Federal Emissions

Code 33 Diagnostic Chart:

Manifold Absolute Pressure (MAP) Sensor Circuit Diagram:

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 1-1.2 volts at idle (high vacuum) to 4-4.8 volts at Wide Open Throttle (WOT) (low vacuum). If the MAP sensor fails, the ECM will substitute a fixed MAP valve and use the TP sensor to control fuel delivery.

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. DTC 33 will set when:
^ No DTC 21 or DTC 22.
^ Engine running.
^ Manifold pressure greater than 74 kPa (A/C "OFF") 83.4 kPa (A/C "ON").
^ Throttle angle less than 2%.
^ Conditions met for 4.8 seconds.

Engine misfire or a low unstable idle may set DTC 33.

2. With the MAP sensor disconnected, the ECM should see a low voltage if the ECM and wiring are OK.


DIAGNOSTIC AIDS:

^ If idle is rough or unstable, refer to "Symptoms," for items which can cause an unstable idle.

^ An open in CKT 802 or the connection will result in a DTC 33.

^ 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. Comparison of this 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 + .4 volt.

^ Check all connections. Make sure electrical connector remains securely fastened.

^ Disconnect sensor from bracket and twist sensor by hand (only) to check for intermittent connections. Output changes greater than .1 volt indicates a bad connector or connections. If OK, replace sensor.

^ Refer to CHART C-1D, MAP sensor voltage output check for further diagnosis. Chart C-1D Manifold Absolute Pressure (MAP) Output Check