Code 33B Manifold Absolute Pressure (MAP) Sensor Circuit

SIGNAL VOLTAGE LOW-HIGH VACUUM




Circuit Description
The Manifold Absolute Pressure (MAP) Sensor responds to changes in Manifold Pressure (vacuum). The Engine Control Module (ECM) receives this information as a signal voltage that will vary from about 0.45 to 0.85 volts, at closed Throttle idle, to 3.8 - 4.5 volts at Wide Open Throttle (low vacuum). If the MAP Sensor fails, the ECM will substitute a fixed MAP value based on Throttle Position Sensor to control Fuel delivery.




Test Description
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This step will determine if Diagnostic Trouble Code (DTC) 33B is the result of a hard failure or an intermittent condition. A DTC 33B will set if:
- Engine speed is greater than 1100 revolution per minute.
- Throttle Position Sensor is greater than 20%.
- MAP signal indicates less then 10.7 kPa.
- These conditions are present for a time longer than 0.125 seconds.
- No DTC 21A or DTC 21B is set.

2. Jumping Harness Terminals "B" to "C", 5 volts to signal, will determine if the Sensor is at fault, or if there is a problem with ECM or wiring.
3. The "Scan Tool" may not display 12 volts. The important thing is that the ECM recognizes the voltage as more than 4 volts, indicating that the ECM and GRY/RED wire are OK.

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 ECM as an indication of Vehicle altitude and is referred to as Barometric pressure. Comparison of this Barometric pressure reading with a known good Vehicle with the same Sensor is a good way to check accuracy of a "suspect" Sensor. Readings should be the same ±0.4 volts.
- A DTC 33B will result if GRY wire is open, or if GRY/RED is shorted to voltage or to BLU/ORG wire.
- If DTC 33B is intermittent, refer to Diagnosis by Symptom. Symptom Related Diagnostic Procedures