Fix the code 78 first and the other two codes may go away.
Code 78 VPWR circuit to ECC was intermittent or the ECC is bad
Possible bad fuse link, wiring, intermittent ECC power relay or computer .
Carefully examine the wiring up near the starter solenoid for damage or signs of the fuse link melting. You may have to unwrap some of the harness to expose the wiring. The computer’s power feed wiring is 14 gauge black/orange. It connects to the fuse link. I believe the fuse link is dark blue.
Diagram courtesy of Tmoss & Stang&2birds
Remove the passenger side kick panel and disconnect the computer connector. There is a bolt with a 10 MM hex head that holds it in place. Check the computer wiring connector for damage or overheating at the black/orange wires.
Replace the ECC power relay after checking the wiring near the starter solenoid and at the computer connector. The ECC relay is mounted on top of the computer: you have to remove the computer to replace it. The computer mount is two 10/32 screws that secure a plastic strap diagonally across the computer case. Removing the passenger side dash speaker may help some. I recommend that you enlist someone with small hands to help you with the relay since the working space is very small.
Diagram courtesy of Tmoss & Stang&2birds
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.
If you have a K&N flat panel filter or other filter that requires oiling, excess oil may coat the MAF sensor element and cause problems.
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).
Changes in RPM causes the airflow to increase or decease, 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. 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 ground. 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
Mustang FAQ - Engine Information
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
Code 98 - basic computer internal tests not passed before it started processing the request to dump codes. Do a Key On Engine Off test before starting the engine. Wait until you see the 11 flash before pressing any buttons if you have a hand operated scanner. The computer is operating in Limp mode, so fix any codes it dumps prior to doing anything else.
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Engine Information Everyone should bookmark this site.
Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif
Complete computer, actuator & sensor wiring diagram for94-95 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/94-95_5.0_EEC_Wiring_Diagram.gif
Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif
TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif
Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif
