12.12 - Fuel Fail 12 : Mass Air Flow sensor out of range ( Brett Gadzinsky,
Output below .2 volts or above 4.5 volts engine rpm 400 to 3000 rpm for 5 crankshaft revolutions.
If FF 12 is flagged, ecu uses throttle position sensor input for engine load indications instead of the maf sensor.
Possible faults :
- Old style unsealed relays for EMS main 1993 year cars and up.
- Open or short in wiring between maf and main ecu.
- High resistance connection between maf and ecu (dirty connector at maf sensor).
- Open circuit in wiring from main relay power (12 volts) to maf.
- Bad maf sensor.
The maf sensor is in the air intake tract, after the air filter and before the throttle. It uses air flow through a side port to cool and change the resistance of sensors, corrected for ambient air temps to measure the air flow through the intake tract. Since it has no moving parts, its reliable, but suffers from having its connector in a bad spot. Its also subject to contamination of the sensors by any type of liquid.
Inside the sensor is a circuit board with voltage regulators, and a lot of parts and IC chips. On the 1988 to 1992 year cars, the base idle mixture trim is built in, but not part of the actual maf circuitry. Sensors to 1991 had a separate ground wire to a stud on the maf.
To test the maf, warm the car up to operating temps. At idle in park (about 740rpm) the output signal should read 1.2 to 1.5 volts, the higher the reading, the higher the indicated air flow. If your maf sensor puts out about 1.2 volts at idle, it is indicating a low air flow, so will adjust the injector pulse time down
to keep the mixture in range. 1.2 volts is on the low side of a normal reading and may cause problems unless you are at a high altitude.
If you have a vacuum leak, the INDICATED air flow into the engine will be low, the ecu will reduce the injector pulse, and you will have a lean mixture and a poor idle.
To test the high air flow, measure the output voltage with the car in drive, full throttle, foot on brake. Only do this test for a very short time, and use great care the car does not get away from you. At full throttle the maf should read about 3.0 to 3.3 volts. This is the torque converter stall reading, actual full throttle high rpm will indicate higher.
The maf sensor has a lot of input on the mixture and indicated load range the engine is operating at. A small change in the signal denotes a large change in mixture.
Under normal operating conditions, the maf sensor and the coolant temp sensor set the mixture and the way the engine runs.