1993 Mustang GT will not display check engine light until it is turned over and will not throw codes...

Stanio

New Member
Oct 31, 2019
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I have been rebuilding a 1993 mustang for about a year now. One of the most significant things about this car's rebuild is that the guy who owned it before me put a wiring harness out of a 1990 or 1991 into the interior without changing the rest of the wiring. I got a 1993 4-cylinder wiring harness, which seemed to fix the myriad of problems I was having....after I fixed the fusible link that was popped when I got the new harness; did I mention I have really bad luck with this car. I have it running fairly well, but when I took it in for a smog the tech noted that the check engine light would not turn on when the key was in the on position; without the engine running of course. After checking further, I discovered that it will not display the 11 or 111 code to my OBD I code reader; like the computer was not communicating with my reader. I did note that when I turned over the motor to start it, the check engine light comes on then immediately goes off when it starts or I let go of the key. Per a thread in this forum, I checked the ohms from the grey with red stripe wire of the diagnostic port to the negative battery cable and got a reading of 1.8. When I checked the ohms between pin 46 of the computer to the same wire on the diagnostic port I got 13.5 ohms, which fluctuated a lot without doing anything to it. The thread says I have a wiring issue but I don't know what to do from here...What should I do?
 
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jrichker

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I have been rebuilding a 1993 mustang for about a year now. One of the most significant things about this car's rebuild is that the guy who owned it before me put a wiring harness out of a 1990 or 1991 into the interior without changing the rest of the wiring. I got a 1993 4-cylinder wiring harness, which seemed to fix the myriad of problems I was having....after I fixed the fusible link that was popped when I got the new harness; did I mention I have really bad luck with this car. I have it running fairly well, but when I took it in for a smog the tech noted that the check engine light would not turn on when the key was in the on position; without the engine running of course. After checking further, I discovered that it will not display the 11 or 111 code to my OBD I code reader; like the computer was not communicating with my reader. I did note that when I turned over the motor to start it, the check engine light comes on then immediately goes off when it starts or I let go of the key. Per a thread in this forum, I checked the ohms from the grey with red stripe wire of the diagnostic port to the negative battery cable and got a reading of 1.8. When I checked the ohms between pin 46 of the computer to the same wire on the diagnostic port I got 13.5 ohms, which fluctuated a lot without doing anything to it. The thread says I have a wiring issue but I don't know what to do from here...What should I do?
Computer will not go into diagnostic mode on 91-95 model 5.0 Mustangs

Revised Dec 23 2017
1.) To clarify signal ground connections on the engine mounted fuel injector wiring harness and add diagram for the engine mounted fuel injector wiring harness
2.) To add warning about using an automatic transmission O2 sensor wiring harness with a A9L manual shift transmission computer.


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. As long as you are successful dumping the codes by using the gray/red wire on the diagnostic connector for the ground when dumping, the computer’s internal ground on pin 46 is good.

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 ohm when measured from anyplace on the engine harness with the battery pigtail ground as the other reference point for the ohmmeter probe.

Engine mounted fuel injector wiring harness sensors for a 5.0 mustang
63347.gif


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.

OR

If an O2 sensor harness from an automatic transmission Mustang is used with an A9L manual shift transmission computer. The 12 volts from the automatic transmission starter circuit will damage the A9L computer.

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 ohm.



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 ohm. More than 1 ohm is a wiring problem. If it reads 1 ohm 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 ohm. More than that and the computer’s internal ground has failed, and the computer needs to be repaired or replaced.
While you have the computer connector disconnected from the computer, turn the ignition switch to the Start position and look for 12 volts on pin 46 of the computer wiring harness. If you see 12 volts then you have an automatic transmission O2 sensor harness. That will damage the A9L manual shift transmission computer.


See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix for the computer internal signal ground.

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 than 1 ohms is a wiring problem

The following is a view from the computer side of the computer wiring connector: it is for an A9L, A9P computer.
eec-iv-computer-connector-for-5-0-mustang-gif.gif


a9x-series-computer-connector-wire-side-view-gif.gif


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.

91-93 5.0 Mustangs




Complete computer, actuator & sensor wiring diagram for 94-95 Mass Air Mustangs
94-95_5.0_EEC_Wiring_Diagram.gif



See the graphic for the 10 pin connector circuit layout.
salt-pepper-10-pin-connectors-65-jpg.jpg



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
 

Stanio

New Member
Oct 31, 2019
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40
Grass Valley, California
The only thing I found that this round of testing said needs further wiring testing (but does not explain how to do) is when I go from the grey and red diagnostic port to pin 46 on the computer harness; I have a lot more than 1 ohm. All of the other testing did as it was supposed to do....I don't have the hot from the O2 sensor, I get less than 1 ohm when testing from pin 46 to pin 40 and 60. And now for some reason, I'm only getting .7 Ohm when I test between the diagnostic port ground and the battery ground. I'm getting no where quick with this. The main issue I have is the light only comes on when turning over the car....
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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Maybe someone can answer this: is a 93 4cyl dash harness compatible with a 93 V8? I would think not but I'm no expert.
Also I've found that when the computer will not give codes there is a problem with the computer (again with the compatibility issue?)
 

jrichker

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Mar 10, 2000
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Maybe someone can answer this: is a 93 4cyl dash harness compatible with a 93 V8? I would think not but I'm no expert.
Also I've found that when the computer will not give codes there is a problem with the computer (again with the compatibility issue?)
This guy has two threads for the same problem. Please do a search on his name and merge the threads or appropriate posts. See https://www.stangnet.com/mustang-forums/threads/check-engine-light.895648/page-5 for the extra post he has made.

jrichker, I checked the other thread and got no where...please see the other thread for further.
I hope that you have better than average electrical troubleshooting skills, because you are going to need them. Teaching basic automotive electrical circuits and troubleshooting is beyond the scope of what I can do here on Stangnet.

The 1993 Mustang wiring diagrams and EVTM manuals will be a necessity, so Google them and find a source to get both of them. That is the first thing you need to acquire. I am going to make a dangerous assumption that you already have a working DVM or Multimeter and know how to use it correctly.

Mix and match wiring harnesses really don't mix and match very well. All of the wiring harnesses should be the same year and engine type to reduce the probability of some wire not being connected to the pin of a different year harness. The 90 computer harness doesn't work in anything but a 90 model car without rework. The 91 and 92-93 wiring harnesses have different physical locations for the fuel pump power relay.

OK, tell me what didn't check out according to the test path below....

Computer will not go into diagnostic mode on 91-95 model 5.0 Mustangs

Revised Dec 23 2017
1.) To clarify signal ground connections on the engine mounted fuel injector wiring harness and add diagram for the engine mounted fuel injector wiring harness
2.) To add warning about using an automatic transmission O2 sensor wiring harness with a A9L manual shift transmission computer.


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. As long as you are successful dumping the codes by using the gray/red wire on the diagnostic connector for the ground when dumping, the computer’s internal ground on pin 46 is good.

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 ohm when measured from anyplace on the engine harness with the battery pigtail ground as the other reference point for the ohmmeter probe.

Engine mounted fuel injector wiring harness sensors for a 5.0 mustang
63347.gif


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.

OR

If an O2 sensor harness from an automatic transmission Mustang is used with an A9L manual shift transmission computer. The 12 volts from the automatic transmission starter circuit will damage the A9L computer.

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 ohm.



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 ohm. More than 1 ohm is a wiring problem. If it reads 1 ohm 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 ohm. More than that and the computer’s internal ground has failed, and the computer needs to be repaired or replaced.
While you have the computer connector disconnected from the computer, turn the ignition switch to the Start position and look for 12 volts on pin 46 of the computer wiring harness. If you see 12 volts then you have an automatic transmission O2 sensor harness. That will damage the A9L manual shift transmission computer.


See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix for the computer internal signal ground.

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 than 1 ohms is a wiring problem

The following is a view from the computer side of the computer wiring connector: it is for an A9L, A9P computer.
eec-iv-computer-connector-for-5-0-mustang-gif.gif


a9x-series-computer-connector-wire-side-view-gif.gif


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.

91-93 5.0 Mustangs




Complete computer, actuator & sensor wiring diagram for 94-95 Mass Air Mustangs
94-95_5.0_EEC_Wiring_Diagram.gif



See the graphic for the 10 pin connector circuit layout.
salt-pepper-10-pin-connectors-65-jpg.jpg





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
 
Last edited:

Boostedpimp

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May 8, 2003
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Maybe someone can answer this: is a 93 4cyl dash harness compatible with a 93 V8? I would think not but I'm no expert.
Also I've found that when the computer will not give codes there is a problem with the computer (again with the compatibility issue?)
yes dash and interior harness is same v8 or 4 banger
 
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