Part 2 of 2

CODE 33 MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR

Code Chart 33B:

MAP Sensor Circuit:

CIRCUIT DESCRIPTION:

The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The ECM receives this information as a signal voltage that will vary from about 1 to 1.5 volts, at closed throttle idle, to 4-4.5 volts at wide open throttle (low vacuum). If the MAP sensor fails, the ECM will substitute a fixed MAP value and use the TPS to control fuel delivery.

TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. This step will determine if Code 33B is the result of a hard failure or an intermittent condition. Code 33B will set if:

^ Engine speed is greater than 1200 rpm.
^ No Code 21 is set.
^ TPS greater than 15%.
^ MAP sensor voltage is too low.

2. Jumpering harness terminals "B" to "C," 5.0 volts to signal, will determine if the sensor is at fault, or if there is a problem with the 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 the 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 vehicles altitude and is referred to as BARO. Comparison of this BARO reading with a known good vehicles with the same sensor is a good way to check accuracy of a suspect sensor. Readings should be the same +/- 0.4 volts. A Code 33B will result if the PPL/ORN wire or GRY/RED wire are open or shorted to ground. If Code 33B is intermittent, Testing and Inspection