Factory Grounds

Discussion in 'Fox 5.0 Mustang Tech' started by buff531, May 31, 2013.

  1. Hey guys,
    was wondering if someone can tell me where all the factory grounds are. Where is the ECM ground, battery negative cable go, lil wire also connected to negative battery cable location... is there a fuel rail ground and where? is there a ground that goes to firewall and where... and is there a chassis ground and where?

    what size cable do you use for the above if you are missing one of the wires. I remember a falt braided ground to firewall when pulling the motor that i never reinstalled. Having issues getting car to start. Have spark, Have fuel... just dont think the injectors are firing correctly, if at all...

    would bad grounds cause issues with the injectors?
  2. kids are going to nap soon, i go out and look for you
  3. oh man that would be a huge help. thanks.
  4. ok. three that i see without getting in real deep. the negative battery cable ground that you know about. there is a braided wire ground that runs from the firewall behind the drivers head to the block(?) or some bolt back there. then there is another that i believe is a ground. thick black coated wire running from somewhere near the battery cables to a bolt on the timing cover/water pump. oh, four actually. the computer has its own ground right next to it in the space it resides in. hope it helps.
  5. yes. thanks. can you clarify where the computer ground is?
  6. passenger side kick panel, remove it and it is below the computer if i remember correctly.
  7. oh ok i thought you meant somewhere in the engine bay.
  8. nope, but i had to check that one too so i added it. i am sure i am missing some somewhere. probably the one thats causing you trouble.:)
  9. I am sure jrichker has this diagram/writeup somewhere.
  10. Grounds

    Revised 28-Oct-2012 to add signal ground description & possible problems if it is bad

    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 as much current, so it needs to be 4 gauge too.

    The picture shows the common ground point for the battery , computer, & 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's main power ground (the one that comes from the battery ground wire) uses pins 40 & 60 for all the things it controls internally: it comes off the ground pigtail on the battery ground wire. Due to its 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 signal ground. This includes the TPS, ACT, EGR, BAP, & VSS sensors. This ground is inside the computer and connects pin 46 to pins 40 & 60, which are the main computer grounds. If this internal computer ground gets damaged, you won't be able to dump codes and the car will have idle/stall/ performance problems

    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.

    See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. Be sure to have the maximum load on a circuit when testing voltage drops across connections. As current across a defective or weak connection, increases so does the voltage drop. A circuit or connection may check out good with no load or minimal load, but show up bad under maximum load conditions. .


    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.