Help me fix and make $50!
- Thread starter 5.0fox
- Start date
LordMustangGT
Member
See first post tfi module is brand new. Coil replaced and no change. Also there hasnt been any changes to the intake tract in well over 2 years.
If it were the ignition switch, it wouldn't crank. Same thing with the starter wire. I would check all of your ground wires, though.
Do the spark plugs have any gas on them after you try to start it?
Ignition switch and tumbler are ordered because they are a little screwed up anyway. plugs do get wet while cranking. Ill check all grounds including injector ground in the morning.
the ignition switch at the key can go bad, and it'll just crank and won't start. It's happened on just about every fox I've had. the switch seperates and begins to fall apart. It looses some of it's contacts, and won't turn the fuel pump on, but will engage the starter.
Don't waste your time looking for an injector ground in the wiring harness because there isn't one.
The injectors have 12 volts on them anytime the ignition switch is in the run postion. The computer
provides a switched ground to trigger the injector.
The orange wire that bolts to ground in the injector harness is the O2 sensor ground.
Remember that the computer does not source power for any actuator or relay, but provides the ground necessary to complete the circuit.
That means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit.
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/
Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
Fuel pump, alternator, ignition & A/C wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
Computer, actuator & sensor wiring
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Fuse panel layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif
Vacuum routing
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
hmm well all grounds are intact and firmly secured to bare metal so i dont think thats the problem. I cleaned the maf sensor and pulled the tps out to look at it while i was under there. The pegs that stick out to catch on the throttlebody shaft are all chewed up and nearly gone so Ill pick another one up today.
tps replaced and no change. Also switched back to my good working stock injectors with no change. Spark checked during the day and looks to be a little weak. msd box bypassed and coil replaced with good working unit. PLugs are ok and wires and cap new! is there any other part of the ignition that could cause weak spark like this?
5pointGO
New Member
no it wont show anything! but I dont think that srelated to the problem cause it hasnt ever displayed codes in the 4 years ive owned it. Even before the mass air conversion or any engine mods nothing would happen. Im getting ready to buy a commuter so putting a carb on this thing wouldnt matter much.
Computer will not go into diagnostic mode 86-90 model 5.0 Mustangs
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
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
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 http://www.veryuseful.com/mustang/tech/engine
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
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
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 http://www.veryuseful.com/mustang/tech/engine
montegoman351
Member
- Jan 12, 2007
- 169
- 1
- 18
Have you tried switching computers? Could the computer be getting too hot? I've seen this before on an older Taurus ('91), it had the same symptoms.
Since you have a weak spark, wet plugs means you have fuel, you obviously have air since it works any time when it is cold.
You have already swapped the Hall Effect, and Ignition module. Also, checked a bunch of other stuff, why not swap the computer? Just take the connector loose and connect it to another computer from a friend, junk yard, whatever, lay it in the passenger's floorboard, and see if the problem goes away.
Since you have a weak spark, wet plugs means you have fuel, you obviously have air since it works any time when it is cold.
You have already swapped the Hall Effect, and Ignition module. Also, checked a bunch of other stuff, why not swap the computer? Just take the connector loose and connect it to another computer from a friend, junk yard, whatever, lay it in the passenger's floorboard, and see if the problem goes away.
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