Battery isolators & alternators

[AlexinPerth]

I've got a battery isolator in the rally car, and it's wired between the negative terminal of the battery and the chassis.

This is all well and good, and it isolates correctly, but it will not kill the engine when the engine is already running, since the alternator is providing a secondary source of power between the positive terminal and the chassis.

My first thought was to modify the alternator so that it connected directly to the negative battery terminal instead of the chassis, but I have concerns that the alternator has it's negative terminal internally connected to the casing. And I don't want to have to mess around with insulating the alternator from it's mounting.

My second idea was to determine when the isolator has been turned off, and cut the ignition using a relay.
The only real way I can think of doing this is to sense the change in voltage between the negative battery terminal and the chassis.

(should see ~2v between these when the engine is running and isolated, since the alternator charges to 13.8V, and the battery will fall to ~12V reasonably quickly?)

I'm working on the assumption that CAMS requires battery isolators to switch off the engine in competition vehicles. Might check that now.

Any other ideas? What does everyone else do?

Alex.

[AlexinPerth]

Seems that the isolator isn't actually required by CAMS, merely a nice thing to have for occasionally used cars.

Alex.

[da9jeff]

although you answered your own thread, ill still tell you how to do it incase anyone else was wondering.

Asuming you have a coil all you need to do is run the alternator directly to the battery, and use a relay to switch the coil instead of a direct feed from the solenoid/battery. The relay is wired up so the trigger feed (with switch in line) is after the cutoff switch so that if you pull the iso the 12v is cut of to the ign relay, causing it to open circuit and killing power to the coil thus stoping the engine.

You'd be surprised how many big dollar racecars have dud kill switches! espec porsche gt1,2,3's

[hotgemini]

Sounds like you bought the cheapy switch rather than buying the right switch. http://www.demontweeks.co.uk/product...pcode=LEMMW002

[AlexinPerth]

Jeff - this is a single pole master isolator. Sounds like you're talking about using a separate light duty isolator to kill the ignition? Or you're thinking that I'm isolating the positive terminal?

hotge: Well, I didn't buy it, but you're right. That switch would solve the problem with the least hassle. Might get one of those.

Do people normally isolate the positive or the negative terminal?


I've attached a diagram showing how it's connected at the moment.

The actual isolator is similar to this, but with a metal lever instead of a key:


Alex.

[10sec rx7]

never had a problem with any of the switches i have put in, id be thinking you have a major earth problem in the car somewhere

[BigMuz]

NAh man- it depends on how your alternator does it's thing I think.

Some kill switches work without worrying about anything. Others need to be fed to the battery by a relay or they keep the car running.

I'm no fucking auto elecco; but you should check yours works after you install the kill switch; and if not fix it.

HTH

Hammered Muz cause I'm three days off the durries and using booze to get me through.

[da9jeff]

sorry ill draw a pic as describing wiring diagrams is something i like to avoid.

[AlexinPerth]

Yeah, that uses a second switch. And you're switching the positive terminal, which is simpler IMHO than switching the negative.

Not to say there's anything wrong with your solution, but I'm trying to avoid having two switches, or completely rewiring the isolator switch.

Oh well, might splurge on the more expensive isolator switch. That and a couple of short wires to the ECU relay will make me a happy camper.

Thanks dudes.

Alex.

[da9jeff]

not to go on about it but the second switch can be the original key or a separate switch on the dash, with a missle switch cover for max effect infact you dont even need a switch or key with that setup if the iso switch is within reach in the cabin, iso on=coil active, iso off=coil dead

[BigMuz]

Kinda defeats the whole purpose of having an external kill switch doesn't it?

[RB30-POWER]

I don't really get why you need that seperate relay but, if you have the alternator + hooked direct to the battery, once you isolate the switch, the engine should stall because it has not alternator or battery to keep the ignition system powered.

[AlexinPerth]

This is all true if you isolate the positive terminal. Mine has the negative isolated.

I wrote that in the first post. I illustrated that in the diagram. I give up trying to explain it again.

Alex.

[10sec rx7]

if you isolate to earth from the battery then you should have no problems at all, i cant understand why it is doing what it is doing, if you cut the earth off all together from the car is should stop
one way to check, have the car running then remove the earth from the battery, if it stops then it is the switch,

i have never seen a altinator that makes its own earth!

[AlexinPerth]

Well, the alternator is a source of power once the engine is running. The current isolator does nothing to separate this source of power from the rest of the circuit.

I always though that it was bad for an alternator to be running without a battery to charge (not enough load), but maybe that's an olden days regulator thing. I've only tested it a couple of times for a couple of seconds though.

Car is a 95 Charade, BTW.

Alex.