No Spark and Checking Codes.

Pull the spout out and then see if you get spark. If no spark then.....

Then it could be a few things. Hall Effect sensor, TFI, coil. Start with seeing if you get a Signal from the top wire of the TFI while cranking. 4-5v is what you would normally see.

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Could be a problem with the code return circuit.

Are you trying to run the codes with the engine off, or running?

Hook the test light up as you normally would to run the codes. But this time, on the small grey plug, run it straight to a good ground, then try and run the codes. See what happens.

The following is for 5.0 Mustangs. It probably won't help much if you are running a 4 cylinder as indicated in your first post.


Computer will not go into diagnostic mode on 91-93 model 5.0 Mustangs

How it is supposed to work:
The grey/red 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. You will have hard starting, low power
and drivability problems. 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 the test connector grey/red wire 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
(grey/red 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 grey/red wire and pin 46 on the computer wiring connector : 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

Check out the diagram and notice all the places the grey/red wire goes. Almost every
sensor on the engine except the MAF is connected to it.


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



If you feel like opening up the computer to attempt to repair the fried wire, see Computer issue? - Mustang Forums at StangNet and joel5.0's comments for more help on fixing the computer innards.

All those codes indicate a problem with the SIGRTN circuit. You need to unplug the ECM, open it up and inspect the back of the board for a burnt trace. let me know what you find out.

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I'm on my phone, but if the main plug was to your right, back of board facing up, then the burnt trace would be to the left of the main plug. You could trace it back to PIN 46.

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Find PIN 46 at the ECM connector. Hook a volt meter to it and a good ground. And do cranking test as you monitor the voltage. ECM out of the car of course.

The voltage should be very very minimal.

Let me know what you find.


Also find a small wire, and just solder it in place of the burnt circuit. Just leave the ECM out till you find out why it got blasted with juice.

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See what color wires on the O2 harness are looped together.

For a manual setup it should be the blue/yellow and purple/ yellow.

If the white/pink is looped to the blue/yellow, then that's why the ECM fried.

Your clear on PIN 46. Solder a wire on the burnt circuit on the board then test the joints for continuity. Plug the ECM back up and then run the KOEO codes. You could be right and someone tried to run codes using the wrong terminal.