Battery cut-off switch installed

[Hal Copple]

When Dennis Olthoff put the full width rollbar on my SPF, it left a hole on the rear deck, just behind the cockpit. So for some months i have had a big amp Moroso battery cut-off switch stuck there, but not wired in. Looked nice, but i don't like non-functional sorts of car things, and at several track and top speed events, the tech inspector would say "good, an external switch", but of course, all he had to do was to peek under the rear of the cockpit, and see that it wasn't hooked up to anything.

So this past weekend, put two grommets in the rear bulkhead, and ran some big gage wire and did it right, with proper metal insulated supports were appropriate, to avoid any chafing and resultant sparks.

Since i always disconnect power before doing any work on my car, it is nice to be able to do it so easily, although i still have the trunk mounted cut-off switch in the circuit.

here is what it looks like.

[Hal Copple]

ok, let me try to make it larger. I don't quite have the hang of this picture stuff yet.

[Rick Parker]

Be sure there are not any fuel vapors that collect in the trunk?????? Dont want any surprises.

Rick

[Jamo]

Darn nice use of the hole, Hal. They've got to be the most convenient thing ever for working on the car. Now you have one more thing to add to that starting sequence.

[G.R.]

Speaking of cut-off switches, I have one I'm planning to install, it's a new Blue Seas that I never got around to installing on my sailboat. What is the best location? I thought about between the seats on the bulkhead just beneath the body rim of the cockpit?

I've read pro and con on wiring in these in this forum--but on boats you always disconnect "power", to me that makes sense because if you disconnect "ground", power is still there and if it finds ground system is hot--right or wrong?

[oldman ohio]

Hal,
Have you tried shutting off the switch with the engine running.
My guess is that the engine won't stop unless you made provision to cut the power to the coil.
jack

[nanard289]

Hi Garry,
About the battery switch location, you have to found a compromise between these 2 conditions:
- the switch must be located as closed as possible of the battery (to minimise the "live part" when the switch is off)
- the switch must be located in a free and easy access area.
Regarding the polarity to be cutted, theorically, it is possible to cut either plus or minus to obtain the same result (as a cooling circuit, you can close either input or output way to stop the flow). For practical reason it is a common use to cut (and to protect by fuses) the positive wire only (to keep the negative wire as short as possible). However, if you accept to use this switch to stop your engine (like me), you have to connect the alternator wire at the upstream side (see sketch here attached) to avoid any alternator overvoltage during the stopping sequence (due to the engine inertia) and to risk to burn alternator diodes
I hope these small informations can help you.
Bernard

[nanard289]

Originally posted by oldman ohio


Hal,
Have you tried shutting off the switch with the engine running.
My guess is that the engine won't stop unless you made provision to cut the power to the coil.
jack

Hi Jack
This case is applicable only if you have a diesel engine because to stop it, you need electrical power to energized the solenoid diesel cut off valve. Consequently, you have to segregate this circuit from the general cut-off switch (like the alternator wire).
Bernard

[Hal Copple]

thought i would answer a few of the above considerations.

First, i need to get another sticker, because i put the big Off sticker on the car when it was a dummy switch, and i rotated it 90 degrees when i installed it, for better cable routing, so now the big Off actually points to the on position.

Next, on SPF's the rear bulkhead is inset a few inches under the rear of the body, so the switch is actually totally in the cockpit and not in the trunk. No real risk of gas fume ignition.

As to turning it off, and having the engine run, to some extent this is true. Bob Olthoff told me that once the engine is running you can take the battery out of the circuit and the motor will run due to "self excitment" of the alternator. But he assured me that this is much higher than my idle, and that my motor would die. But this is not a consideration for me, as the Moroso switch i bought has a spare lug on it, that will ground the coil wire too at the off position, if i run a coil wire to it.

I wanted a switch that would carry a big amp load, and not have a removable handle, so it wouldn't disappear on me.

fun little 150 mile trip today, with Becky riding shotgun.

[G.R.]

nanard289--thanks for the diagram.

On a boat(granted it was diesel) with battery cutoff switch, all power was disconnected--house bank, start, etc. wired to the main power lead from the batteries--you could shut the switch to
"off" and there was nothing, if the engine was running, being a diesel, it would run for a few minutes then die because the injector circuit would not get power.

So if a car circuit--then wouldn't it reasonable to cut off the power at the positive with a jumper lead from the alternator to kill the engine and all circuits. That way once the switch is"off" there is nothing to power the car or it's accessories?