DTC 33

CODE 33

MASS AIR FLOW (MAF) SENSOR CIRCUIT






Circuit Description

The mass air flow (MAF) sensor measures the amount of air which passes through it. The ECM uses this information to determine the operating condition of the engine, to control fuel delivery. For a detailed description of the MAF sensor operation refer to: Description and Operation

The oil pressure switch or the ECM, through control of the fuel pump relay, will provide 12 volts for the MAF power relay which provides the 12 volts needed by the MAF sensor.

The ECM provides a current limiting 5V on the signal line (CKT 998). The MAF sensor then changes the signal by dropping the voltage so that with low air flow the ECM sees a low voltage and a high air flow will cause the ECM to see near the 5 volt supply.

Test Description: Numbers below refer to circled numbers on the diagnostic chart.

CODE 33 indicates: ECM has seen flow in excess of 45 grams per second (above about 2.2 volts) for one second when:
- Engine is first started.
OR
- TPS is less than 1/4 throttle.
- RPM is less than 2000.

Due to the 5 volt pull-up resistor in the ECM, if CKT 998 becomes open, the ECM will see a high voltage signal and set a CODE 33.

1. This test will determine if the conditions to set the code still exist.
2. Will the ALDL terminal "G" jumpered to 12 volts, there should be 12 volts at the sensor. If no voltage is present, make sure that the fuel pump is running. If not repair, repair fuel pump circuit.
3. If a burn off signal is present at the MAF sensor with the engine running, a Code 33 will set. Be sure no voltage is present on CKT 994 during the first 25 seconds that the fuel pump runs or after the 2 second period.
4. The ECM sources a voltage (4-6 volts) to the MAF sensor on CKT 998. This test checks for that voltage.

Diagnostic Aids:

Intermittent: By jumpering the fuel pump test terminal ("G" term. of ALDL) to 12 volts, the MAF sensor will stay powered up and the signal line should see a low voltage, less that 250 mV or displayed as low grams per second on a "SCAN" tool. By wiggling the related wiring the intermittent may be detected. Also, an erratic signal with the engine running may indicated faulty wiring or components. A burn-off signal to the MAF sensor, while running, may result in a code 33. Be sure relay connections are clean and check CKT 994 for a potential short to voltage.