Electrical Injector pulse and 5v reference

zombiestang

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Sep 25, 2022
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Hello this is my first post. I just got a 92 5.0 convertible with automatic trans. The car supposedly ran when parked approximately 10 yes ago. I bought it along with a 88 gt hatch with no motor. My plan was to put the motor and trans from the 91 into the 88 but I wanted to get it running first..
The car will crank and will run with starting fluid. The fuel pump runs constantly with key on and there is a very faint CEL. The CEL is bright and constant while cranking. I have checked the pip with a led light probed on the first wire of the tfi and I get flashes as engine cranks over. I then went to check for 5v ref at the salt and pepper plugs on top of the engine. I have no voltage on pin 2 but have 5.35 volts on pins 7 and 8 which are for the oil and temp gauges.
I have no idea why there would be voltage on those pins. I have tried to pull engine codes using the CEL and using a test light and can't get it to give any codes. I disconnected the brown plug at driver side fender and ran a jumper for the red/green tracer wire to power the computer and there is no longer 5 volts on those pins but nothing on pin 2 either. I have tried a different ecm with the same results. Any help would be greatly appreciated.
 

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Hello this is my first post. I just got a 92 5.0 convertible with automatic trans. The car supposedly ran when parked approximately 10 yes ago. I bought it along with a 88 gt hatch with no motor. My plan was to put the motor and trans from the 91 into the 88 but I wanted to get it running first..
The car will crank and will run with starting fluid. The fuel pump runs constantly with key on and there is a very faint CEL. The CEL is bright and constant while cranking. I have checked the pip with a led light probed on the first wire of the tfi and I get flashes as engine cranks over. I then went to check for 5v ref at the salt and pepper plugs on top of the engine. I have no voltage on pin 2 but have 5.35 volts on pins 7 and 8 which are for the oil and temp gauges.
I have no idea why there would be voltage on those pins. I have tried to pull engine codes using the CEL and using a test light and can't get it to give any codes. I disconnected the brown plug at driver side fender and ran a jumper for the red/green tracer wire to power the computer and there is no longer 5 volts on those pins but nothing on pin 2 either. I have tried a different ecm with the same results. Any help would be greatly appreciated.

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.

attachments\57945


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.

See image below for help finding the burnt signal ground trace.
?hash=146a243133771eeba54f17b17d721b1f.jpg

The fix is some careful soldering of a small jumper wire across the burnt section of copper trace.

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.

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
91-93_5.0_EEC_Wiring_Diagram.gif




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
 
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zombiestang

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Sep 25, 2022
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Thanks for the information! I saw this posted on another thread and did some of the tests but was jumping around to different things. I have printed this out and will follow the instructions and post the results. Thanks again for the help.
 
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zombiestang

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Sep 25, 2022
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Sorry for posting so late but I did the tests with the following results
Grey/red at test connector to batt ground 0.0 ohms.
Grey/red to pin 46 0.4 ohms.
Pin 46 to pin 40 0.3 ohms.
Pin 46 to pin 60 0.3 ohms.
Voltage on pin 46 at cranking 4.6 mV.
STI grey connector to pin 48 .6 ohms.
I have battery voltage at pin 1 of black connector.
I have 0 volts on pin 2 of white connector.
Pin 2 white connector to pin 26 1.4 ohms.

I don't know if you saw in my original post but I have 5.35 volts on pins 7 and 8 of the white connector which doesn't make sense because those are for the oil and temp gauges. I have triple checked and it's definitely there.

What's really confusing is that there is no continuity between pin 26 and pins 7 or 8 of the white connector. No continuity between pin 2 and pins 7 or 8 either. There is 59.7 ohms of resistance between pins 7 and 8.

I'm at a loss and not sure what to try next but I'm considering opening up the wiring harness to look for shorts. Any insight would be greatly appreciated.
 

zombiestang

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Sep 25, 2022
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Just wanted to add the computer that's in it now is an A9p. I'm assuming it's stock because there is a sticker on the door jamb with the same number. Fuel pump runs constantly, CEL is only on while cranking and still can't get the computer to give any codes.
 
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General karthief

wonder how much it would cost to ship you a pair
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Just wanted to add the computer that's in it now is an A9p. I'm assuming it's stock because there is a sticker on the door jamb with the same number. Fuel pump runs constantly, CEL is only on while cranking and still can't get the computer to give any codes.
The computer is compromised, send it to the ECUExchange, they are on ebay. Very affordable, I send all my stuff to them.
 

zombiestang

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Sep 25, 2022
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I don't have 5 volt ref on any sensors or pin 26. Its looking like I may actually have 2 bad ecms. I will be sending the original to the ebay store mentioned above when I have the funds. Thanks for the reply!
 
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Mustang5L5

Put lubricant all over the balls
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I don’t think it will work from pin 26 to battery ground. You should go to sensor ground.


Easiest place to check is at the tps sensor. Unplug it. Turn the key to on. Black wire on the engine side harness is your pin 46 ground and orange is pin 26.

Use multimeter to test between the two and see if you have 5v there.

If you don’t have it, then you can start going down to the ECU and backprobing the pins on the connector. A little trickier to do which is why I suggest starting at the tps sensor first.
 

zombiestang

Member
Sep 25, 2022
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.75 volts at tps connector on orange and black wires as well as at ecm connector on 1 ecm and 0 volts with the other one so I'm guessing both are bad. And the voltage on the sending unit wires was coming from the gauges I think because with the brown plug at driver side firewall disconnected the voltage goes away.

I really appreciate everyone's help and will be sending the original ecm to get repaired. I will update the post when I get it but may be a month or two. Thanks again for all the help!
 
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