Wiring a Battery Cut Off Switch in a 3.6 Conversion

[Duc Hunter]

I am thinking of putting a battery cut off switch in my 3.6 conversion for a few reasons.

1. They "look the part." Let's be honest, most of us who put these in street cars do it for this reason only.
2. The car's wiring is a bit messy, and OLD. I would like to put it in just incase there is an electrical problem/fire. And yes, a cable pull to the interior will be set up.
3. It does add slightly more security, as in slowing down someone who might want to steel the car.
4. When my cars are in storage I always disconnect the battery's so they are fresh when I go back.

Since I have a 3.6 though this gets a little more interesting. I have learned that once I cut power to the DME the car becomes VERY hard to start, until I let the ECU "re-learn" the engine. To get it started to do that can be a struggle. I want to avoid struggle, esp when I put the car in storage for a month or so, or when I travel (as I kill the battery when I am gone for a week or more). So I was thinking of wiring my 6 pole kill switch as you normally would, except I would leave the DME with key on power to maintain its memory. Yes it will drain the battery slightly in storage, but that should not be much at all. Thoughts/issues? To do this I think I would need to identify the DME feed wire that should be connected lower side of the fuse block in the "always on" area, and run it directly to the battery with an inline fuse?

The other question I have, which I could not find through searching, is where do I get the wire for the ignition switch to the coil/ignition system from? I know the idea of this wire is to kill spark to the motor and shut it down. I would assume I get it from the fuse block? Meaning at some point a wire must go from the battery to the top of the fuse block for "always on" power, and then a wire from the bottom of the the fuse blocks "always on" area feeds the ignition switch, which then has a wire that goes back to the top of the fuse block and feeds the fuse block's "key-on" power area. From there I would think there would be a wire out of the bottom of the fuse block to the coil or ignition system, and that is the wire I need to run through the kill switch right? Some people seem to wire part of their switch into their fuel pump circuit instead of their ignition too, which I could also do I suppose. Thoughts?

[betterair]

on my 3.6,I put an interupt switch on the fuel pump relay.
So,cranking on it till hell freezes over won't even start the car.

Ill take the dead battery in exchange for a discusted thief who couldn't steal the porsche.

[Duc Hunter]

That makes sense. Thought I might get a few more responses too. Guess everyone is still enjoying their Sunday coffee.

[dad911]

As I recall, car long gone, I believe I wired the secondary poles to the fuel pump relay.

You could jump the main cutoff with an 'expendable' fuse, probably 5 amp would work. Then if someone tried to 'steal' the car, fuse will blow.

[sandiego]

In my 3.6 conversion into a 1984 911, when I have a dead battery I always disconnect the ground before putting the charger on. When the ground is reconnected I have never had a hard start problem.
Have you ever experienced a hard start problem or just heard rumors?

[betterair]

dad911 ,
yes , of course.
Thanks ,great idea,as a double safeguard - theft deterrent AND to save the battery

[sandiego]

I disconnect the ground only because I heard rumors that some of the delicate electronics could be damaged by a voltage spike from the charger.

[rattlsnak]

The purpose of a cut off switch is to have the engine cut off when it is running by twisting the switch. Best way to do that on a DME car is to interrupt power going to the DME relay which then shuts off ignition and fuel. You do NOT want to simply disconnect the main battery from the car/alt.

[Duc Hunter]

Originally Posted by sandiego

In my 3.6 conversion into a 1984 911, when I have a dead battery I always disconnect the ground before putting the charger on. When the ground is reconnected I have never had a hard start problem.
Have you ever experienced a hard start problem or just heard rumors?

I have suffered through these problems in a big, big way. Had a mechanic re-wire the DME chasing other issues, and he did so without constant power, and I really struggled to get it started. For weeks this went on, even int eh warm Fl summer. Different mechanic "fixed it" and it works fine. If I disconnect the battery it will struggle a to start, an once you let it die for about 10 min, its totally fine. One of my Indy;s is an ex factory mechanic, and says the 964 has to learn the ICV etc, and it is a standard procedure on those cars to let the car learn after a battery change etc.

As to the others who mentioned not wanting to disconnect the positive while the alternator is turning, you are correct. That is why the resistor is there, so you do not fry the alternator when you kill the power. I am not sure why everyone puts these not the + side for racing, and racing sanctioning body require it that way, vs the -. There must be a good reason though.

Sleep in mind I want those for storage purposes, and incase I have an electrical prob while I am driving the car too.

[Tremelune]

What you want is a battery tender.

Battery kill switches are a 10 second theft deterrent. A better setup would be something hidden, but you risk them tearing stuff apart to get at the ignition wiring. A fuel cutoff is another good one...They only get a mile down the road.

[aston@ultrasw.c]

I have a 3.6 conversion with a battery disconnect.

Never had a problem starting after a disconnect, which is almost every time I use the car.

I use the disconnect as it eliminates any parasitic draw when the ignition is off.

I also have the resistor to protect the alternator.