89 stang 5.0 codes 98 and 66

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.

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.

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 - Wiring & Engine Info

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

This is a rather long story so get your reading glasses. This winter I converted my 1965 mustang over the efi. I used an RJM wiring harness (awesome quality) and a used 1988 California 5.0 (E303 cam, BBK 76 mm MAF, 30lb injectors, typhoon intake and 70mm TB) with the 5 speed.

I ended up plugging the main power body plug on the mustang backwards (the one that feeds the rjm harness) and fried the computer. This computer was chipped and dyno tuned, I didn’t know if a stock A9L would run the motor with all the extras but I bought and installed an autozone one anyway. The car ran exactly how you described arcticman with a code 66 and 98. I ran through a couple of posts and copied and pasted bullet comments on what to check to help me stay focused. There’s a lot of good information out there, due to guys like JRICHKER (THANK YOU!!!).

I went through and started checking the suggested wires/sensors and was almost through the checklist, when I realized that the RJM harness had originally came with the 89 MAF plug (as it should have). My MAF needed the 94 plug, I used butt connecters to wire it in. I checked them and all were tight and looked good. So for some reason I decided to cut them out and solder the connections as I should have from the get go. I cleared the codes and started the car and it ran a lot better (idle still wanders about 200 rpms I think that may be due to the 30lb’ers) but best of all, both of those codes are gone at least during the first KOEO test (ran out of time for the KOER test but ran the motor for a while and it seems to be fixed). Long story short, it’s usually the stupid little things that we look at a hundred times that get us. Think back and if there’s anything that you’ve tinkered with in the past, give it a quick once over.

My borrowed checklist
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. PASS

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. PASS

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). CLEANED

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 49k ohms.

Inspect the connectors for damaged wires and pushed or corroded pins. PASS
Check the resistance from MAF terminal D and pin 50 of the ECA 1 ohm; it should be less than 5 ohms. Check the resistance of MAF terminal C to pin 9 1 ohm; it should be less than 5 ohms. If these resistances are okay, check the resistance of each wire to ground; it should be infinite Tan MAF RTN has 7 ohms to ground.

Start the engine and allow it to idle. Check the voltage on ECA pin 50. If the voltage is between 0.2 and 1.5 volts, replace the ECA. If the voltage is not within these specs, replace the MAF sensor. Haven’t checked yet

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 Haven’t checked yet

I forgot to mention Larrym1961, he had some useful tips as well.
Thanks you guys

arcticman said:

i ran threw the check list and everything is fine except for the voltage at pin 50 or on harness near maf sensor the voltage i got was 1.30 to 1.35 volts (idling) so do i need to replace the computer or can i check anything else.

Click to expand...

Replacing the computer will not change the output of the MAF sensor. The voltage from the MAF definitely seems to be high.