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 4Likes
05-15-2021, 02:59 PM
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CC Member
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Join Date: Jun 2014
Location: Dallas,
TX
Cobra Make, Engine: ERA #812 427/482 FE
Posts: 495
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Not Ranked
Battery Disconnect Switch
Hey all, I need some help. I tried to start my car this morning and got nothing. I checked the battery voltage at 12.74V, and I keep a battery tender on it. This is a newish Optima red top. I’m confident the battery is good.
I have the trunk mounted battery set up. Next to the battery is a disconnect switch, which has the positive battery cable attached, From the other side of the switch the cable goes to the starter.
I checked voltage at the cable on the starter and get about half a volt. The only thing in between is the disconnect switch.
Is it possible the disconnect switch has gone bad? I use it quite a bit but have never had an issue before.
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Chris
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05-15-2021, 03:29 PM
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CC Member
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Join Date: Jan 2009
Location: Las Vegas,
NV
Cobra Make, Engine: Shelby CSX4005LA, Roush 427IR
Posts: 5,584
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Not Ranked
Did you just bypass the disconnect switch?
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Cheers,
Tony
CSX4005LA
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05-15-2021, 04:15 PM
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Half-Ass Member
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Join Date: Jun 2005
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
Posts: 22,005
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Not Ranked
Is it a simple, moron-proof switch like mine? Or is it fancy and complicated....
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05-15-2021, 04:33 PM
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CC Member
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Join Date: Jun 2014
Location: Dallas,
TX
Cobra Make, Engine: ERA #812 427/482 FE
Posts: 495
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Not Ranked
I bypassed the switch and everything came to life. It is hard to get to, and the old man paper thin skin on my arm is Now bruised and bleeding.
It is the disconnect that ERA installed with a key. I guess I’ll talk to Peter P. On Monday to get a new one, unless someone has a better suggestion/brand.
I never thought a simple part like this would fail.
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Chris
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05-15-2021, 04:59 PM
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Half-Ass Member
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Join Date: Jun 2005
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
Posts: 22,005
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Not Ranked
Summit Racing has dozens of battery disconnect switches, most sound like the style you have. https://www.summitracing.com/search/...nnect-switches Now, considering I don't have a trunk mounted battery, and you can see my simple knife switch above, others will have better input. But I do like a switch where I never have to guess if it's open or closed.  |
05-15-2021, 06:07 PM
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CC Member
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Join Date: Dec 2010
Cobra Make, Engine:
Posts: 53
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Surprised at you guys, especially Patrick. Disconnects should always be on the battery ground cable. Drag, road race, Nascar.
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05-15-2021, 06:29 PM
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Half-Ass Member
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Join Date: Jun 2005
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
Posts: 22,005
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Not Ranked
Quote:
Originally Posted by patty442
Surprised at you guys, especially Patrick. Disconnects should always be on the battery ground cable. Drag, road race, Nascar.
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I have one on the negative side too.  |
05-16-2021, 07:08 AM
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CC Member
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Join Date: Apr 2009
Location: Little Rock area,
AR
Cobra Make, Engine: ERA Street Roadster #782 with 459 cu in FE KC engine, toploader, 3.31
Posts: 4,521
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Not Ranked
If it’s one of those green knob disconnects, some of them do go bad over time and loose continuity when engaged. Others I’ve had have worked well. So quality is sort of hit or miss. The knife type like in Patrick’s picture are pretty much dead reliable.
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05-16-2021, 10:08 AM
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CC Member
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Join Date: Dec 2003
Location: Gurnee,
IL
Cobra Make, Engine: Kirkham #259
Posts: 1,396
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Quote:
Originally Posted by DanEC
The knife type like in Patrick’s picture are pretty much dead reliable.
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And dangerous as well.....any time you have a large open switch like that you need a cover to prevent an accid3nt.....that could never pass a Trans Am or SCCA tech inspection.... 
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Morris
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05-16-2021, 10:34 AM
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Join Date: May 2002
Location: SF, Bay Area,
CA
Cobra Make, Engine: SPF832, 466cid
Posts: 501
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Quote:
Originally Posted by Morris
And dangerous as well.....any time you have a large open switch like that you need a cover to prevent an accid3nt.....that could never pass a Trans Am or SCCA tech inspection.... |
Or military, medical device. |
05-16-2021, 10:38 AM
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Half-Ass Member
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Join Date: Jun 2005
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
Posts: 22,005
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Naaah, you just slip the inspector a couple of C-Notes and you're good to go. |
05-23-2021, 09:28 AM
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CC Member
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Join Date: Sep 2018
Cobra Make, Engine:
Posts: 30
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I've had two of the "green knob" go bad. They refuse to provide the proper ground and make it look like a bad battery. And yes, these should be between the negative terminal and ground earth. NEVER put on the hot side.
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05-23-2021, 10:07 AM
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Half-Ass Member
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Join Date: Jun 2005
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
Posts: 22,005
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Not Ranked
Quote:
Originally Posted by panteralee
NEVER put on the hot side.
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OK, how do I reconcile that with the NHRA rules?  Uhhh, like this one:
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05-23-2021, 10:38 AM
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CC Member
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Join Date: Dec 2015
Posts: 1,442
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Quote:
Originally Posted by patty442
Surprised at you guys, especially Patrick. Disconnects should always be on the battery ground cable. Drag, road race, Nascar.
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I would read up on those rules again. I just checked Nascar, SCCA, NASA, NHRA and all say positive. I did come across a kid on a Bemmer site that said negative so there's that. |
05-23-2021, 10:49 AM
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Half-Ass Member
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Join Date: Jun 2005
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
Posts: 22,005
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Not Ranked
Quote:
Originally Posted by spdbrake
I would read up on those rules again. I just checked Nascar, SCCA, NASA, NHRA and all say positive. I did come across a kid on a Bemmer site that said negative so there's that.
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What?!? You're double checking my cites on me?   |
05-23-2021, 11:08 AM
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CC Member
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Join Date: Dec 2015
Posts: 1,442
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I would assume the 58-67 VW 6vdc positive ground cars would have a negative cutoff switch.
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05-23-2021, 11:33 AM
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CC Member
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Join Date: Aug 2013
Location: Canandaigua,
NY
Cobra Make, Engine: SPF MKII Riverside Racer FIA
Posts: 2,499
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My race car is set to SCCA and Vintage racing standards, and yes the kill switch is to the positive terminal.
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05-23-2021, 11:39 AM
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Half-Ass Member
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Join Date: Jun 2005
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
Posts: 22,005
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Not Ranked
Quote:
Originally Posted by 1795
... Vintage racing standards
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Well what could be more vintage than dropping a spanner on an open-air cutoff knife switch that is mounted inches away from a grounded aluminum firewall? |
05-23-2021, 12:10 PM
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CC Member
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Join Date: Dec 2003
Location: Gurnee,
IL
Cobra Make, Engine: Kirkham #259
Posts: 1,396
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If you disconnect the negative cable... you will not shut off the motor.....because the alternator will use its chassis to have a ground and keep the motor running....
That is why the a Sanction bodies want the positive side disconnected so that you kill the power coming from the Battery and on those disconnect switches is a smaller set of contacts that disconnect the power to the alternator so the motor shuts off.
That’s the first thing they check when you go to the tech shed....
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Morris
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05-23-2021, 02:39 PM
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CC Member
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Join Date: Aug 2013
Location: Canandaigua,
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Cobra Make, Engine: SPF MKII Riverside Racer FIA
Posts: 2,499
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As Morris states, if you do not wire the battery disconnect with correctly, when you engage the kill switch on a running engine, it will keep running. The first time that I wired the kill switch I tested it on a non-running engine and the engine wouldn't start and I thought that I had done it right. Then when the SCCA tech session came and I had to use the battery disconnect to stop a running engine it did not, as the alternator kept the engine running. Back to the paddock and time to re-wire. Not making that mistake again.
Jim
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