Without Speed Density Update

Chart - Code 34 Mass Air Flow Sensor (Signal Frequency Low):

Wiring Diagram for MAT & MAF Sensors.:

CODE 34 MASS AIR FLOW (MAF) SENSOR CIRCUIT (GM/SEC LOW)



Circuit Description:
The MAF sensor measures the flow of air entering the engine. The sensor produces a frequency output between 32 and 150 hertz (3gm/sec to 150gm/sec). A large quantity (high frequency) indicates acceleration, and a small quantity (low frequency) indicates deceleration or idle. This information is used by the ECM for fuel control and is converted by a "Scan" tool to read out the air flow in grams per second. A normal reading is about 4-7 grams per second at idle and increase with rpm. The MAF sensor is powered up by a 7.5 amp fuse which is located underhood in the right side relay center and the sensor should have power supplied to it anytime the ignition is "ON." The MAF and MAT sensor are combined into one assembly located in the air duct.


Test Description:Numbers below refer to circled numbers on the diagnostic chart.
1. Code 34 will set if-
Engine running.
Check for vacuum leaks.
MAF sensor disconnected, OR MAF signal
Circuit shorted to ground. OR
Air flow less than 2 gm/sec (low frequency)
With engine running. A loose or damaged air duct can set Code 34. This test checks to see if ECM recognizes a problem. A light "OFF" at this point indicates an intermittent problem.
2. Checks to see if 5 volt reference signal from ECM is at MAF sensor harness connector.
3. Checks continuity of electrical circuit at MAF sensor.
4. Checks for open in 12 volt supply.


Diagnostic Aids:

The "Scan" tool is not of much use in diagnosing this code because when the code sets, the value displayed will be the default value. It may be useful in comparing the signal of a problem vehicle with that of a known good running one. Check for loose or damaged air duct . Check for any vacuum leaks. Inspect sensor connections as an open will result in a Code 34. Refer to "Intermittents" in "DIAGNOSIS BY SYMPTOM - NO TROUBLE CODE STORED".


*** NOTE: Before replacing MAF sensor, see TSB #886E31 DATED JUNE, 1990