Master disconnect and EFI ECU

[notenoughcash]

I plan on installing a master disconnect switch on the '74 to avoid any tech issues when I eventually head to the track. Have already seated and read up on various ways to do this.

What I'm unsure of is the main power feed for the EFI ECU and Ignition box. Both sets of install directions are adamant about having the non-ignition side straight to the battery. If I put them on the 'car' side of the disconnect switch would there not be the potential for damage to the units if/when the master switch is used ? I know the mechanical switches are not exactly instant on the connect/disconnect and that makes me think spikes etc getting to these units.

Am I over thinking this and worrying about nothing or should I look to get something inline to protect those feeds ? In the process of drawing up the disconnect/fuel pump relay bracket and wiring diagram(s) since I what to have it all neatly mounted on the battery box.

Thanks

 

[67Dart273]

This is a tough one. Do they want battery to insure enough voltage/ amperage to the EFI, or do they need it to maintain memory settings in the processor? That would be the FIRST thing to find out.

IF you don't need the EFI constantly connected for memory reasons, you could do this with a constant duty solenoid (looks sorta like a Ford solenoid) and a 4 terminal (DPST) Cole Hearse disconnect

You could ground one of the disconnect small terminals, run the other small terminal up front on a no 14 wire, and hook that to one of the small terminals on the solenoid. Hook the other solenoid small terminal to "switched ignition"

Of course the large disconnect terminals disconnect the battery hot cable

Then wire one large terminal of the solenoid to your battery buss, and the other large terminal to your EFI battery feed. Also connect the feed to your ignition system to this solenoid terminal.

In fact, you can hook ALL switched devices to the cold side of the solenoid, fans, pump, etc, just make sure the alternator is on the battery side of the switch.

So if EITHER the ignition switch OR the disconnect is off, the solenoid drops out, and kills power to the EFI and the ignition system

One last thing you could do is find yourself a BIG zener diode somewhere in the 16 volt range and install to ground as close to the EFI as you can, and on the EFI side of the fuse. So IF you were to get a big enough spike AND if anyone ever hooked the system up backwards (booster cables) the zener would clamp the voltage to whatever it's rating is, or if reversed, would blow the EFI fuse.

 

[inkjunkie]

The way I had it explained to me, and this is not going to help you much is that if you just wire a disconnect switch into a vehicle with a computer in it and hit the kill switch, you will end up with a voltage surge because the alternator is still spinning that has no where to go, potentially frying your computer. There was a lot more to it than that, might want to call Mad Electrical, there site is down right now, about it. He was the one I spoke to about it.....

 

[notenoughcash]

I'll dig into the why tomorrow, hopefully it's a quick phone call. WIll also cal MAD to see what they say.

I may have a big zener around now that you mention it from a previous project for a pyro firing system. Thanks for the brain kickstart !

Will update here when I think I have it figured out for another set of eyes to go over