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Join Date: Mar 2016
Location: SoCal Agua Dulce
Posts: 1,193
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What’s this resistor for?
X race car.
I see this resistor looking thing that’s grounded and the other end hooked up to this cutoff switch. Any ideas why this would exist? Thanks  
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1975 911 3.4 outlaw/hotrod? lsd 1988 535i 1987 535is 1973 bronco ranger |
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08-01-2025, 01:54 PM
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Never seen a battery cutoff switch with the 4 spade terminals. What are they for, no idea.
And the resistor looking thing is a 10W or 25W ceramic resistor. Can you make out its value? It should be printed on the ceramic housing. Something like 10R or similar. It’ll tell you how much power it will turn into heat: For example a 10 Ohm resistor will result in 1.4 Amp at 14 Volt battery voltage (engine running and alternator charging) and this will burn 1.4A * 14V = 19.6 Watt of power and get pretty hot in the process. If you leave that resistor connected to the battery it’ll drain a fully charged battery in about a day to the point where it won’t crank anymore. Again, not clear what this is for. In old Kettering ignition systems there was a resistor to prevent the coil from burning up when the points were closed and engine not running. But I can’t fathom what it would do related to the battery cutoff.
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1974 Targa 3.6, 2001 C4 (sold), 2019 GT3RS, 2000 ML430 I repair/rebuild Bosch CDI Boxes and Porsche Motronic DMEs Porsche "Hammer" or Porsche PST2, PIWIS III - I can help!! How about a NoBadDays DualChip for 964 or '95 993 |
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08-01-2025, 02:47 PM
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Navin Johnson
Join Date: Mar 2002
Location: Wantagh, NY
Posts: 8,777
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Thats basically so you don't fry your alternator if you shut the car down with the cut-off switch
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Don't feed the trolls. Don't quote the trolls  http://www.southshoreperformanceny.com '69 911 GT-5 '75 914 GT-3 and others |
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08-01-2025, 02:50 PM
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Navin Johnson
Join Date: Mar 2002
Location: Wantagh, NY
Posts: 8,777
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Don't feed the trolls. Don't quote the trolls http://www.southshoreperformanceny.com '69 911 GT-5 '75 914 GT-3 and others |
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08-01-2025, 02:53 PM
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Registered
Join Date: Mar 2016
Location: SoCal Agua Dulce
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Gotcha. I had to replace that switch because it was glitchy. Got one without the four terminals. How else can I wire this up?
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1975 911 3.4 outlaw/hotrod? lsd 1988 535i 1987 535is 1973 bronco ranger |
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08-01-2025, 03:00 PM
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So this is only beneficial when cutting power via cutoff switch? If one uses the ignition first to cut power then the cutoff, then there is no risk of frying the alternator?
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1975 911 3.4 outlaw/hotrod? lsd 1988 535i 1987 535is 1973 bronco ranger |
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08-02-2025, 03:09 PM
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1974 Porsche 911 Targa 3.6
