freediver

I am adding a battery cut-off switch to my 928 and am wondering how to cut off the alternator field so the engine does not keep running. Any thoughts? Please don't respond with "cut the field wire" because I see a lot more than one wire coming out of this thing. Thanks.

checkmate1996

Well on mine, if you do a battery cutoff at the rear of the car, there IS NO power running to anything. The long B+ cable goes to starter then alternator.

Mrmerlin

if you add the cut off then it will open the whole electrical system best if added to the rear of the car near the battery.
Dont run the car with the battery disconnected

tailpipe

i put mine where the battery ground cable attaches to the body on my 79 this way i only have to remove the back pannel to turn the battery off i used one of the battery cuttout switches from a side post battery just bolted the cable in one side and the body bolt through the other side just spin the **** and no circuit i got mine at princess auto here in canada but i'm sure harbour frieght or some place like that would carry them

freediver

Ok. I probably should have mentioned a few, er, changes we've done. It is a '79 (25th production car built) and so might be a little different. All of the stuff that was unnecessary for racing (heated seats, all soundproofing, heater, a/c, dashboard, etc) has been discarded. The battery and tremendously heavy battery box have also gotten the heave-ho. Battery was replaced with an Optima in a special mount behind the passenger seat.

I am hoping that someone with a bit more specialized knowledge of the 928 charging system can direct me to the specific wire to interrupt. Just cutting the power from the battery does not stop the engine.

Alan

You cannot do what you asked about on a 928... since the regulator is integrated in the alternator you cannot control the field current.

Never try to use a battery isolator on the battery ground to turn off the engine - it won't work (alternator will still run the car) and may damage your ECU's due to overvoltage - the battery is an important ballast element in the power generation circuit - switching it out of circuit when running is a very bad idea...

Your best option is to place a cut-out switch between the alternator/battery and the CE panel - e.g. in series at the jump post is a good place

On a race car its easy to rewire the ECU feeds to place such a switch at the rear by relocating the wiring to the ECU's via the battery or remotely by using a relay to control the ECU's and using a field disconnect terminal set to drive the relay - this way has the advantage of battery isolation also

I do know of an overvoltage protection circuit that "crowbars" the exciter circuit to shut down the alternator. However its designed only as a last line of defense to protect the ECU/electronics and if it were activated it would very likely destroy all the field generation diodes in the alternator. Bit too much of a one time solution to be practical...

Alan

RKD in OKC

When I got my GTS it had a battery cut off at the jump post. Pretty sure is was to disconnect the battery for storage. The connection to the cut off kept coming loose and shutting down the car so I had it yanked and everything wired up like stock.

freediver

Thanks, Alan. The rewiring project is slated to take place this winter in my newly heated garage. There is still a lot of excess wire here and there (20 lbs or more?) and I will tackle rewiring the ECU to the cut-off then. I guess its time to start drawing up a new wiring diagram.

Alan

This is not an ideal location physically or functionally. Its hot and corrosive and there is little space, it also will only isolate the battery from the fuse panel - not the alternator, ABS, PSD, ECU's etc. exact items depends on year - but many.

It does have the advantage that it will stop the engine "safely" (electrically).
However if its installed poorly this fact may also be a problem (as here) - since it isn't really all that safe to loose the engine when travelling at speed....

Alan

2002maniac

I think i know how it should be wired. Someone please correct me if i'm wrong. Because it is a 4-pole cutoff switch. You need to wire the 2 small poles in the ignition circuit. Thus, when you turn the switch, the battery is disconnected and the ignition circuit is broken (same as turning the key to the off position). Obviously this switch would only be used in an on-track emergency.