Part 1 of 2

Code Chart 33 (Part 1 Of 2):

MAP Sensor:

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 33 is the result of a hard failure or an intermittent condition. Code 33 will set if:

^ MAP signal indicates greater than 84 kPa (low vacuum).
^ No Code 21 is set.
^ TPS less than 5%.
^ These conditions are present longer than 5 seconds.

2. This step simulates conditions for a Code 33 (signal voltage low-high vacuum). If the ECM recognizes the change, the ECM and sensor ground wire and signal circuit wire, are OK. If the 5.0 volt reference wire is open, there may also, be a stored Code 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 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 33 will result if the ground wire is open, or if the signal wire is shorted to voltage or to the 5.0 volt reference wire. If Code 33 is intermittent, Testing and Inspection