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Master switch wiring

Practical Motorsport Wiring - Club Level

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I'm wiring a drift car from scratch, I connected the battery postive to the starter motor and there's a power cable connecting the starter motor to the alternator and another power cable from the starter going back inside the car to the master switch and everything takes main power from the master switch, first I started the car up with alternator not charging the master switch was working probably when I connected the alternator lamp and the alternator started charging the master switch is not getting the car switched off, I couldn't figure what's wrong

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Once the alternator is producing power, disconnecting the battery (which your master switch does) is not sufficient to stop the engine. You either need to use a master switch that disconnects the ignition signal (run/stop) so the ECU will stop the engine. These switches will have another terminal that closes, you connect that to a high-wattage resistor to give the alternator a load while the coil and injectors are no longer drawing any power.

If your ECU doesn't have a Run/Stop / Ignition signal, then you can just use the switch to disconnect the power to the Coils and/or Injectors which will have the same effect -- the engine (and thus alternator) will stop producing power. You still need the load dump resistor.

Here is an example of the wiring instructions for such a switch:


I looked at your wiring diagram. I bet the alternator light is attached to the other side of the master switch and that is what keeps the power on for the car. I bet it will die if you disconnect that. I still recommend you follow the instructions given above and use a proper alternator capable master switch.

No the alternator light is powered from an AUX relay and all the fuse box is powered from the master switch, i know the 6 pin master switch is better than what I'm using and I'll add a switch to cut the ECU relay trigger 12V and all relay are triggered by ECU 12V and this will shut the car off but i can't find an explanation for what's going on

Please describe how the aux relay is wired. Are you sure it's not on the switched side of the master switch?

I use the ECU power to trigger all the relays in the haltech fuse box to ensure that nothing will have power untill the ECU has power first and the ECU relay is triggered from the master switch I know if i added a normal switch to this wire it'll turn the car off

There are some detail differences with different master/isolator switches, just as there are with alternators and ECU's wiring - exactly which switch,d alternator and ECU are you using?

As has been suggested, it seems you have it wired in such a way the alternator is still powering the ECU.

The alternator warning light is extinguished by the running alternator producing ~12V to give a net ~0V across it (sometimes it isn't quite that and the light can be seen glowing faintly at night if the alt' is higher. It's possible that 'back feed' is sufficient to keep the ECU 'on' and the engine running.

If so, try removing the warning light's bulb/globe to kill that and see if the engine kill works correctly, if so, a simple diode can be placed in that circuit to block the reverse current.

Oh, with the kill switch in the "off" position, if the engine can't be started, that would further point to the warning light wiring being the problem.

As an aside, what are the specific regulations for the kill switch? Some motorsport bodies require only the engine to be stopped, others anything engine related - such as fuel pumps (should be ECU controlled, anyway) and fans, and others all the electrical systems.

The regulations for the kill switch it should get the engine off only and yes if the switch is on off position the engine can't run and the ECU is off the problem is with alternator charging so which diode I should install and how it should be installed

IF (!) that's the problem, fit it so the diode passes current from the bulb to the alternator, but blocks it from the alternator back to the bulb.

If you look at a diode, you will see a line, or band, around one end, like this -


The band/stripe end is wired towards the alternator.

I would expect a half amp, or higher, with 20, or higher, rating should work well - anyone in your local electronics' shop should be able to show you your options if you tell them what you want to use it for.

It's been a while, so if anyone wishes to comment/correct this...?