this occured about 2 months ago. was trying to pull my codes and jumped from the hood light terminal to one of the terminals in the 6 pin connector for self diagnostics. the light doesnt come on when u turn the key on. when u crank it is does but thats all. tried the voltmeter way, test light way. i bought a breakout box to try and diagnose this and havent got very far yet. tried some jrichker suggestions on other threads to see if the ecu was bad but most pass. any help appreciated. thanks
no way to pull codes. tried evertything.
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No info about your car's model year, EFI or carb, no mods listed. Makes it hard to troubleshoot when we don't know what you have.
Therefore, no clue as to which wiring diagram you need. The wiring changed several times between 79 & 95.
That's why it is a good idea to use the sig under the user control panel (User CP) option. It allows you to post your car year & mods, which help greatly when troubleshooting things. No, it is not there for us to snoop and see any "Secrets" you have hidden away under the hood. Be a good stangnetter and update you sig for future reference & don't keep us guessing.
Therefore, no clue as to which wiring diagram you need. The wiring changed several times between 79 & 95.
That's why it is a good idea to use the sig under the user control panel (User CP) option. It allows you to post your car year & mods, which help greatly when troubleshooting things. No, it is not there for us to snoop and see any "Secrets" you have hidden away under the hood. Be a good stangnetter and update you sig for future reference & don't keep us guessing.
sorry bout that. its an 89 lx 5.0. 5 speed. bbk headers. catless x pipe. i got the egr blocked off. i was reading some of your stuff today and yeasterday. and i have another question. is the black white white supposed to have an external ground or only ground through the ecu? cause when i got the car they had a ground wire from it to the firewall off the tps sensor harness and it was loose so i just regrounded it to the fender.
The black/white wire is a signal ground that goes to the computer.
Computer will not go into diagnostic mode on 86-90 model 5.0 Mustangs
Disconnect the battery positive terminal before making any resistance checks.
The small voltage drop in the battery cables will cause incorrect resistance readings.
Computer diagnostic connector:
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. 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 black/white wire gets jumpered to power.
There is a dark brown connector with a black/orange wire near the diagnostic connector. It is
the 12 volt power to the under the hood light. If this happens, it 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. The STI
has a gray connector shell and a white/red wire 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 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 wiring connector.
Diagram courtesy of Tmoss & Stang&2birds
Check out the diagram and notice all the places the black/white 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
Computer will not go into diagnostic mode on 86-90 model 5.0 Mustangs
Disconnect the battery positive terminal before making any resistance checks.
The small voltage drop in the battery cables will cause incorrect resistance readings.
Computer diagnostic connector:
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. 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 black/white wire gets jumpered to power.
There is a dark brown connector with a black/orange wire near the diagnostic connector. It is
the 12 volt power to the under the hood light. If this happens, it 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. The STI
has a gray connector shell and a white/red wire 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 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 wiring connector.
Diagram courtesy of Tmoss & Stang&2birds
Check out the diagram and notice all the places the black/white 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
with my ground hooked from tps sensor harness to fender, the 46, 40, and 60 tests pass. but if i unhook it, they fail.
between the black/white wire and pin 46, it reads less than 1.5 ohms.
and between the pin 48 and the red white wire is less than 1.5 ohms as well.
so i think the ecu may have been harmed when i went from hoodlight power to the black/white wire. or maybe it has been all along and i just made it worse.
between the black/white wire and pin 46, it reads less than 1.5 ohms.
and between the pin 48 and the red white wire is less than 1.5 ohms as well.
so i think the ecu may have been harmed when i went from hoodlight power to the black/white wire. or maybe it has been all along and i just made it worse.
You are correct, the ECU has been damaged.
Depending on your level of electical skill, it might be possible to repair it. See http://forums.stangnet.com/749974-computer-issue.html for more help on fixing the computer innards.
With the age of these cars and the number of times they have changed hands, anything is possible.
You would need to disconnect the computer and check each wire for voltage when the ignition is in the Run position. Take note of any pin that has voltage on it and compare it to the computer wiring diagram and computer wiring connector diagram I posted. It would be a minimum of 2-4 hours worth of work.
just wanted to let you know i got an used c3w1 ecu (from some guy i will never do business with again and never went anybody to deal with) and my check engine light works now. car seems a lil sluggish but im guessing it could take a few days to get acustom to my changes. thank you for all your help jrichker
