Alternator Question for the EE's out there

304065 · 01-24-2002, 09:12 AM

Ladies and Gentlemen:

I have to install a battery cutoff switch for PCA Club Racing. But I know that if I just interrupt the battery positive lead with the engine running, the massive voltage spike will destroy the alternator.

So, I have obtained a battery cutoff switch that has four poles, and when the throw is rotated, first one pair opens, then the other. The first pair of contacts is for the alternator, so that the field is killed, THEN the battery is cut off with the second pair of contacts, killing the ignition.

Now, the QUESTION: I have converted to a later, 84-89 Carrera Alternator that is internally regulated. If I were using an EXTERNALLY regulated alternator, I would just run the DF lead (alternator field) and D+/61 lead (exciter/warning light) through the switch, and that would kill the field. But with the internally regulated version, there is no external DF lead, only the D+/61 lead. If I interrupt that circuit, it won't matter, because the alternator field is being energized by the current coming off the diode trio ("bootstrapping"). What if I ground it, bypassing the field?

I think I need to open up the alternator and cut into one of the leads to a brush, and run that to the switch. I'd need a wiring diagram for the Valeo alternator for this.

Does anyone have other guidance? Have any of you "F" stock club racers, Carrera folks in particular, wired up a cutoff switch?

Thanks in advance for any replies.

stormcrow · stormcrow's Garage

Just curious. Is the battery cut off switch purpose to kill the engine?

Steve

304065 · 01-24-2002, 12:13 PM

Yes, the rules state that cars must be equipped with an electrical cutoff switch that will shut off the engine when it is running well above idle. In case you flip over, etc. The electrical cutoff feature will also shut off the fuel pump, if you have the presence of mind to pull the handle as you go off-track, but I don't expect that I would, and will be installing an inertia switch, like the one smartracing sells, in series with the fuel pump.

So, another way to skin the cat is to install a resistor to take up the load from the alternator-- see attached from SCCA's Northern New Jersey Web Site. . . .http://www.scca-nnjr.org/spec-corner/technotes-01.html

The easiest way to wire an OMP 6 Pole switch, (available at Driving Impressions), on most cars is as follows:
Remove the battery ground, then disconnect the positive battery cable from the starter. This will be connected to one of the large terminals on the switch. Find the smaller auxiliary lead on the positive battery cable near the battery terminal and cut it off close to the cable. This lead will be connected to the second large terminal of the switch. Attach another battery cable to this second terminal and run this cable to the starter. Most auto parts stores will make this cable up for you, just let them know how long you need it and what size studs each end attaches to. Also, from the same terminal, run a 12 gauge wire to one of the terminals marked "1" on the bottom of the switch. The two terminals marked "1" are the Normally Closed contacts of the switch. The other "1" terminal will be connected to the supplied resistor. The other side of this resistor is connected to a good ground. This resistor is what protects your alternator diodes. The resistor should be tie wrapped to the other wires and the leads insulated. You have now succeeded in disconnecting your battery and protecting your alternator, now for shutting off your motor. There are two terminals left on the switch, these are marked "2". These terminals are for the Normally Open contacts of the switch. Disconnect the wire going to the "+" terminal of your ignition coil and attach this to one of the terminals marked "2". Run a 12 gauge wire from the remaining "2" terminal to the ignition coil "+" terminal. Now all that is left to do is to mount the switch and reconnect your battery ground cable.

Not as elegant as shutting down the field, but probably better than cracking the alternator case open.

Interstingly, this problem is most often faced by boaters-- they are switching batteries from time to time, and the literature on the battery switches that include an alternator field disconnect says that they won't work with internally regulated alternators.

John

'71 E
'88 Carrera

emcon5 · emcon5's Garage

On a related note, you can score an inertia fuel cutoff switch from most junkyards. Mid-late 80's Fords came with one. On my 86 Mustang it was located right next to the latch for the hatchback, against the rear sheetmetal. It has been a long time, but I am pretty sure it was a white plastic box with a black reset button on it.

Tom

stormcrow · stormcrow's Garage

John,

Why not go with a fuel cut off switch. All electrical components would be in tact and if the fuel was cut off to the engine, it would stop running. Unless they require the electrical system to be shut down, I think the fuel cut off switch would be the best way to go.

Steve

304065 · 01-25-2002, 06:46 AM

Steve,

The rules are specific that the battery has to be shut off-- the organizers don't want a severed battery cable flopping around and creating sparks against the body sheetmetal-- giving new meaning to the phrase "ignition points."

I like the idea of a cheap inertia switch- the one from smartracing is about $60, do you know what the triggering g is? A 911 race car can probably generate over 1.0g in braking and close to that laterally-- wouldn't want the switch to shut off in normal operation.

John

emcon5 · emcon5's Garage

I don't know what the specs are, but I doubt it would cut off without significant impact. I had a close encounter with a guardrail, that did a decent amount of damage to the car, (front bumper, one fender, wrinkled hood) that didn't set it off.

Anyone have a Ford that they can check the owners manual?

Tom

911pcars · 01-25-2002, 10:25 AM

I'd think an effective fuel cutoff device would work like an air bag G-sensor; something that switches upon rapid deceleration (rather than steady state G's). Perhaps the g-forces needn't be as high as that required for an airbag. In addition, it might be important to mount it in a location and in a manner to properly sense a collision impact. And would you need another in the rear of the car as well for those tail-swapping hits?

I'm not sure the devices on the market address these issues. However, a rollover device to shut off the electric fuel pump would be a good idea.

Sherwood Lee
http://members.rennlist.org/911pcars

304065 · 01-25-2002, 11:07 AM

A quick cruise of the WWW indicates that there are many different types out there, mostly used in the aero$pace industry. Wasn't able to find the spec for the Ford switch but I will.

One other thing that occurs to me is the way that ELT's are triggered in light aircraft-- there's a conventional DPST toggle switch mounted with a metal pendulum dangling down that fits over the top of the swtich. Any significant acceleration will cause the pendulum to flip the switch on, activating the ELT. A toggle switch with a four-way throw mounted with the toggle vertical would provide not only convenient on/off in the cockpit, but I would think the toggle would slap sideways under its own weight in any kind of significant impact, (e.g. one big enough to tear through the aeroquip lines in my fuel system) and shut off the fuel. I'll look into it.

John