check engine light won't work when doing koeo test

Does the light come on when you turn the ignition switch to Run before you crank the engine?

Improved diagram for the 86-88 cars...

IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no
functional check engine light on the 86-88's except possibly the Cali Mass Air cars.




If that does not work, try this...

Computer will not go into diagnostic mode

How it is supposed to work:
The black/white wire (pin 46) is signal ground for the computer. It provides a dedicated
ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the
computer into self test mode. If this ground is bad, none of the sensors mentioned will work
properly. That will severely affect the car's performance. Since it is a dedicated ground,
it passes through the computer on its way to the computer main power ground that terminates at the
battery pigtail ground. It should read less than 1.5 ohms when measured from anyplace on the
engine harness with the battery pigtail ground as the other reference point for the ohmmeter probe.
What sometimes happens is that it gets jumpered to power which either burns up the wiring or
burns the trace off the pc board inside the computer. That trace connects pins 46 to pins 40 & 60.
Only an experienced electronics technician can open the computer up & repair the trace if it burns
up and creates an open circuit.

The STI (Self Test Input ) is jumpered to ground to put the computer into test mode. Jumpering
it to power can produce unknown results, including damage to the computer. The ohm test
simply verifies that there are no breaks in the wiring between the test connector and the computer input.

How to test the wiring :
With the power off, measure the resistance between the computer test ground
(black/white wire) on the self test connector and battery ground. You should see less than
1.5 ohms.



If that check fails, remove the passenger side kick panel and disconnect the computer
connector. There is a 10 MM bolt that holds it in place. Measure the resistance between
the black/white wire and pin 46: it should be less than 1.5 ohms. More that 1.5 ohms is a
wiring problem. If it reads 1.5 ohms or less, then the computer is suspect. On the
computer, measure the resistance between pin 46 and pins 40 & 60: it should be less than
1.5 ohms. More that that and the computer’s internal ground has failed, and the computer
needs to be replaced.

If the first ground check was good, there are other wires to check. Measure the
resistance between the STI computer self test connector (red/white wire) and pin 48 on
the computer main connector: it should be less than 1.5 ohms. More that 1.5 ohms is a
wiring problem

The following is a view from the computer side of the computer connector.


Diagram courtesy of Tmoss & Stang&2birds



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