Code 66. 1967 With 87 Harness And Maf

Discussion in 'Fox 5.0 Mustang Tech' started by JohnnyK81, Sep 15, 2013.

  1. I bought an 87ish donor car, and upgraded the wiring for MAF.

    The car starts, runs, drives, but surges a little bit, and once it hits operating temperature, runs worse. ALso, VERY rich. Burns your eyes to be behind it.

    I went through the MAF checklist that others have posted for code 66.. At idle, I'm reading about 1.2ish volts.. maybe slightly under (was a couple weeks ago I took the measurement). According to that checklist, that is correct for the signal wire, and the problem must lie in the ECU?

    And other things to check (wire continuity is good, 12 volts is good, etc).

    SHOULDN'T be anything wrong with using the original harness and adding the 4 wires if it's going to an A9L I wouldn't think.
  2. Also meant to add.. When it's running and up to operating system.. If you unplug the maf sensor, it does NOTHING.. acts like you didn't touch anything.. Like the sensor wasn't there to begin with.
  3. 66 - , MAF signal below minimum test voltage of .4vdc (O,CM,R), Transmission Fluid Temperature Circuit Grounded
  4. Yup. As I said, reading approx 1 volt at idle between the signal wires. MAF has 12v hooked up through a relay, signal wires go to ECU as they should. Code 66.
  5. Do you have another mass air u can use to see if its good or not ?
  6. You got about 1/8 the information on code 66 that you need to understand and diagnose the problem. Here's the whole banana...

    Code 66 MAF below minimum test voltage.

    Revised 29-Aug-2013 to add description of differential circuitry on MAF outputs C & D

    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).

    89-90 Model cars: 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.

    91-93 Model cars: Measure the MAF output at pins C & D on the MAF connector light blue/red 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.

    Actually MAF pins C & D float with reference to ground. The signal output of the MAF is a differential amplifier setup. Pins C & D both carry the output signal, but one pin's output is inverted from the other. The difference in signal between C & D is what the computer's input circuit is looking for. The difference in the two outputs helps cancel out electrical noise generated by the ignition system and other components. Since the noise will be of the same polarity, wave shape and magnitude, the differential input of the computer electronically subtracts it from the signal. Then it passes the signal on to an Analog to Digital converter section inside the computer's CPU chip.

    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring

    Ignition switch wiring

    Fuel pump, alternator, ignition & A/C wiring

    Computer,. actuator & sensor wiring

    Fuse panel layout

    Vacuum routing

    MASS air conversion instructions from FREE
    The conversion harness seems to work well for most folks. It avoids the compatibility problems in using a harness from the junkyard. Simple and cheap, actually less work that swapping the wiring harness.
    Conversion Harness kits & parts
    Also see for help with the harness part

    If the car was converted to a V8, it may still have the 4 cylinder O2 Sensor harness and 4 cylinder4 wire O2 sensors Which are NOT compatible with a 5.0 harness or computer.

    O2 Sensor harness interchange and modification

    Originally Posted by 302EFI

    Revised 16-Oct-2011 to add O2 sensor harness warnings
    The wires for the 02's and low oil did not change throughout the years, they are all in the same place.
    The main ones you need to worry about are (on the harness end (ECU) that plugs into the 02 plug) is:
    \- 1. Lightblue / yellow
    - 2. White / Purple
    - 3. Purple / Yellow
    The White/Purple & Purple/Yellow gets looped for a automatic ECU
    The Purple/Yellow & Lightblue/Yellow for a manual ECU

    See for more O2 sensor wiring harness info

    Basic premise to use with transmission swaps:
    Only run a 5 speed trans O2 harness with an A9L. Do not run an Auto O2 sensor harness with an A9L. Doing so will damage the computer’s internal signal ground.
    Only run an Auto trans O2 sensor harness with an A9P in a car that has an Auto trans. Using a 5 speed trans O2 sensor harness with an Auto trans will cause no crank problems.
    See for Joel5.0’s fix to the computer internal signal ground.

    90 model year harness only works with 90 model cars without inspection/rework.
    The 4 cylinder O2 harness uses 4 wire O2 sensors. It probably won’t work correctly without modifying it.
  7. Nah, I've gone over that checklist many times.

    I swapped computers today with another of unknown origin. What I've noticed is at idle, MAF jumps from about .9 to 2.0volts.. Which is high.. but code 66 is insufficient voltage? Definitely continuity to the end of the harness.
  8. I didnt see this thread until now. Are you sure you have the MAF wires going to the correct PINs at the ECM? If you do, what is the MAF signal voltage at the ECM itself? Continuity doesnt really mean that much when dealing with voltage issues.
  9. Unfortunately i tried all day, but it's crammed into a 1967 and I really can't probe it, try as I might.

    BUT, tonight, checked voltage again for the signal at the sensor.. 3.0 up to 8v with a slight rev?!

    This a sensor issue?
  10. Just checked voltage WITHOUT the sensor hooked up (At the harness at the sensor). 5.0v
  11. MAF voltage is messed up. Its technically a 5v sensor. You should under no circumstances see any voltage above 5v on the signal wire. The MAF Pins are labeled A B C D. Which pin are you measuring for the signal wire? The unplugged voltage on the signal wire should be close to 0v. So it seems the wiring is messed up. Or you are mesuring the wire.
  12. I thought signal wire sends 5v to the sensor? Sensor knocks it down, no? I was measuring 5v between C and D unplugged.. A and B have the 12v.

    Wiring CAN'T be messed up.. I ran a straight shot from C and D straight to ecu.
  13. Signal is not sent to the sensor. It is sent from the MAF to the computer. If youre getting 8v out of the MAF, you have issues.
  14. It certainly appears that way doesn't it. I'll grab a new one today, and hopefully if it's the ECU, it doesn't fry it.
  15. Why is it a 2 wire signal system then? I believe that ecu should send 5 volts to maf, and maf sends back, no? Otherwise, if what you are saying is correct, it would be a 3 wire sensor system.

    HOWEVER, I DO find it strange that unplugged, I am getting 5v to the sensor (which sounds like it could be ok), but then after it goes through the sensor, the voltage spikes. I wonder if there is some sort of short in the sensor.
  16. No, the maf is powered by 12v. The electronics in the Maf convert the signal sent to the ECM to a 0v-5v scale depending on the amount of air going through it. I would first make sure the pins you used for the signal and signal return are in correct location on the ECM 60 pin connector. Many guys have wired it wrong because they were looking at the 60 pin in the orientation when compared to pictures seen on the internet.
  17. He may be right. See the diagram I posted for the wire side of the computer connector in my Code 66 MAF post above.
  18. It's a bosch sensor. Believe it needs a 5v signal wire (Just because it doesn't get it from vref doesn't mean it doesn't get 5v).

    Put new mass in today. Seemed to run much better. Intermittent CEL now. Have to check the code tomorrow. Here's to hoping it's not 66 and 98!
  19. Your miss informed. The 5v signal is not sent to the Mass Air sensor. Its sent from the Mass Air Sensor to the ECM. So the ECM can determine the engines Fuel and Load state. So again..... The 0v-5v signal is sent OUT of the Mass Air. Not INTO the Mass Air.

    VREF powers the Mass Air so it can send the Signal OUT! Its an output signal. Also, code 98 is because of code 66. So once code 66 is fixed, 98 will go away as well.
  20. Welp as I said, replaced maf this morning.. have to a run a KOER tomorrow and see if it's still there..

    But, if a signal is only sent OUT, why is it a 4 wire sensor? Should be a 3 wire sensor.

    You'd be a dear if you measured voltage across C and D with your MAF disconnected... we'd know for sure then.