Battery Cut-off Switch In Trunk

Discussion in 'Fox 5.0 Mustang Tech' started by FoxMustangLvr, Feb 19, 2014.

  1. I recently acquired an 88' Notchback 5.0. It has the battery relocated to the trunk with a NHRA approved battery tray and a cut-off switch that exits out between the license plate and tail light on the passenger side. I don't have a pic yet.

    What type of racing requires this setup?
    Is this required for anyone w/ a trunk mounted battery at the drag strip?

    Please enlighten me, I've never "legally" drag raced. Thanks
  2. Any rear mounted battery needs a disconnect/cut off switch to kill the car in the event of an accident as per NHRA rules.
  3. Thanks Nick, that's the answer I was hoping for. I guess the PO had plans to drag race it at the track. There's only one place in San Diego that has a dedicated 1/8 mile strip.
  4. You have to make sure to wire in a high amp cut off solenoid so the car will actually shut off when you push the kill switch in. Without the high amp cut off solenoid, the car will stay running because the alternator is putting out voltage to the vehicle. Think of it this way, start the car and then unhook the battery cables......what happens? The car stays running because of the alternator generating voltage. The high amp cut off solenoid requires the alternator power wire feed thru it so that when the battery kill switch is pushed, all power is terminated to the vehicle, alternator included.
  5. I'll double check the wires before I fire it up and test it out. That's a long ways away though, no engine or trans in the car right now, need to swap k-member first and install flaming river manual rack. So many things need to be done I don't know where to start sometimes.

    Thanks for the info guys
  6. For a battery cut off switch, see
    is the switch 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 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 Everyone should bookmark this site.

    Ignition switch wiring

    Fuel, alternator, A/C and ignition wiring

    Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

    Vacuum diagram 89-93 Mustangs

    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..

    Picture courtesy timewarped1972 [​IMG]

    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.
  7. I have mine wired with a 0 gauge ground to the body, another 0 gauge ground to the engine(this was instructed in the holley EFI manual), then i have a 0 gauge wire from the + terminal to the solenoid. To shut the alternator off i ran a 2 guage wire from the cutoff switch to the alternator post. All of the hot wires are fused and this shuts the car off right away and passes NHRA tech.

    A friend has his wire to kill the fuel pump instead of the alternator. This works very well also. I do like the idea of running the small gauge wire from the alternator plug to the switch too.
  8. The idea behind the alternator shutdown saves $$ and reduces the amount of heavy gauge wire you need to run. The alternator uses a small current (green wire) to turn the regulator on and make it output voltage. Turn off the power on the (green wire) to the regulator and the alternator shuts down. Very easy to run the 18 gauge wire and it is very reliable.