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 64 – ACT (Air Charge Temperature) signal low or
grounded. Bad sensor, sensor wiring on pin 25 shorted to
ground, or bad computer.
Note that all resistance tests must be done with power
off. Measuring resistance with a circuit powered on will
give false readings and possibly damage the meter.
The ACT is located in the #5 intake runner on Mustangs
built from 86-93. Disconnect the sensor by removing the
sensor wiring connector. Measure the resistance across the
sensor pins.
Ohms measures at the computer with the computer
disconnected, or at the sensor with the sensor
disconnected.
50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms
If the sensor resistances read good according to the table,
remove the passenger side kick panel and disconnect the
computer connector. A single 10 MM bolt secures the
connector in place. With the sensor connector still
disconnected, measure the resistance between pin 25 (lt
green/pink wire) and ground. If you wish, you can make
your measurements at the ACT sensor end. This will save
you from counting the computer connector pins to locate
the correct one. However, you will still have to disconnect
the computer connector. You should see 100K Ohms or
greater between pin 25 and ground. Less than that
indicates wiring problems with the pin 25 lt green/pink wire
shorted to ground. Note that the following diagram on the
connector displays the pins viewed from the computer side.
See 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/
http://www.veryuseful.com/mustang/tech/engine/images/88-91eecPinout.gif
Diagram courtesy of Tmoss & Stang&2birds
If the resistance readings for the ACT sensor and the ACT
wiring on pin 25 read OK, then you have a bad computer.
Do not replace the computer unless the two test steps I
have described pass OK. Computers seldom fail and are
very rugged. It takes a lot to kill one while it is bolted in
place in the car.