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An Easy and Inexpensive Ignition Cut-Off Switch
 
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Pelican Technical Article:

An Easy and Inexpensive Ignition Cut-Off Switch

Time:

1 hour1 hr

Tab:

$30

Talent:

**

Tools:

wire cutter, pliers, screwdriver

Applicable Models:

Porsche 356A (1956-59)
Porsche 356A Super (1956-59)
Porsche 356B (1960-63)
Porsche 356B Super (1960-62)
Porsche 356B Super 90 (1960-62)
Porsche 356C (1964-65)
Porsche 356SC (1964-65)
Porsche 914 (1970-76)

Parts Required:

SPDT switch

Performance Gain:

Fast engine cutoff, theft prevention
When I first acquired my 356B it had a 912 engine installed along with a mechanical tachometer. Seeing how the mechanical tachometer will not work with the 912 engine, I quickly set out to convert it to electric. After studying the electrical diagrams, I soon realized that the tachometer line connects to the output from the points that drive the ignition coil. Thinking about this in detail, I soon realized that the points that trigger the coil would not work if the tachometer cable was accidentally grounded. With this in mind, it became apparent that if I placed an electrical switch that swapped the electric tachometer cable between the gauge and ground, I would have a handy ignition cutoff switch with very little extraneous wiring.

The basic electrical ignition wiring diagram for the 914, early 911 and 356 is shown in Figure 1. As you can see from the diagram, placing an in-line switch that grounds the tachometer signal will prevent the coil from firing. A thief may enter your car and be able to hot-wire the starter, but he will not be able to get the car started because the points will be grounded. The starter will turn over and over, but the car will not fire up because there will be no spark for ignition. This basically has the same effect as disconnecting the coil.

I chose to use a simple SPDT switch from Radio Shack that allows me to alter the tachometer signal cable from the gauge to ground. I placed this switch behind the dashboard so that it couldn't easily be seen from the drivers seat, or from underneath the dash. The switch works really well, and will prevent anyone from starting the car who does not know it's there.

Comments, questions or feedback?  Feel free to drop us a line.

Here's an additional thought on the subject from one of our readers, 914 owner Bill Kohnke:

Just a thought for the safety of the car and a little theft prevention.  I am a Ford salesman and through the years have to explain to many customers about fuel shut off switches that are installed in all Fords. They work in an accident situation by popping up mechanically from the force of the impact and interrupting the fuel pump circuit. Just push the switch down to reset.

I have one mounted to the firewall because if I ever oops & tag the ditch, tree, etc. I don't want the electric fuel pump to empty the tank into an already bad situation.

For theft prevention a little direct tap from a screwdriver handle and the carbs/FI will cease to run very soon or until the switch is reset. This is great because who would be looking for a 1988 - 1998 Ford part stashed above the relay box and can't be seen. The 19 year old has no idea why the car can't run long enough to leave the cellar either.

Simple Wiring Diagram
Figure 1

Simple Wiring Diagram

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Comments and Suggestions:
air cooled engine guru! Comments: 86 928s 32 valve car needs a power window motor right side and I remember that 944 cars also use a window motor just like it. does anyone know if a 944 window motor will work? and which side? same number of drive teeth?
May 23, 2016
  Followup from the Pelican Staff: I don't believe so. Give The Pelican Parts parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 
Javisan28 Comments: Would this work on a 1977 911? I've found a switch behind central console, not connected, that seems had this function and I'm planning to restore it's functionallity.
November 21, 2015
  Followup from the Pelican Staff: The article applies to:
Porsche 356A (1956-59)
Porsche 356A Super (1956-59)
Porsche 356B (1960-63)
Porsche 356B Super (1960-62)
Porsche 356B Super 90 (1960-62)
Porsche 356C (1964-65)
Porsche 356SC (1964-65)
Porsche 914 (1970-76)
- Nick at Pelican Parts
 
James Comments: i know its 2014, but i just bought a 1985late 944 Porsche...would this work on it ?
November 24, 2014
  Followup from the Pelican Staff: I think this is 911 only. - Nick at Pelican Parts  
mistachin Comments: Would this also work for a 87 911? I have an intermittent cut out where the tach drops to zero and the engine bucks as it cuts out and catches back. Cut out lasts for a second to three seconds.
November 15, 2014
  Followup from the Pelican Staff: I don't this applies to your vehicle.

Have you confirmed it is spark dropping out? I would start there. - Nick at Pelican Parts
 
bmr Comments: The horn on my 2000 Boxster goes off without me pressing the horn / air bag button. It goes off when I hit a bump or when I step on the brakes, sometimes it goes off by itself without me doing anything. Is there a horn relay that need to be replaced or could it be something broken inside the horn / airbag unit on the steering wheel. Any advise is greatly appreciated.
July 8, 2013
  Followup from the Pelican Staff: The clock spring/slip ring assembly is probably the problem - Nick at Pelican Parts  
24 Bobby Comments: i live in cali and have a 1985 grey market carrera cabrio i am buying in a lien sale. The 2 vin numbers were removed by thiefs. Is there another location where I can find a vin, besides under the front bonnet and the windsheild piller?
December 27, 2011
  Followup from the Pelican Staff: Which VINs were removed? Have you looked at the firewall? - Nick at Pelican Parts  
mclmk8d Comments: Does the wiring diagram apply to a 1987 Carrera?
October 29, 2011
  Followup from the Pelican Staff: No, it looks like it is for early cars. - Nick at Pelican Parts  
WB Comments: Wayne,
thanks for the 18Aug comeback!
WB
August 19, 2010
  Followup from the Pelican Staff: No sweat. - Wayne at Pelican Parts  
wkbergman Comments: Wayne,
thanks for the comeback 4Aug.
Yes, the length of two wires between engine and dash is an issue unless installed like in my case during a rip-apart restoration. The voltage correctly grounding is confirmed by my meter, but in fact I've not used it yet.
Is there any risk to my Bosch CD ignition from my setup, by the way?
Or from yours, as I see you installed it in a 356?
Cheers
WB
August 18, 2010
  Followup from the Pelican Staff: If all you're doing is grounding the points signal, then you should be fine. - Wayne at Pelican Parts  
wkbergman Comments: Will this work on a car [1970 911E] that had the points replaced with electronics?
I ran a double wire from the dash-mounted switch to get in between the distributor and its +12v source.
thanks
WB
August 4, 2010
  Followup from the Pelican Staff: Your double-wire should work fine, although the signal is traveling all the way to the front and then back, which might degrade it, but I'm not sure. Try it and see... - Wayne at Pelican Parts  
Porscheboy Comments: Its not as quick as flipping a switch, but on cars with a distributor I raise the engine hood, snap off the distributor cap, remove the rotor and put it in my pocket. Then I replace the cap and close the hood.
July 25, 2010
  Followup from the Pelican Staff: Yup, that works great too. Another good one is to remove the fuse or relay to the fuel pump. Sometimes that can be easier than removing the rotor. - Wayne at Pelican Parts  
Danskeren Comments: Nice trick. Will that also work on the Tacho wirering on a 993 engine ?
June 6, 2010
  Followup from the Pelican Staff: Sorry no. This will only work on cars with points. - Wayne at Pelican Parts  
W Bergman Comments: Interesting...I just spent a good bit of time putting in a somewhat hidden switch between the 12 volt lead to the distributor and its source, the little fuse panel in the left rear of the [911] engine compartment. During restoration, I had laid spare wires from front to rear, so it was not that hard. But this one is EASY! Thanks, good tip
May 17, 2010
  Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
 
scpilot3112 Comments: the dash lights stay on when the key is removed. how do you know if this is an electrical proble or the switch is bad the only way i can cut off the car is pull the fuel pump regulator NOT TOO GOOD WHAT DI I NEED TO FIX THIS and do i need to take out the switch to make the repair
John
August 26, 2009
  Followup from the Pelican Staff: which car are you talking about? - Wayne at Pelican Parts  
Richie Rich Comments: "I placed this switch behind the dashboard so that it couldn't easily be seen from the drivers seat, or from underneath the dash."

For practical purposes, how do u reach the switch to turn it off/on every time u pull into a parking lot?

Ed
May 22, 2009
  Followup from the Pelican Staff: It was all within reach, just under the steering wheel. - Wayne at Pelican Parts  

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