Ignition Cut Out and Alternator Question

Kev · Post by **Kev** » Sun Apr 03, 05 12:29 pm

Me and Blue are having trouble getting our head around an electrical problem. We need a cut out switch that kills the engine. The standard cut out switch only cuts the battery so we need one that cuts the alternator as well. This is the one we intend using;
http://www.moroso.com/catalog/categoryd ... code=42225
If you look at the instructions the alternator cut out is rated at a max of 20amps. Now with a fuel pump, water pump, fan and everything else this is a draw of more than 30amps. With, say, a 100amp alternator, when is it putting out 100 amps? Are we in danger of frying our wiring? It seems the only NHRA kill switchs available are this Moroso one and a Flaming River one. What (true) kill switch are you racers using? Looking at my Valiant wiring diagram the alternator feed wire goes through the standard ignition switch so I assume it won't be a problem, we just need to know if an alternator can put out 100amps, when does it do this and why can the relatively small gauge wiring take the strain? TIA!

Anonymous · Post by **Anonymous** » Sun Apr 03, 05 3:10 pm

I would have said the best way to control an alternator is with the voltage reg, the way to do it depends on what alternator you have.

On a one pin the reg controls the power side of the field current so if you find a switch that has at least 2 seperate on/off circuits, battery to one and reg wire to the other
On a 2 pin the reg controls the ground side through the second pin, the easiest way to do this is fit an electronic reg, ground the second pin to the case and then do as above.

This way you wouldn't have to bother about what the alternator is capable of producing as you're using the reg wiring to do it

Thats as i understand it but check the wiring diagrams first as i may be wrong

And i would think that switching the alternator output wouldn't actually stop the alternator from producing current.

Pete · Post by **Pete** » Sun Apr 03, 05 3:26 pm

Can't you use a switch that just grounds the amplifier ?

Alex · Post by **Alex** » Sun Apr 03, 05 3:28 pm

FIA switches wire up so you kill the battery, the coil live and drop the alt live to ground through a resistor to prevent spikes etc, I will pull the diagram from my race files tomorrow if you want?

Anonymous · Post by **Anonymous** » Sun Apr 03, 05 3:36 pm

I've had another thought.

You could fit a single terminal switch (really heavy duty ones are sold by caterpillar dealers ie Finnings for around ?45) and use a relay to switch the voltage reg off/on powered from the switchable side

Kev · Post by **Kev** » Sun Apr 03, 05 5:13 pm

So if it's so complicated does this mean the diagram for the Moroso one doesn't work? And if not how come it's NHRA sanctioned?

Kev · Post by **Kev** » Sun Apr 03, 05 5:15 pm

Alex. The diagram would be good, but where does one buy a switch that does all that in one hit? Obviously these things MUST be available to satisfy Sanctioning body rules. "An external method of shutting the engine down."

Alex · Post by **Alex** » Sun Apr 03, 05 5:19 pm

Demon Tweeks supply an FIA spec master, I will pull the diagram tomorrow

Kev · Post by **Kev** » Sun Apr 03, 05 5:38 pm

Here's one. But I don't want no steenkin' red key sticking out me boot!
http://www.pegasusautoracing.com/Produc ... RecId=1464

Dave-R · Post by **Dave-R** » Sun Apr 03, 05 6:53 pm

I thought the idea was you had to have a visable means to isolate the battery so that if you were knocked out, on your roof, and with the doors jammed, the emergency services can kill the engine from the outside and make the car safe for them to use their cutting gear without electrical mishap.

That usually means a big red key clearly marked and sticking out the back of your car.

Alex · Post by **Alex** » Sun Apr 03, 05 7:00 pm

Correct Dave, although in drag racing the rules seem slightly different but in any FIA or MSA sanctioned event the red key set up is the norm, it must kill all power and shut down the engine, they also like you to wrap yellow tape around the ground lead so they know which to chop with cutters if required, don't ask me what the logic is when the master is on the +ve side !

Kev · Post by **Kev** » Sun Apr 03, 05 7:00 pm

Yeah, Oi understand that but I was hoping to go for this; http://www.moroso.com/catalog/categoryd ... code=42225
with the push/pull rod below the switch. Just push to disconnect. A lot nicer looking than a big red key that some chav might mistake for a cigarette lighter

http://performanceunlimited.com/cobrava ... witch.html

Kev · Post by **Kev** » Sun Apr 03, 05 7:51 pm

MSA Santa Pod Rule Book; "Plastic or Keyed switches prohibited." But most racers use them......

Dave-R · Post by **Dave-R** » Sun Apr 03, 05 8:04 pm

Hang on. You said it only had a rating of 20amps? The one you are looking at is rated at 300amps?

So whats your problem?

Kev · Post by **Kev** » Sun Apr 03, 05 8:57 pm

If you look at the instructions the alternator cut out is rated at a max of 20amps

The diagram shows the output of the alternator going straight to the battery through the auxilary contacts. Now if you have a 100amp alternator giving full output won't it fry the switch?