Kill switch/alt protection/early car

logician · 03:27 PM

How to wire kill switch in an early 911? I know this is a common question, and I thought i had it right. However, another blown alternator in my '70 911 race car makes me suspect otherwise. This vintage had the Marchal (sp?) alternator with external mechanical regulator.

I'm using a six pole switch from Smart Racing. The big poles interrupt the hot lead going to the battery. One set of small poles interrupt the power to the ignition. The second set of small poles are wired to a load resistor that connects to ground when the switch is turned off.

I suspect that provides insufficient protection to the alternator The problem I see is that the alternator supplies it's own field power through the D+/61 pin. So it continues to generate power as long as its spinning. I'm not convinced that the load resistor is sufficient to reduce the voltage spikes for the early alternator/regulator configuration.

Do those that run these early style race cars use a different wiring scheme? I'm thinking a safer method would be to interrupt the field current. Any suggestions from those of you familiar with this problem in early cars?

Another question that I have is why do you bother to switch the load resistor? Why not just use a constantly connected load, either a resistor or light bulb? What is the value of that resistor anyway?

-Juan

Mahler9th · 02-20-2007, 06:07 PM

I have mine wired basically the same way yours has been, except I cut one of the fuel pumps with the small pair of NC contacts. No issues for years. I have spoken to many folks about this. You don't really need to bother with the resistor protection, but since the poles are there, what's the harm? If you alternator has failed, it isn't likey due to the kill switch.

There are some Pelican BB threads on good alternator shops-- one is in Campbell by Rich and Jerry and I think there is one in SF.

I carry a spare, and I think a few other PRC racers do as well. I am not sure what DOES cause the alternators to fail. Between Rich, Jerry and Mat, you can easily get an answer that you know is rock solid. But it won't be the switch.

logician · 02-20-2007, 07:10 PM

Mike,

Thanks for the reply. Do you have the Marchal alternator with the external regulator? I'm thinking that the later alternators might be more robust, and not have the same issues.

-Juan

Eagledriver · 02-20-2007, 07:10 PM

Juan,

I think the resistor is a fairly low resistance and so it would get hot if it was on all the time. Mine is hooked up as you describe. I'm not sure what the other end of my resistor switch is hooked up to but it needs to be something that is on the alternator output when the switch is activated. I'm running a later alternator with the attached regulator. Makes a nice simple 2 wire hook up on the back of the alternator.

-Andy

Mahler9th · 02-21-2007, 07:43 AM

I have an early alternator with an external regulator. That is one way to save weigt... I do not need a 964 or 993 alternator. As I said, if you have been having alternator failures, I would look beyond the kill switch. I know of one PRC car that runs the simple 2-pole Bosch kill switch (the fellow like the "period correct" look-- can you guess who it is?). So he has no alternator protection. No issues.

Walt Fricke · 02-22-2007, 10:19 PM

The period correct 2 pole kill switch also won't kill anything in a 911. The alternator will keep everything going.

I think that originally those switches were intended only to be battery disconnect switches, which they will do, and not combined battery/kill switches.

Alternatively, if it is rigged up to kill the engine, it won't disconnect the battery from much.

Hence the 6 pole switches.

I suppose you could isolate the alternator B+ and run it all the way up to the battery without any switching or branching off, bypassing the kill switch. But then you'd have a fairly stout wire always hot and not disconnectable running the length of the car. Almost like the cable to the starter/alternator.

Walt Fricke

turboteener · 02-27-2007, 06:29 PM

Why not just run the hot lead from the alternator to the battery side of the kill switch? When the switch is killed the alternator sends any voltage back to the battery thus not killing the diodes. But the engine will cease to run.

logician · 02-27-2007, 06:32 PM

Quote:

Originally posted by turboteener
Why not just run the hot lead from the alternator to the battery side of the kill switch? When the switch is killed the alternator sends any voltage back to the battery thus not killing the diodes. But the engine will cease to run.

That doesn't satisfy the requirement that the cutoff switch kill all power. You still have a very thick hot lead going to the alternator.

-Juan

barney911rs · barney911rs's Garage

I made mine very simple, no alternator. I crewed for a guy with a 914-6 SCCA GT car for several years that never ran an alternator, and we never had any issues. So, when I built my car, no alternator. Made the cut off switch very simple and reliable. I've run the car a whole day and a few 90 minute enduros with out recharging the battery. I usually bring a generator to put the car on the charger between sessions. That and the switch to the yellow top Optima, never had a problem.

Walt Fricke · 02-27-2007, 11:22 PM

John

That sure would deal with the kill switch, though by using the 6 pole switch it isn't an issue, so the question is what other benefits are there from the alternator delete configuration.

I suppose you save the HP needed to turn out current from the alternator (though one can do that with an alternator cut out switch - @WOT you open the field - or do you ground it, I forget). Seems like the fan may be the larger HP hog, though.

Did you also gut the alternator or rig up some substitute to serve as a bearing for the fan? Less weight, and less inertia to deal with?

And is the trick here the capacious Optima yellow battery? Which is going to outweigh any ordinary weight saving from gutting an alternator, no? Of course, if you have to meet a weight limit, and need ballast, heavy batteries substituting for other things sound attractive.

Walt

turboteener · 02-28-2007, 09:56 AM

But the engine won't continue to run. When the ignition is cut the engine will rotate a little longer and having no where for the voltage from the alternator to go will burn out the diodes.

barney911rs · barney911rs's Garage

It's a gutted alternator, he already had one sitting around his shop. It was primarily to keep the wiring simple. When the battery is cut, there is no power to anything. Had nothing to do with HP or weight.

The best thing I could do to save some weight in the car would be to have someone smaller then me drive it.

logician · 02-28-2007, 08:48 PM

I'd be curious how well the ignition systems work on the reduced voltage available from only a battery. The alternator will provide a two to three more volts than the battery. So the spark plugs might not fire as hot as they would otherwise.

-Juan

barney911rs · barney911rs's Garage

I have an MSD 6AL. I started off with a regular Interstate battery, and once in awhile I'd have a problem, but since switching to the Optima, runs like a champ.