Code 66 Low Maf Voltage- All Checks Ok. Ecm? Help!

Buko

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Oct 17, 1999
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Mtop PA
www.ATVillustrated.com
Have been chasing down a code 66 for days (low MAF voltage). Did all the trouble shooting, ohmed out all the wires, 12 VDC OK, ground OK, etc. Replaced MAF-same issue!!! My idle volts are like .124 VDC, if I rev the car I can see them climb to ~.185 VDC in a linear fashion. So, it looks like the MAF is reading flow but the voltage is way down.

Does the ECM generate this voltage? I'm guessing it has to. What should it be at the MAFSIG on the connecter with MAF unplugged? Is ECM the next logical route? I don't think I have anything left?
 
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jrichker

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The mass air sensor is a wire heater in close proximity to a precision resistor. Some MAF sensors combine the heater and the precision resistor into a single resistor. Either way, the airflow across the heater/resistor causes it to cool, which reduces the resistance, and causes a change in voltage. More airflow across the sensor means more voltage, while less airflow means less voltage. The 12 volts that powers the heater part of the circuit also powers the sensor part of the circuit. The computer just reads the voltage output from the MAF and converts the analog voltage into a digital signal so that the computer can process it.

Code 66 MAF below minimum test voltage.
Insufficient or no voltage from MAF. Dirty MAF element, bad MAF, bad MAF wiring, missing power to MAF. Check for missing +12 volts on this circuit. Check the two links for a wiring diagram to help you find the red wire for computer power relay switched +12 volts. Check for 12 volts between the red and black wires on the MAF heater (usually pins A & B). while the connector is plugged into the MAF. This may require the use of a couple of safety pins to probe the MAF connector from the back side of it.

There are three parts in a MAF: the heater, the sensor element and the amplifier. The heater heats the MAF sensor element causing the resistance to increase. The amplifier buffers the MAF output signal and has a resistor that is laser trimmed to provide an output range compatible with the computer's load tables. Changes in RPM causes the airflow to increase or decrease, changing the voltage output.. The increase of air across the MAF sensor element causes it to cool, allowing more voltage to pass and telling the computer to increase the fuel flow. A decrease in airflow causes the MAF sensor element to get warmer, decreasing the voltage and reducing the fuel flow.

The MAF element is secured by 2 screws & has 1 wiring connector. To clean the element, remove it from the MAF housing and spray it down with electronic parts cleaner or non-inflammable brake parts cleaner (same stuff in a bigger can and cheaper too).


Measure the MAF output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins 50 & 9 on the computer. Be sure to measure the sensor output by measuring across the pins and not between the pins and ground.

At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt

Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the computer (dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF and pin 9 on the computer (tan/light blue wire) should be less than 2 ohms.

There should be a minimum of 10K ohms between either pin C or D on the MAF wiring connector and pins A or B. Make your measurement with the MAF disconnected from the wiring harness.


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel pump, alternator, ignition & A/C wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Computer,. actuator & sensor wiring
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Fuse panel layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

Vacuum routing
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
 

Buko

Founding Member
Oct 17, 1999
151
0
16
Mtop PA
www.ATVillustrated.com
If the ECM does not generate the signal I'm guess it comes from the 12v MAF feed?

All tests pass except this part. I get .12 to .18 VDC with the new MAF.

<<Measure the MAF output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins 50 & 9 on the computer. Be sure to measure the sensor output by measuring across the pins and not between the pins and ground.

At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt
 

jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
27,426
2,758
234
75
Dublin GA
lowendmac.com
If the ECM does not generate the signal I'm guess it comes from the 12v MAF feed?

All tests pass except this part. I get .12 to .18 VDC with the new MAF.

<<Measure the MAF output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins 50 & 9 on the computer. Be sure to measure the sensor output by measuring across the pins and not between the pins and ground.

At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt

Since you sig metions a supercharger, what is the location of the MAf? Before the supercharger or between the superchager outlet and the throttle body?

Have you tried clocking the MA? That means rotating the MAF housing when the engine is running and observing changes in the engine behavoir and voltage output from the MAF.