Code 66 Help!!!

BossmanPJ

Active Member
May 19, 2017
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10
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All,

I have a 90 Gt that I have completed a top end rebuild on. All new gaskets and hardware. When I got the car done it had a hell of a surging idle. Cleaning of the 10 pin connectors finally tamed that down to an acceptable level. The car has always had a code 66 for MAF voltage low since I have had it.

Here is the issue. The wiring for the MAF is good and shows no resistance to the computer. The signals all check out based on the surging idle checklist numbers. The power and ground are good. This is a new MAF stock replacement. The one on the car when I got It was also a stock unit. I replaced it to make sure that was not the issue and it was not. The MAF operates and raises voltage based on throttle as it should. I am banging my head against the wall trying to figure it out and need to see if someone else has had this issue.

When I unplug the MAF connector the car runs the same. The car is in FMEM and will not complete a KOER test. Every time I clear the KOEO codes and start the car the code 66 comes back. Could the computer be fried on the MAF circuit? Someone please give me some advice. Also, I am not beyond the thinking that there may not be enough fuel pressure and it is making the MAF run haywire. But I am not sure that would cause this or not.

As a side note on the condition of the car when I got it, everything was basically wrong. Firing order was backwards, intakes leaked, timing cover leaked, unplugged vacuum lines, the works.
 
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What codes other than the 66 are you getting?
What are you getting for MAF readings when you measure the MAF output voltages at the computer?


Code 66 or 157 MAF below minimum test voltage.

Revised 10-Feb-2014 to add 95-95 Mustang code 157 and 94-95 ECC diagram

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.

Computer wiring harness connector, wire side
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Computer wiring harness connector, computer side
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Diagrams courtesy of Tmoss and Stang&2Birds

ECC Diagram for 88-90 5.0 Mustangs
88-91_5.0_EEC_Wiring_Diagram.gif



How the MAF works

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.

91-95 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 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
 

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Last edited:
I will measure the output on pins 50 and 9 today. Thanks. I am assuming that if they match it verifies the wiring is good.

I will also be returning the replacement MAF for a new unit to rule that out as well. It's free, why not? Also on a side note, I have cleaned the connector and made sure the pins are tight.

Code 66 is the only KOEO code I have. As stated, it will not allow me to run KOER. Wish it would. Essentially, I believe my car is running without the mass air functioning. As if it were unplugged. The strange thing is that it runs great while driving. No misses, no hesitation, perfect throttle response.
 
No code 67?


Code 67
Revised 18-Mar-2017 to include warning about the necessity of having a 5 speed O2 Sensor wiring harness when bypassing the wiring for test purposes

Cause of problem:
Clutch not depressed (5 speed) or car not in neutral (5 speed and auto) or not in park (auto) or A/C in On position when codes where dumped. Possible neutral safety switch or wiring problem. This code will prevent you from running the Key On Engine On tests.

External evidence from other sources claims that a code 67 can cause an idle surge condition. Do try to find and fix any issues with the switch and wiring if you get a code 67.

What the NSS (Neutral Safety Switch) does:
5 speed transmission: It has no connection with the starter, and the engine can be cranked without it being connected.
Auto transmission: It is the safety interlock that prevents the starter from cranking the engine with the transmission in gear.
What it does for both 5 speed and auto transmission cars:
The computer wants to make sure the A/C is off due to the added load on the engine for the engine running computer diagnostic tests. It also checks to see that the transmission is in Neutral (5 speed and auto transmission) and the clutch depressed (T5, T56, Tremec 3550 & TKO)). This prevents the diagnostics from being run when the car is driven. Key On Engine Running test mode takes the throttle control away from the driver for several tests. This could prove hazardous if the computer was jumpered into test mode and then driven.

The following is for 5 speed cars only. Do not do this unless you are sure that you have a 5 speed O2 Sensor harness!!!! Smoke, sparks and expensive pain in the wallet may ensue if you don’t.
The NSS code 67 can be bypassed for testing. You will need to temporarily ground computer pin 30 to the chassis. Computer pin 30 uses a Lt blue/yellow wire. Remove the passenger side kick panel and then remove the plastic cover from the computer wiring connector. Use a safety pin to probe the connector from the rear. Jumper the safety pin to the ground near the computer.
Be sure to remove the jumper BEFORE attempting to drive the car!!!

a9x-series-computer-connector-wire-side-view-gif.71316
 
No code 67 that I have seen. But that is extremely helpful. When I bought the car it went through a span of no start and I figured it was the NSS. But then it started working properly again so I let it go for the time being.

Didn't know it would disable KOER. I will ground the computer as instructed and see if it works.
 
No code 67 that I have seen. But that is extremely helpful. When I bought the car it went through a span of no start and I figured it was the NSS. But then it started working properly again so I let it go for the time being.

Didn't know it would disable KOER. I will ground the computer as instructed and see if it works.
 
Ok, for some reason the OHM readings are not working on my meter. The voltage at idle is 1.40 which I thought was strange.

Thoughts? Everything else seems to be fine.
 
Ok final update from his morning.

I have verified all of the wiring readings on the 4 MAF pins. They are:

Pin A - 12.83v. Good
Pin B - Ground. Good
Pin C - 1.40v at idle, 8K ohms or so. Possible issue.
Pin D - 1.40v at idle, 12K ohms. Possible issue.

At this point I will be getting the MAF replaced for free to rule out getting a bad new unit. I feel the MAF connector works because I resistance tested it on both sides and it zeros out. Readings are the same at MAF plug and computer pins.

I like to think I have an above basic knowledge of wiring diagrams. I know the 12v red wire spiders to everything from the injectors to the relays and MAF. I do not suspect it has an issue anywhere because everything else works properly that needs power from it.

If I am thinking correctly the only issues can be the MAF itself, the ECM, or the connector/wires. I do not think any other variable can give me this code 66.

Thoughts based on my findings?
 
Ok final update from his morning.

I have verified all of the wiring readings on the 4 MAF pins. They are:

Pin A - 12.83v. Good
Pin B - Ground. Good
Pin C - 1.40v at idle, 8K ohms or so. Possible issue.
Pin D - 1.40v at idle, 12K ohms. Possible issue.

At this point I will be getting the MAF replaced for free to rule out getting a bad new unit. I feel the MAF connector works because I resistance tested it on both sides and it zeros out. Readings are the same at MAF plug and computer pins.

I like to think I have an above basic knowledge of wiring diagrams. I know the 12v red wire spiders to everything from the injectors to the relays and MAF. I do not suspect it has an issue anywhere because everything else works properly that needs power from it.

If I am thinking correctly the only issues can be the MAF itself, the ECM, or the connector/wires. I do not think any other variable can give me this code 66.

Thoughts based on my findings?
The MAF voltages are never measured to ground.
To do it right, the proper way is to put one meter lead on pin C and the other meter lead on pin D; it is not polarity sensitive with a good meter.

If you make the measurements inside the car at the computer, use pin 9 and 50. it is not polarity sensitive with a good meter.
Again, do not measure from either pin to ground.


For those of you who have an electronics background, here's why.
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.
 
That is what I meant to type. The 1.40v at idle was back probing the c and d pins together. Also pins 50 and 9 on the computer. They are the same voltages in both spots. The new MAF will be here tomorrow and I got a new connector as well because one locking tab is broken on mine.

Will update thread when I change parts out. At this point I hope this fixes it or I am lost.
 
That is what I meant to type. The 1.40v at idle was back probing the c and d pins together. Also pins 50 and 9 on the computer. They are the same voltages in both spots. The new MAF will be here tomorrow and I got a new connector as well because one locking tab is broken on mine.

Will update thread when I change parts out. At this point I hope this fixes it or I am lost.
have you got any updates?