installing a master cut off switch with a 3.6 transplant

[aston@ultrasw.c]

I installed a master cut-off switch and gave it a test.

The engine keeps on running when the switch turned off.

I know it can happen because of the alternator etc...

What is the best and convenient place to splice into the ignition low voltage?

The coils have 3 wires green, green/red and black.

Can I take the black (common) to the master switch and back

Or do I take both green and green/red?

Or can I pick up a power feed to the ignition control unit?

Or power to the DME. I have read that switching #86 on the DME relay is the way to go. Can anyone verify this.

BTW this is a 3.6 non v stuffed into an 86

TIA

[donporfi]

What type of cut off switch are you using?
Single pole or three pole ?

By single pole I mean something like this


By three pole something like this, which has two extra poles one for the alternator and one for the ignition.

[aston@ultrasw.c]

3 pole

battery and alternator resistor installed

[Walt Fricke]

First off, study the circuit diagram (very simple) which came with your switch. With the big battery switch on, one of the little switches is also on, and one off.

The "off" one is the one you use connect the resistor from big positive away from battery to ground when you turn the master off. This is said to cushion the alternator going off, since it can't in a sense drain into the battery with the master disconnecting the battery - something like that.

Don't mess with the ignition.

The key is the switch which is on in when the master is also on, and off when it is off.

Run the DME relay black/white wire which connects pin 20 of the DME itself to the 85b terminal of the DME relay through this little switch. That way when you turn the master off, the fuel pump relay is de-energized, it opens, no current can flow to the fuel pump, and the fuel pump stops. You engine will promptly stop when the fuel pump stops.

Relay coils are low current devices, so they are what you want to turn off with the small switch. The fuel pump fuse is 25 amps, which gives you an idea of its current draw. Relays are made to handle that. The little switch under the master cut-off is not.

[aston@ultrasw.c]

thanks Walt that worked great!

[ischmitz]

Most modern cars interrupt the +12V to the coils as that cuts spark and instantly kills the engine.

I found that for good measure (safety) the coils are powered directly through the ignition switch while the rest of things that are on with the key in RUN (aka: Term. 15) is done by a relay.

The reason I don't like the idea of interrupting the fuel pump (DME relay 2nd stage) is that the engine will run on for a little while and deplete fuel pressure in the rails. Both are rather undesired. Hot start will be more difficult due to vapor lock if the fuel pressure in the rails is gone.

Ingo

[Walt Fricke]

Well, the 80-83 3.0s used fuel pump cutoff as the rev limit. No problems there, and it sure is quick - pressure drops below injector open pressure pretty much instantly. Just as fast as ignition cutoffs of various sorts.

I don't know as much about 3.6 EFI, but I am dubious that there is any significant overrun. However, where ignition is powered directly from the ignition switch, I'd put a relay into that circuit if I wanted to use that as a cut-off. Way too much current flow to want to run through that little switch.

Here is something else to consider. Our cars have a nice cut-off switch - the key. The only use for these battery/engine cutoffs is racing, so corner workers can shut off the battery so sparks don't create trouble after a crash., fuel pumps can't keep running, and so on. And, incidentally, to turn off the engine should it happen still to be running.

I don't think hard starting is going to be a practical issue. For most of us, this switch is used on a running engine on those rare occasions where race officials are checking safety equipment.

Carbs are a different matter when it comes to cutting the fuel pump off - they will run on, so that's not an option - the engine has to stop promptly.

[aston@ultrasw.c]

My cut-out scheme needs to pass a racing inspection.

The regs don't say how quickly the engine should shut down.

The engine does run for 2-3 seconds as fuel pressure depletes.

[Walt Fricke]

I'm not sure anyone's regs give a specific time. Inspectors are mostly used to seeing the engine die at once, or just keep running and running and running. Carb bowls emptying would seem too long to an inspector. But 2-3 seconds?

Shows Ingo knows his DME cars. CIS dies instantly with the cutting off of the fuel pump.