltage or to CKT 416.
^ If DTC 33 is intermittent, refer to "DIAGNOSIS BY SYMPTOM/INTERMITTENTS" Testing and Inspection
^ Check all connections.
^ Disconnect sensor from bracket and twist sensor by hand (only) to check for intermittent connections. Output changes greater than 0.1 volt indicates a bad connector or connection. If OK, replace sensor.

NOTE: Make sure electrical connector remains securely fastened.

^ Refer to "C-CHARTS/CHART C-1D for further diagnosis. Chart C-1D Manifold Absolute Pressure (MAP) Output Check

NOTE: After repairs use Tech-1 scan tool "Fuel Trim Reset" function, to reset long term fuel trim to 128.