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Thread: cut-off switch

  1. #1
    Jedi Hopeful Drone Dog's Avatar
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    cut-off switch

    why the third wire to the cut-off switch in the trunk? (white/black) is this because of some charge left in the coil?

    Thanks

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    Jedi Hopeful Drone Dog's Avatar
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    Re: cut-off switch

    i found the answer in a search. sorry could not find it the first search.

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    Great Pumpkin Keoke's Avatar
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    Re: cut-off switch

    Quote Originally Posted by Drone Dog View Post
    why the third wire to the cut-off switch in the trunk? (white/black) is this because of some charge left in the coil?

    Thanks
    NO it grounds the coil so the engine can not be started until the switch is set back to off----Safety device OK
    1966 Daimler V8 Saloon; Safely Fast, Built to Last & and; Smooth as Glass.
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    Jedi Warrior andrea's Avatar
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    Re: cut-off switch

    on my car I have rebuild the CUT off switch BUT at the first road test having some strange power (cut off cut on) during the driving I have prefer to skip it, connecting directly the battery to ground
    and disconnecting and insulating the black white wire - the power problems disappeared , confirming the bad reputation of this switch, In my opinion affordability and safety preval, I Think if undesired power cut off occur on a highway - skip, aid also when mysterious issue occur, easing and reducing the reasons, examination -- better a CUT OFF Directly from the battery with a more affordable and secure switch
    https://photos.app.goo.gl/w3uIwfPGLI6mf6hD3
    https://photos.app.goo.gl/BHPueEThqeUkBxi13

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    Jedi Hopeful Drone Dog's Avatar
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    Re: cut-off switch

    i have read in a few places about their reliability. i am surprised someone has not made a look-a-like replica that is more dependable.

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    Darth Vader John Turney's Avatar
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    Re: cut-off switch

    Quote Originally Posted by Drone Dog View Post
    i have read in a few places about their reliability. i am surprised someone has not made a look-a-like replica that is more dependable.
    I think most of the switches that are unreliable are at least 50 years old. I replaced my old one ~20 years ago with a genuine Lucas switch and it has been fine.
    John, BN4

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    Yoda
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    Re: cut-off switch

    They can be 'restored.' They're just a pair of spring-loaded rotating/sliding copper plates inside. I think the copper plates just get corroded over the years; a little cleaning and some dielectric grease should do the trick.

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    Jedi Hopeful Drone Dog's Avatar
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    Re: cut-off switch

    thanks

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    Great Pumpkin Keoke's Avatar
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    Re: cut-off switch

    Quote Originally Posted by Bob_Spidell View Post
    They can be 'restored.' They're just a pair of spring-loaded rotating/sliding copper plates inside. I think the copper plates just get corroded over the years; a little cleaning and some dielectric grease should do the trick.
    Yep John I agree, and this can make them almost like new.
    1966 Daimler V8 Saloon; Safely Fast, Built to Last & and; Smooth as Glass.
    1966 BJ8 [ 2 ] Lil Red & Miss bLU
    1985 XJ6 Saloon
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    Yoda
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    Re: cut-off switch

    The key thing to remember if you 'go in' is that the battery ground circuit and the coil ground circuit are connected oppositely. When the switch is in the 'ON' position the battery circuit to ground is closed, but the circuit that grounds the coil is open; in the 'OFF' position the opposite is true (the coil is grounded so that it cannot produce a spark even if the ignition switch is hot-wired--ridiculously easy to do--and someone tries to push start the car).

    Note if you use a Pertronix and want to retain the 'anti-theft' capability, as I do, the 'theory' is different: The white/black wire from the coil needs to be connected to the same terminal as the battery ground circuit. This makes a convenient ground for the hot wire->Pertronix->coil->GND circuit, so there's no additional wiring required.

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    Jedi Hopeful Drone Dog's Avatar
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    Re: cut-off switch

    I cleaned the switch today. Good thing. Took a few minutes to figure out how it goes back together as it popped apart. But I got it. Since I do not have the engine in yet, I left the white/black wire off. I will probably go with the pertronix when I get there. I like it on my TR6.
    Also want to change to negative ground and an alternator. But that is probably closer to winter for that.

    finished up the back end wiring. Man those tail lights are a pain to get in. Glad I checked the bulbs and the connections first. Maybe there is some trick I don't know....

    really want want to thank everyone for the help. Can't imagine trying doing this without the help and pics/info I have gotten from this site.

    now back to the main harness. See other post.

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    Jedi Knight RAC68's Avatar
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    Re: cut-off switch

    Hi All,

    About 20 years ago, I took my round switch apart for cleaning and installed a liberal amount of Dielectric Grease in all internal areas for lubrication and anti-flash. Although this original switch had some contact burning, after this process the switch has performed flawlessly. Based upon my success, I would recommend everyone still using an original switch dismantle and check its deterioration after its years of use. As I have had such good success from the application of dielectric grease, I would suggest others also consider this application.

    Just some thoughts,
    Ray(64BJ8P1)

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    Darth Vader Michael Oritt's Avatar
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    Re: cut-off switch

    Certainly the original switches can be repaired but they will still be susceptible to the same possibility/likelihood of failure, specifically due to internal corrosion on the copper parts which are naturally prone to go green. High quality shut-off switches--particularly those for marine applications--have anti-corrosion plated conductors, just as the best quality wiring is tinned.

    If one likes the look of originality--but does not want it to extend to being broken down alongside the road--keep the original POS switch installed with no current-carrying wires and have another functional one concealed either in the boot or behind the bulkhead, and in any case disconnect the b/w wire that grounds the coil as it can only lead to trouble.

    If running an alternator consider getting a switch such as those sold at Pegasus, etc. that provides a circuit to ground/protect the alternator when the power is cut off.

    Best--Michael Oritt
    1954 Austin-Healey 100 Lemans
    1958 Elva Courier
    1959 Elva MK IV Sports Racer
    1961 Ginetta G4

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    Darth Vader John Turney's Avatar
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    Re: cut-off switch

    Quote Originally Posted by Drone Dog View Post
    I cleaned the switch today. Good thing. Took a few minutes to figure out how it goes back together as it popped apart. But I got it. Since I do not have the engine in yet, I left the white/black wire off. ...
    Be sure you tape up the end of that white/black wire so it won't touch ground/earth. Otherwise you will suddenly stop.
    John, BN4

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    Re: cut-off switch

    will do, i actually have put it in the terminal and noted it in my things to do. i just did not strip the wire.

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    Re: cut-off switch

    Just noticed Michael's comment :-
    "If running an alternator consider getting a switch such as those sold at Pegasus, etc. that provides a circuit to ground/protect the alternator when the power is cut off."
    Could you please explain in a bit more detail. I have an alternator fitted to my car. I was a bit concerned about the cut-off switch and decided not to use it. It's permanently left in the ON position. I fitted a battery terminal isolating switch, the type with a knob on the top that opens the ground connection when you unscrew it. I wasn't aware that I might need another switch for the alternator.

    AJ

  17. #17
    Darth Vader Michael Oritt's Avatar
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    Re: cut-off switch

    AJ--

    Go here:

    https://www.pegasusautoracing.com/pr...caAgIJEALw_wcB

    You'll see that there is a separate circuit within the switch (not a separate switch) that grounds the alternator and protects it in the event the switch is activated while the engine is running.

    This is a safety requirement and while such an event is possible in a race car (a shutoff switch that can be easily accessed both by the driver and someone standing alongside the car is required by all the sanctioning I know of) it is quite unlikely that such a situation will occur in a street car, esp. when the switch is located in the boot.
    Nevertheless it costs little to do things the right way by getting such a switch and if for whatever reason you must ever kill the engine with the shutoff switch the alternator is protected.

    Note also the seller's comment about the internal contacts being plated, versus being bare copper. this is what I was referring to in an earlier post as to why I think the Lucas switch needs to be thrown away versus rebuilt. It will always be an inferior piece of equipment likely to fail as is borne out by the frequency of our having to talk about it.

    Best--Michael Oritt
    1954 Austin-Healey 100 Lemans
    1958 Elva Courier
    1959 Elva MK IV Sports Racer
    1961 Ginetta G4

  18. #18
    Yoda
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    Re: cut-off switch

    Quote Originally Posted by Michael Oritt View Post
    ... I think the Lucas switch needs to be thrown away versus rebuilt. It will always be an inferior piece of equipment likely to fail as is borne out by the frequency of our having to talk about it.
    Please don't tell my Lucas switch as it's been fine for almost 200K miles and I don't want it getting any ideas.

    Just had a thought: Could some of the problems be caused by non-use? I hardly ever use it, but on our long road trips my navigator always turns it off at the end of the driving day; maybe regular use prevents a corrosion or gunk buildup?

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    Re: cut-off switch

    Quote Originally Posted by Michael Oritt View Post
    Certainly the original switches can be repaired but they will still be susceptible to the same possibility/likelihood of failure, specifically due to internal corrosion on the copper parts which are naturally prone to go green. High quality shut-off switches--particularly those for marine applications--have anti-corrosion plated conductors, just as the best quality wiring is tinned.

    If one likes the look of originality--but does not want it to extend to being broken down alongside the road--keep the original POS switch installed with no current-carrying wires and have another functional one concealed either in the boot or behind the bulkhead, and in any case disconnect the b/w wire that grounds the coil as it can only lead to trouble.

    If running an alternator consider getting a switch such as those sold at Pegasus, etc. that provides a circuit to ground/protect the alternator when the power is cut off.
    This one won't look original but should be fairly bullet proof: https://www.pegasusautoracing.com/pr...asp?RecID=1464

    Cheers,
    Dan M.

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    Re: cut-off switch

    Quote Originally Posted by DTDuck View Post
    This one won't look original but should be fairly bullet proof: https://www.pegasusautoracing.com/pr...asp?RecID=1464

    Cheers,
    Dan M.
    Moss Motors now makes a good "original" replacement if your original cannot be rebuilt.

    Excerpt from the moss Description: "We have our switches made by a company that is owned by an electrical engineer, and we have tested these switches with a 200 amp load for 5 minutes. The switch remained cool to the touch."

    https://mossmotors.com/media/instructions/145-771.pdf

    https://mossmotors.com/switch-battery-master?assoc=5371

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