Electrical CEL always on, after reset, KOEO code 66 and KOER code 98 & 66


5 Year Member
Jan 19, 2019
Houston, TX
I have 1990 Ford Mustang GT Hatchback, stock wiring, second owner for 25 years, bought from close friend with 25,000 mile and drove another 200,000 mile. Last three years, engine and T5 manual transmission, exhaust, 2.5 inch, cat system, drive train converted to 3.5 gear, suspension upgraded mostly MMS. Factory harness is good shape, car was always in garage for last 30 years. No rust.

Engine rebuilt, piston 4.03, Comp Cam XE264HR, head AFR 186 CC, combustion chamber 58 CC, intake trick flow track heat, BBK throttle body 70 MM, 30 Lbs. injectors, BBK MAF calibrated to 30 Lbs. Vacuum Leak check, no leaks, all emission control system connected and functional.

I started the engine, timing set at 12 degree w/o SPOUT. TPS 0.98 Volt, Idle vacuum 14 inch, hot idle rpm fluctuate 710 -735 rpm, oil and temp normal, Fuel pressure set at 44 psi w/o vacuum hose, idle pressure 39 psi.

Car starts and runs smooth, does not die once started cold and drive 10 to 12 mile, city driving, does not die at stop lights.

Problem: Check Engine Light Always On, code 66 MAF low voltage and code 98 limp mode.

MAF electrical data: MAF Pin A,B,C, D from right to left
Pin A Red - Pin B Black at KOEO - 12.1 volt
Pin C - tan/light and pin D dark blue/orange - KOER - 0.91 Volt
Harness EEC to MAF wiring - pin C and EEC Pin 9 - 0.3 ohm, pid and EEC pin 50 0.3 ohm, pic and ground 28 k ohm, pin D and ground 98 k ohm.

EEC IV - E9ZF 12A650 A2A - replace all three capacitor, still code 66 and 98.

Like to hear member's comments, suggestion, advise on further troubleshooting. My options - should I send EEC for further test and repair, or install tuner chip, or buy another MAF.


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Yes, I did follow the MAF testing steps outlined in this web site.

Also w/o connecting to wiring in BBK MAF, pin C and D resistance is 10.1 k ohm ( this is calibrated to 30 lbs.)

My Ford Stock MAF Pin C/D = resistance 3.8 k ohm (Stock for 19 lbs. Ford does not calibrate their MAF?)

Both when installed and run, I get CEL ON each case, and code 66 in each case.

This lead me to believe that my EEC MAF signal is grounded some where. Let me know if my understanding is correct.
I would test down at the ECU on the pins for the MaF out to the MAF harness. There might be a damaged wire or a grounded wire that is affecting the readings that the ECU is seeing.

If you can get your hands on a breakout box, it would be invaluable here to see what voltages the ECU is seeing when it is running.
Thanks, I got some unexpected voltage reading at EEC - when car on idle:

Voltage between Pin 9 and Pin 50 - 5.9 Volt. (Does not varies when I pressed accelerator)

I checked voltage at pin C and D again I got 0.92 Volt at idle and it increases when bump the TB lever to 1.3 volt (car on neutral).

I checked Ohm reading again as follows
Pin C and Pin 9 - 0.3 ohm
Pin D and Pin 50 - 0.3 ohm
Pin 50 and ground - 99.3 K ohm; Pin D and ground - 100 K ohm;
Pin 9 and ground - 28.2 K ohm; Pin C and Ground - 27.9 K ohm;

Let me know your comments, suggestions, advise as applicable......Thanks again
Code 66 or 157 MAF below minimum test voltage.

Revised 2 Nov 2019 to add details on MAF testing

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.

Computer wiring harness connector, computer side.

Diagrams courtesy of Tmoss and Stang&2Birds

ECC Diagram for 88-90 5.0 Mustangs

ECC Diagram for 91-93 5.0 Mustangs

94-95 Diagram for 94-95 5.0 Mustangs[/b]

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.

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.

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.

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.

MAF output readings: Use the computer connector diagram to help choose the proper pin connection on the computer when measuring the MAF output voltage. The idling voltage check can the done with the voltmeter directly stuck in the backside of the MAF connector.

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

If the output of the C&D pins exceeds the specs above, there are two possible problems:

1.) The MAF sensor is defective and needs to be replaced.
2.) The MAF sensor is installed in a different housing than the one it was designed for. The sensor is designed to work with a specific MAF part number or model MAF housing.

Check the resistance of the MAF signal wiring
For the next 2 checks make your measurement with the MAF disconnected from the wiring harness.

Pin D on the MAF wiring harness and pin 50 on the computer (dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF wiring harness and pin 9 on the computer (tan/light blue wire) should be less than 2 ohms.[/B]

There should be a minimum of 10K ohms between either pin C or D on the MAF wiring connector and pins A or B.

Reconnect the MAF to the wiring harness and proceed to the next section.

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

Fuel pump, alternator, ignition & A/C wiring

Computer, actuator & sensor wiring

Fuse panel layout

Vacuum routing


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Thanks, I will do these test again this weekend, and report back to you all.

My previous MAF test for A - red ,B - black ( 12.1 volt ), C& D with KOER = 0.9 to 2.2 volt. readings are stable and it varies with rpm change
It also meets the resistance values.

I have to re-check EEC Pin 9 and 50 again and let you all know.


After EEC repaired, my car runs good specially when it goes into closed loop. No check engine light and no codes.

Now I am ready go for state safety inspection and drive around.

Thanks for stangnet.com and associated team members for help.
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