On this system, it is not enough to use the 12V ground and signal from the three pot connections. You also must jumper the sensor reference low harness connection to the chassis ground harness connection as shown in Figure 2 in green. During normal connection to the MAF sensor, the ECM picks up the sensor reference low through the MAF sensor internally to the chassis harness ground.
Use your service information troubleshooting guides or ECM terminal voltage charts to determine where the airflow signal terminal voltage originates. Always start this test procedure by first testing the MAF sensor power supply, ground, reference voltage (if used) and signal wire. Without good circuitry to the MAF sensor, it cannot do its job correctly, even if the sensor itself is without defect.
If you are experiencing an unsteady idle or surge at steady, no load higher RPM states, we are going to be using the test pot to feed the ECM a steady MAF signal. The pot will be feeding the ECM a MAF signal that is not dependent on what the actual airflow rate going into the engine is. If the engine surge ceases when we use our steady pot-driven MAF signal, then the vehicle's MAF sensor was the actual cause of the surge, not some other component or defect condition.
Checking for low engine vacuum, leaking EGR systems, faulty IAC systems and steady fuel pressure also are checks to be done to address an unsteady idle complaint if the MAF substitution does not steady the RPM. Again, this test is to be conducted only at idle or steady state no load throttle positions. It is not possible to use a potentiometer to accurately create a substitute MAF signal under dynamic throttle conditions.
The pot does take a bit of adjustment to get an acceptable MAF signal that creates a steady RPM and a normal fuel trim. Make extremely small adjustments with your pot, waiting a few seconds to a minute between adjustments for the fuel trims to react and steady out. If you adjust your pot too quickly or at too large an increment, the ECM's fuel trims will not be able to react fast enough or wide enough. This will cause the ECM to go into an Open Loop Fault state.
Watch the Loop Status on your scanner to be sure your adjustments are not too wide. Without fuel trim feedback after each pot adjustment, it becomes extremely difficult to steady the RPM or tell what is a good MAF reading for that state.
Once you have a steady RPM and acceptable fuel trims, you then will know what a good MAF rate is for that RPM. Compare this value to the ones you were getting from the actual MAF. You will find that a MAF that is out of calibration by as little as 0.07V (70mV) can cause unsteady RPM and fuel trims skewed 25 percent.
When you consider the published idle MAF specs for Nissan vehicles range from 1.0 to 1.7V, a 700mV difference, you can see this range is pretty much useless as a test of the MAF. The vehicle will stall long before the MAF changes 0.5V (500mV).
Don't trust the scan tool MAF voltage readings Key On Engine Off (KOEO). Many ECMs do not report actual MAF readings to the scanner unless the engine is running.
Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.
