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Theft Prevention
By Steve Grosekemper
You have gone out for the evening and parked you car in a secure, well lit parking structure. As you walk to your destination, you hear an alarm screaming in the distance. Do you run towards the sound
of the alarm? No, you ignore it and just keep on walking. Don't kid yourself and think anyone else is going over to investigate the alarm either. Twenty years ago, when an alarm went off, people
noticed (and even came running to investigate). Today, every car from a brand new 996 to a 15 year old Yugo have got alarms, and no one is paying them any attention.
The truth of the matter is that today's car thief is smarter and better equipped. If he wants your car bad enough, he will get it! Fortunately, he is also lazy and has a short attention span. My plan
is to make your car too much trouble to bother with. If he can't get your car started, or even better, thinks it is broken, he will move on to the cars of those guys who didn't read this article, and
steal it.
The problem with most alarms is that they will fail and keep you from driving you car, more times than the thief. I have come up with a few devices that will keep your car from being started when you
don't want it to be started.
The first version--kill switch--is my favorite because when activated, it gives the impression that the car is broken. This version is for '78-'83 911SCs. The over rev protection for this car is
controlled by the fuel pump relay and engine speed control unit. When the engine reaches 6800 rpm, the speed control unit sends a ground signal to the fuel pump relay, shutting off the flow of fuel.
If we tap into this same wire and feed a ground signal to the relay, the car won't run. It tries to start, but dies when the key is released from the start position. This is how it works:
When you turn the key to the start position, voltage from the starter enters the relay at pin #87 (see diagram #1).
It then travels through the activated relay to the fuel pump. The relay gets activated from the ignition switch voltage at pin #87a and 86, along with the ground provided by the air flow meter
switch. The engine is now running and pulls enough air through the fuel injection airflow sensor flap to open the grounding switch. With the relay no longer activated, power from the ignition switch
enters the relay at pin #87a and travels to the fuel pump (pin #30), making the car run.
If we create another ground at pin #85, the relay will never energize and the car will start and die... start and die...start and die. This will quickly frustrate any thief and he will move on to that
other guy's car.
Note: '76-'77 911s and all 930s use this same relay without the speed control unit and can be similarly wired.
The actual wiring is very simple. Locate the fuel pump relay and cut open the outer insulation to expose the individual wires. Locate the two brown wires with red traces. These wires come from the
speed control unit and air box switch and go to relay pin #85. Solder in a ground wire, re-insulate it and run the wire to a hidden ground switch. When the ground switch is activated, the engine will
start and die... start and die.
The second version of switch is even easier to install but does not give the broken car impression. It is for '84-'89 Carreras. The fuel injection relay is energized from the ignition switch along
with the ignition coil. This connection takes place at a power lug on the fuse box between fuses 10 and 11. Locate the two wires. They are black, one 2.5mm wire powering up the coil and one 1mm wire
powering up the DME relay (see diagram #2). Separate the wires and reconnect the 2.5mm wire to the power hub
along with a new black wire. Run this new wire to a small switch in the car. Run another black wire back to the remaining black wire at the fuse box (1mm), solder and insulate this wire. When this
switch is off (open position), the car will not start. It will only crank and crank until the thief moves on to, well, that other guy's car.
This same principle can also be applied to other DME controlled cars like 944s and later 911 versions.
Earlier 911s with CIS fuel injection ('73-75) have no fuel pump relay. They are directly wired from the ignition switch to the fuel pump. This third version kill-switch can be used on these cars as
well as most other later cars were you will want to shut off the fuel pump (914, 944, 928, etc.).
For this version we will use a factory Porsche relay kit. This consists of a relay plug (part #901.612.333.00 - $28.49) and a round black relay (part # 911.615.109.01 - $19.20). When installed, this
will have a factory appearance as seen in pre-1989 911s fuse boxes.
When this relay is installed power will flow through the relay from pin #87a to pin #30. If the fuel pump is connected to pin #30 and power to the pump is connected at pin #87a, every thing will
operate normally (see diagram #3). However, if pin #86 and #87a are connected together and pin #85 is
grounded, the fuel pump (or whatever is connected at Pin #30) will not operate. Pin #87 should not be connected to anything (see diagram #4).
A grounding switch can be installed to the interior of the car to activate the relay when desired. The biggest advantage to this type of switching is that a bad relay can never keep you from starting
your car as the relay off position starts the car.
For earlier cars that have carburetors or mechanical fuel injection we must turn off the ignition. The residual fuel left in the carburetors/fuel injection is more than enough to make a sufficient get
away. If the car in question has an electric tachometer, a wire may be run from the tach signal wire (black/purple) to a ground switch. When activated the ignition will not operate. For cars with
other after-market ignition systems (or no electric tachometer) the version three relay kit mentioned above may be utilized. This relay would then be used to turn off power to the ignition coil
instead of a fuel pump.
WARNING: Repair or modification of electrical systems should be left to qualified personnel. If you do not feel comfortable doing this repair yourself, consult your Porsche professional.
Good Luck!
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