The odd looking connector is the EFI power ground.
Grounds
Grounds are important to any electrical system, and especially to computer controlled engines. In an automobile, the ground is the return path for power to get back to the alternator and battery.
1.) The main power ground is from engine block to battery: it is the power ground for the starter & alternator.
2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges.
Any car that has a 3G or high output current alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects. The 3G has a 130 amp capacity, so you wire the power side with 4 gauge wire. It stands to reason that the ground side handles just a much current, so it needs to be 4 gauge too.
The picture shows the common ground point for the battery & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.
The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
Picture courtesy timewarped1972
Correct negative battery ground cable.
3.) The computer has its own dedicated power ground that comes off the ground pigtail
on the battery ground wire. Due to it's proximity to the battery, it may become
corroded by acid fumes from the battery.
In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white wire.
You'll find it up next to the starter solenoid where the wire goes into the wiring harness.
4.) All the sensors have a common separate ground. This includes the TPS, ACT, EGE, BAP, & VSS sensors.
5.) The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is an orange wire with a ring terminal on it. It is located in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or bolt on back of the cylinder head.
6.) The TFI module has 2 grounds: one for the foil shield around the wires and another for the module itself. The TFI module ground terminates inside the computer.
7.) The computer takes the shield ground for the TFI module and runs it from pin 20 to the chassis near the computer.
8.) The computer's main power ground (the one that comes from the battery ground wire) uses pins 40 & 60 for all the things it controls internally.
See Automotive Test Tools for help for help troubleshooting voltage drops across connections and components. .
Extra grounds are like the reserve parachute for a sky diver. If the main one fails, there is always your reserve.
The best plan is to have all the grounds meet at one central spot and connect together there. That eliminates any voltage drops from grounds connected at different places. A voltage drop between the computer ground and the alternator power ground will effectively reduce the voltage available to the computer by the amount of the drop.
Starter solenoid wiring:
Regular starter on 86-91 Mustangs:
High torque mini starter or 92-93 Mustangs:
Grounds
Grounds are important to any electrical system, and especially to computer controlled engines. In an automobile, the ground is the return path for power to get back to the alternator and battery.
1.) The main power ground is from engine block to battery: it is the power ground for the starter & alternator.
2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges.
Any car that has a 3G or high output current alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects. The 3G has a 130 amp capacity, so you wire the power side with 4 gauge wire. It stands to reason that the ground side handles just a much current, so it needs to be 4 gauge too.
The picture shows the common ground point for the battery & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.
The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
Picture courtesy timewarped1972
Correct negative battery ground cable.
3.) The computer has its own dedicated power ground that comes off the ground pigtail
on the battery ground wire. Due to it's proximity to the battery, it may become
corroded by acid fumes from the battery.
In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white wire.
You'll find it up next to the starter solenoid where the wire goes into the wiring harness.
4.) All the sensors have a common separate ground. This includes the TPS, ACT, EGE, BAP, & VSS sensors.
5.) The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is an orange wire with a ring terminal on it. It is located in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or bolt on back of the cylinder head.
6.) The TFI module has 2 grounds: one for the foil shield around the wires and another for the module itself. The TFI module ground terminates inside the computer.
7.) The computer takes the shield ground for the TFI module and runs it from pin 20 to the chassis near the computer.
8.) The computer's main power ground (the one that comes from the battery ground wire) uses pins 40 & 60 for all the things it controls internally.
See Automotive Test Tools for help for help troubleshooting voltage drops across connections and components. .
Extra grounds are like the reserve parachute for a sky diver. If the main one fails, there is always your reserve.
The best plan is to have all the grounds meet at one central spot and connect together there. That eliminates any voltage drops from grounds connected at different places. A voltage drop between the computer ground and the alternator power ground will effectively reduce the voltage available to the computer by the amount of the drop.
Starter solenoid wiring:
Regular starter on 86-91 Mustangs:
High torque mini starter or 92-93 Mustangs:
The black connector is the computer power ground. It is the third most important ground on a EFI car. The OEM battery cable (see drawing in my previous post) plugs into the connector to provide a ground for the computer. If your battery cable does not have the connector, ground the black/white pigtail to the gound point by the windshield washer filler neck (see the picture in my previous post). Make sure that the extra ground wire on the battery cable connects to the same place.
Just cut the connector off, crimp a terminal on it and ground it to the ground point by the windshield washer filler neck.
Since you are doing a rear mounted battery, here's some rear mounted battery recommendations.
Rear mounted battery ground wiring. Follow this plan and you will have zero
ground problems.
One 1 gauge or 1/0 gauge wire from battery negative post to a clean shiny spot
on the chassis near the battery. Use a 5/16” bolt and bolt it down to make the
rear ground. Use a 1 gauge or 1/0 gauge wire from the rear ground bolt to a clean
shiny spot on the block.
One 4 gauge wire from the block where you connected the battery ground wire to
the chassis ground where the battery was mounted up front. Use a 5/16” bolt
and bolt down the 4 gauge engine to chassis ground, make sure that it the metal
around the bolt is clean & shiny. This is the alternator power ground.
The computer has a dedicated power ground wire with a cylindrical quick connect
(about 2 ½”long by 1” diameter. It comes out of the wiring harness near the
ignition coil & starter solenoid (or relay). Be sure to bolt it to the chassis ground
in the same place as you bolted the alternator power ground. This is an
absolute don’t overlook it item for EFI cars
Note: The quick disconnect may have fallen victim to damage or removal by
a previous owner. However, it is still of utmost importance that the black/green
wires have a high quality ground..
The screwdriver points to the common body ground for the alternator, battery and computer.
Picture courtesy timewarped1972
Crimp or even better, solder the lugs on the all the wire. The local auto stereo
shop will have them if the auto parts store doesn't. Use some heat shrink tubing
to cover the lugs and make things look nice.
For a battery cut off switch, see Moroso : Category Display
is the switch http://www.moroso.com/catalog/images/74102_inst.pdf is the installation instructions.
Use the super duty switch and the following tech note to wire it and you will
be good to go.
Use the Moroso plan for the alternator wiring and you risk a fire. The 10
gauge wire they recommend is even less adequate that the stock Mustang
wiring.
There is a solution, but it will require about 40' of 18 gauge green wire.
Wire the battery to the two 1/2" posts as shown in the diagram.
The alternator requires a different approach. On the small alternator plug
there is a green wire. It is the sense lead that turns the regulator on when
the ignition switch is in the run position. Cut the green wire and solder the
40' of green wire between the two pieces. Use some heat shrink to cover the
splices. See Ford Fuel Injection How To Solder Like a Pro for some excellent
help on soldering & using heat shrink tubing.
Run the green wire back to the Moroso switch and cut off the excess wire.
Try to run the green wire inside the car and protect it from getting cut or
chaffed. Crimp a 18 gauge ring terminal (red is 18 gauge color code for the crimp
on terminals) on each wire. Bolt one ring terminal to each of the 3/16" studs.
Do not add the jumper between the 1/2" stud and the 3/16" stud as shown it the
Moroso diagram.
How it works:
The green wire is the ignition on sense feed to the regulator. It supplies a turn
on signal to the regulator when the ignition switch is in the Run position. Turn the
Moroso switch to off, and the sense voltage goes away, the voltage
regulator shuts off and the alternator quits making power.
The fuse & wiring in the following diagram are for a 3G alternator. The stock alternator uses a dark green fuse link wire that connects to 2 black/orange wires. Always leave them connected to the starter solenoid even if you have a 3G alternator.
See the following website for some help from Tmoss (diagram designer)
& Stang&2Birds (website host) for help on 88-95 wiring
Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.
Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
Since you are doing a rear mounted battery, here's some rear mounted battery recommendations.
Rear mounted battery ground wiring. Follow this plan and you will have zero
ground problems.
One 1 gauge or 1/0 gauge wire from battery negative post to a clean shiny spot
on the chassis near the battery. Use a 5/16” bolt and bolt it down to make the
rear ground. Use a 1 gauge or 1/0 gauge wire from the rear ground bolt to a clean
shiny spot on the block.
One 4 gauge wire from the block where you connected the battery ground wire to
the chassis ground where the battery was mounted up front. Use a 5/16” bolt
and bolt down the 4 gauge engine to chassis ground, make sure that it the metal
around the bolt is clean & shiny. This is the alternator power ground.
The computer has a dedicated power ground wire with a cylindrical quick connect
(about 2 ½”long by 1” diameter. It comes out of the wiring harness near the
ignition coil & starter solenoid (or relay). Be sure to bolt it to the chassis ground
in the same place as you bolted the alternator power ground. This is an
absolute don’t overlook it item for EFI cars
Note: The quick disconnect may have fallen victim to damage or removal by
a previous owner. However, it is still of utmost importance that the black/green
wires have a high quality ground..
The screwdriver points to the common body ground for the alternator, battery and computer.
Picture courtesy timewarped1972
Crimp or even better, solder the lugs on the all the wire. The local auto stereo
shop will have them if the auto parts store doesn't. Use some heat shrink tubing
to cover the lugs and make things look nice.
For a battery cut off switch, see Moroso : Category Display
is the switch http://www.moroso.com/catalog/images/74102_inst.pdf is the installation instructions.
Use the super duty switch and the following tech note to wire it and you will
be good to go.
Use the Moroso plan for the alternator wiring and you risk a fire. The 10
gauge wire they recommend is even less adequate that the stock Mustang
wiring.
There is a solution, but it will require about 40' of 18 gauge green wire.
Wire the battery to the two 1/2" posts as shown in the diagram.
The alternator requires a different approach. On the small alternator plug
there is a green wire. It is the sense lead that turns the regulator on when
the ignition switch is in the run position. Cut the green wire and solder the
40' of green wire between the two pieces. Use some heat shrink to cover the
splices. See Ford Fuel Injection How To Solder Like a Pro for some excellent
help on soldering & using heat shrink tubing.
Run the green wire back to the Moroso switch and cut off the excess wire.
Try to run the green wire inside the car and protect it from getting cut or
chaffed. Crimp a 18 gauge ring terminal (red is 18 gauge color code for the crimp
on terminals) on each wire. Bolt one ring terminal to each of the 3/16" studs.
Do not add the jumper between the 1/2" stud and the 3/16" stud as shown it the
Moroso diagram.
How it works:
The green wire is the ignition on sense feed to the regulator. It supplies a turn
on signal to the regulator when the ignition switch is in the Run position. Turn the
Moroso switch to off, and the sense voltage goes away, the voltage
regulator shuts off and the alternator quits making power.
The fuse & wiring in the following diagram are for a 3G alternator. The stock alternator uses a dark green fuse link wire that connects to 2 black/orange wires. Always leave them connected to the starter solenoid even if you have a 3G alternator.
See the following website for some help from Tmoss (diagram designer)
& Stang&2Birds (website host) for help on 88-95 wiring
Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.
Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
The ground is the main ground for the computer. If it is disconnected, you would be lucky to get any computer functions to work.
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. It is best if an experienced electronics technician opens the computer up & repairs 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 - Wiring & Engine Info
See Computer issue? - Mustang Forums at StangNet for more help on fixing the computer innards.
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. It is best if an experienced electronics technician opens the computer up & repairs 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 - Wiring & Engine Info
See Computer issue? - Mustang Forums at StangNet for more help on fixing the computer innards.