[Larry_A]

OK, OK, so it's not a MkIV specific question, but I figure you guys might have a good clue on this one...so help me out.


For all you hardcore drag-racer-type guys who've wired in a batt cut-off switch:

How the hell did you wire it up?

Wiring up between the batt & the starter won't shut the car off, which means anothr circuit must be included to accomplish this. I have a 20A nominal, 125A peak switch...so I'm limited to how it's connected.

Any help provided will be greatly appreciated...or no drags this wknd.

Thnx!

 

[Larry_A]

Given I needed an answer right away, I figured it out myself.

Hooking the wire that runs from the starter to the batt, looks to be a waste of time. We don't need to turn the fricken starter off.

The circuit that [@ 1st] makes sense to put the cut-off switch on, is the ign switch wire [White-Red] that heads into the ign switch, coming out as IG2 [Black-Orange]. This 30A fusible linked circuit supports both igniter & the fuel injectors...so a switch in-between would def cut the engine, if turned off.

However, given the 'rules' say that power must be cut from everything, I see anothr circuit is needed. This takes us to the othr 1/2 of the Ig switch [Access/Ig], where [white] goes into the Ig switch, coming out as [Purple-Light Blue] for Acces/Ig, & {Black-Yellow] for ECU/Eng/Turn/Wipr/etc. on a 40A fusible linked circuit.

So, IG2 has a 30A fusible link in it, while the Ig/Access has a 40A fusible link in it...that tells us all we need to know about the future of the cut-off switch I have. The cut-off switch's nominal rated capacity of 20A/125A peak rating is insufficient.

We haven't even mentioned the ~18' of extra wire [round trip] to do this, & having to implement a fusible link somewhere along the way, too- if wired directly from the Ig switch [er, aye kapten].

Foregoing use of the cut-off switch on hand, is in order; & I need one that has a higher rated Amperage capacity, period. Such is life.

Anyone need/want a brand new, never used Moroso batt cut-off switch [20A/125A peak]? Includes packaging, unused stickers, & instructions that don't apply to Supras.
$20 + shipping. Any rice boys out there wanna look cool w/a non-func cut-off switch poking out the back? Ha! LOL.

Bottomline:
No racing this wknd.

If I missed something, pls advise. If not, log this one in the "How To Race-Prep Your Supra" book.

 

[AZWildcat]

Larry,

I don't think my post on the AZ list went through, so I'll try it again...

I have a strong suspision that these wires are not current carrying wires,
but simply signal wires. Ignition works by rapidly changing the current in a
primary winding (generally via a switch on an inductor, remember
Voltage=inductance*change in current/change in time). So, if the current
(any amount of current) could be cut instaneously, where dt=0, you would have infinite voltage. Obviously the primary winding doesn't cut it instantly,
but it cuts it fast enough to make a large amount of voltage. Then through
mutual inductance, the voltage is stepped up further through a secondary
winding. What does all of this mean? I don't think it's drawing any 120
amps.

If you're REALLY that worried, simply buy a few 30 amp relays to fix the
problem. Wire the relays so they're on when the switch is in the "safe"
position and cut when the switch is thrown.

 

[Larry_A]

.....If you're REALLY that worried, simply buy a few 30 amp relays to fix the problem. Wire the relays so they're on when the switch is in the "safe" position and cut when the switch is thrown.

I thought of relays, but the solution infers having an energized relay pretty much 100% of the time.

The 'right' way to approach this, is to have a switch capable of bypassing the batt AND the Alt completely...which is what I plan to do. Flaming River has a 250A switch that even has a rod attached, to help locate the actual switch 'closer' to everything. That looks like the ticket.

I DO apprec your input, however.

 

[AZWildcat]

I thought of relays, but the solution infers having an energized relay pretty much 100% of the time.

The 'right' way to approach this, is to have a switch capable of bypassing the batt AND the Alt completely...which is what I plan to do. Flaming River has a 250A switch that even has a rod attached, to help locate the actual switch 'closer' to everything. That looks like the ticket.

I DO apprec your input, however.

Can only lead a horse to water... good luck.

 

[Larry_A]

Can only lead a horse to water... good luck.

Ha! LOL.

The wiring scheme you suggested would've worked to shut the eng down, BUT, the rules want the whole shebang to go dead [ie, EVERYTHING], so I really had no choice but to get a higher-rated switch.

I've got a 300A nominal/2000A peak switch yest, & it's going in now. It looks like I'll be able to hit the track on Sat aftr all!

Now, all I have to worry about is breaking something.