Register now to get rid of these ads!

Technical Car won't shut off with battery cut off switch

Discussion in 'Traditional Hot Rods' started by 55onefifty, Jun 15, 2020.

  1. Mr48chev
    Joined: Dec 28, 2007
    Posts: 26,954


    I believe they have the answer you need.
    You not only have to cut the power to the switch ( by turning the shut off switch) you have to cut off the power from the alternator because with the ignition switch still on the alternator is still in on mode with the exciter wire hooked up or as a one wire.

    Running the "batt" wire from the alternator to the battery side of the cut off switch should stop the feedback to the system. That is a few bucks worth of 8 gauge wire and two terminals. That way it would not supply power to the electrical system when the switch is thrown but everything works when the switch is in the on position.
    I was thinking relay in the exciter wire with the power wire connected to the battery cable but feedback from the alternator might keep the relay triggered and not be effective.
    As mentioned previously if you have an electric fan you may also have to eliminate feedback from it. Some turn into a generator when the key is turned off and they are still spinning.
    anthony myrick likes this.
  2. Mimilan
    Joined: Jun 13, 2019
    Posts: 569


    I'll add to my ^^^ reply.
    You can also do this with a Ford Starter solenoid at the battery [and bridge the solenoid on the starter motor]
    You'll need to run a wire from the starter button/switch back to the solenoid.

    The Master Key /Isolator switch is in series with the Ign switch [but at the rear]

    This is how my old touring car was wired
    The main starter cable is as "dead as a dodo" unless the engine is being started
    [It is the safest method that is used in road racing cars]

    here is a simple scribble of the basic wiring [the arrow goes to engine /accessories etc]
    Blues4U and rod1 like this.
  3. Blues4U
    Joined: Oct 1, 2015
    Posts: 4,681

    from So Cal

    Nicely done. The isolator switch does not need to be nearly as heavy duty either, it only has to carry a much smaller load, the Ford solenoid is carrying the heavy starting load. I would mount the solenoid under the hood, keeping the run from the solenoid to the starter short, as Ford did.

    My question is, where to mount the isolator switch? What does NHRA require?
  4. greybeard360
    Joined: Feb 28, 2008
    Posts: 1,469


    If you are using an MSD box this is a common problem with them. I had a 7 AL box in my race car and it wouldn't shut off with the key or the power switch in the back. You have to put some resistance in the small power wire going to the box to bleed off residual voltage coming from the alternator.

  5. RmK57
    Joined: Dec 31, 2008
    Posts: 1,464


    Rear of the car. The switch itself can be mounted underneath or inside the trunk but has to accessed from the outside the vehicle usually with a push / pull lever. It also has to be clearly marked ON/ OFF or PUSH OFF / PULL ON.
  6. Mimilan
    Joined: Jun 13, 2019
    Posts: 569


    What I showed in the schematic is up to FIA standards [ it must kill a running engine , and also everything else as well]
    The FIA require the cut-off to be at the base of the windshield [usually on a fender] and requires this decal to I.D it.

    Having the cut-off at the windshield is a pain with trunk mounted batteries [hence the fusible link]

    This system would be up to NHRA requirements, but switch placement and decals would be to their regulations.

    I've personally seen the consequences of having a regular cut-off [Non-Ford solenoid] system.
    20 years ago the series 1 to 3 Maxda RX7's were cheap and popular with club racers.
    A lot of them would lighten the flywheel and clutch pressure ring in a lathe.
    They would grenade themselves right through the alloy bellhousing.
    Usually resulting in cutting the fuel line and H/D battery cables, exhaust etc . And a dead short would cause a fire [and the electric pump would feed it]
    This ^^^^ is why we prefer the solenoid close to the battery.

    A Ford type solenoid would prevent this type of short circuit.
    If it cut through the main power feed to the cut-off switch , the fusible link would blow.

    Edit: This solenoid cut-off would be dead easy to do with the Chevy starter "heat soak" problem that is remedied with a Ford Solenoid.
    Last edited: Jun 16, 2020
  7. Blues4U
    Joined: Oct 1, 2015
    Posts: 4,681

    from So Cal

    So it can be inside the trunk? That helps with aesthetics.
  8. I forgot we are dealing with a race car here, thanks.

    Sent from my iPad using H.A.M.B.

Share This Page

Register now to get rid of these ads!


Copyright © 1995-2020 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.