shiners780

CHANGE OF DIRECTION - DIFFERENT SWITCH DISCUSSION STARTING AT POST 25

I am installing one of these Longacre 4 post electrical cutoff switches http://www.longacreracing.com/catalo...d=477&catid=14

1) Some directions/threads/online info I have read show using a resistor, but the Longacre directions don't mention anything about a resistor. Do I use one or not? What would it be used for?

2) If I cut the power to the coil to stop the engine, what about the alternator power surge? Do I need to include the alternator in my wiring of the switch, or will simply cutting the coil power eliminate any potential alternator problems with power surges during shutdown?

I have read every online source for wiring one of these things, and all I hear is the "wah wah wah" sound the adults make on the Peanuts cartoons. Electrical work continues to baffle me....

Any assistance getting this wired up would be greatly appreciated.

Scootin159

Where does the "Field wire" (I'm assuming this is the alternator exciter wire) on our cars connect to? I imagine you're fine to "break" the main power lead to the (+) terminal of the battery, and the exciter wire to the alternator.... but if you know where the other end of the alternator exciter wire is, this could make wiring a bit easier.

shiners780

I dunno.

I'm hoping to be able to use the coil, since the wiring would be a cinch to work with due to it's location. I'm not sure I even need to connect to the alternator, and I guess that is what I am questioning.

joes

I just run the power to the coil through the switch, it works fine. The way to check is rev to 3000 RPM throw the switch if the engine dies game over.

Scootin159

I guess I don't see what using the coil will do for you. The whole purpose of the cutoff switch is to kill electrical power to everything in the car (fuel pump, etc.). There's two sources of power in the car, the battery and alternator. Kill both, and there's no electrical power in the car. The battery's easy enough (disconnect the (+) terminal from everything)... so I'll skip that.

I'm guessing that by killing the coil wire, you're going to kill the engine, which will kill the alternator. My only problems with this is the inherent delay in the mechanical engine stopping (especially if you're still moving). The other is that the coil wire has MUCH higher voltages in it than the switch is designed for... this might have unforeseen issues.

Seems easier to just 'unplug' the alternator. This can be done by either disconnecting the exciter wire or the main (+) feed.

shiners780

...hence my confusion

I have no idea, so you guys keep explaining it until (if) I grasp something I can comprehend....

jpk

As Scott mentioned, there are two power sources in the car. If you kill just the battery, the alternator can provide enough power to keep the car running. If you kill just the alternator, the battery provides the power.

The way the car is normally wired, you have several connections to the battery + terminal; a big cable running directly to the starter, and one or more smaller wires feeding the fuse panel and ultimately everything else electrical in the car.

The alternator output is effectively directly connected to the battery + as well, but it makes this connection at the starter. The first thing you need to do is re-route that wire from the connection on the starter to wherever you will mount your switch.

Wire it up according to the top picture in the installation PDF. The alternator output gets it's own terminal, and everything else (big starter cable and all other battery connections) goes to the other output terminal.

On the battery side of the switch, you can simply jumper the two connections (it shows two wires going to the battery + terminal)

When the switch is closed (on) everything gets power from the battery, and the alternator can supply power back into the system.

When the switch is open (off) nothing gets power. You're killing power to the fuel pump and DME, so the engine immediately dies. You're also killing excitation voltage to the alternator, so it immediately stops generating power. No surges, no power spikes, nothing - the car will just die.

shiners780

OK John, your explanation is starting to make sense...thank you. Let me see if I have this right:

Using the top pdf diagram...

1) Attach all the + cables/wires currently on the car to the large output terminal at the 3 o'clock position on the switch.

2) Make a large gauge jumper wire from the battery + to the large terminal at the 9 o'clock position on the switch.

3) The 6 o'clock position on the switch will be empty.

4) Run a wire from the 12 o'clock position on the switch to the alternator? To which terminal on the alternator? And what is the purpose of this wire again?

So if all the above is correct, the current factory wiring to/from the alternator remains unchanged, with the exception of the wire added as coming from the switch?

edit: and back to my original question - no resistor needed with this setup?

924RACR

This might help... sorry, I can't relate personal usage, 'cause I've got a crappy old 2.0L car...
http://www.improvedtouring.com/forum...ad.php?t=20130

shiners780

OK Vaughan, while I appreciate the link, all it did was make my brain hurt.

If I can get clarification on my post above, I think I might be close to having a plan.

Scootin159

Yes and no

#1 - yes, almost (see #4)
#2 - yes
#3 - no, connect 9 o'clock to 6 o'clock
#4 - connect the large lead running from the battery to the starter to 12 o'clock

*** Note: the above is true ONLY if there are no other (+) connections at the starter except the starter itself and the alternator.

jpk

Jim,

Sorry if this confuses things, but Scott has the #4 connection incorrect (no offense Scott, notice I didn't use the word wrong )

1) All wires currently connected to the battery + get moved to the 3 o'clock terminal on the switch. This means a new end for the 4 gauge starter lead.

2) New heavy gauge wire (2 or 4 gauge) from battery + to 9 o'clock.

3) 6 o'clock position needs to be jumpered to the 9 o'clock. This needs to be heavy enough to handle alternator output, at least 10 gauge, 8 gauge is better. You can use a solid bar as well, as long as it doesn't touch anything else (it'll probably be easier than a really short wire)

4) Alternator output needs to be re-routed to the 12 o'clock. Currently, this is the 8 gauge wire running from the big lug on the alternator to the big lug on the starter. Disconnect it from the starter and connect it to the switch 12 o'clock. Unfortunately, it's likey to be too short to run it up near the battery tray, so you will need a new, longer cable.

Internally, the switch connects (or disconnnects) the 9 o'clock to 3 o'clock with one set of contacts. This will essentially disconnect the battery from the rest of the electrical system. This will have to handle 300+ amps momentarilly while the starter cranks, but the switch is rated to do that (1000 amps intermitant).

The 6 o'clock and 12 o'clock are connected with another seperate set of contacts. This will disconnect the alternator from the rest of the system. You MUST disconnect the alternator independently of everything else, or it will back feed the DME and the car will keep running. These wires only need to handle alternator output (115 amps max).

Oh yeah - no resistor needed. I hope this clears things up.

shiners780

Awesome info Scott and John, thank you!!!

Question: in addition to the larger 8 gauge wire on the larger lug on the rear of the alternator, there is a smaller gauge wire. Is this a ground? It's wrapped with the 8 gauge wire once it leaves the alternator's smaller lug. Where does it go to? Since it runs into the black hole under the intake I can't follow where it goes. I must keep this right?

Thanks again, this is a huge help!

jpk

The second smaller wire on the alternator is required to excite the field; you need to leave it connected, otherwise the alternator won't work at all.

shiners780

Where does it go to? I'm thinking I will need to remove it with the larger wire since they are wrapped together in a heat shrink tube, then reconnect. Where's the other end go to? Or do they both go to the starter? In which case do I leave that smaller wire connected to the starter, or move it to the switch as well on the same 12 o'clock post?

edit: Like I said, I have no friggin' clue when it comes to wiring.....

edit x 2: On my wiring diagram it appears the smaller wire from the alternator runs into the central electric (fuse box)? That sound correct?