Adding a ignition switch controlled accessory outlet or other accessory to a 993

The existing cigarette lighter outlet on a 993 is live all the time even when the ignition is off. This is a great feature if you want to charge or maintain your battery through the outlet. 

Unfortunately, it is the older larger diameter outlet that modern accessories fit poorly into and sometimes have trouble making a reliable connection to. Also not being switched makes it a real bother as anything plugged into it needs to be unplugged when parking the car if you want it to shut off.

The simple route to adding a switched outlet is to attach an aftermarket surface mount outlet somewhere out of sight under the dash similar to the one pictured to the right. Then attach it to a SWITCHED high-amperage circuit already present in the vehicle. How to simply wire such an outlet is described below.

Putting an outlet in the unused Dash position on USA-delivered cars:

Some folks have put an outlet under the dash others have rigged up something that goes inside the ashtray both good minimally invasive options. 

Here are the steps to put an outlet in the dash in the European foglight switch position not used on USA-delivered cars, a very handy location:

• Remove the radio and its mounting frame, the hardest part, who would think? The mounting frame may get bent up in the process. Don't worry it can be straightened out with large pliers before reusing it.
• Drill a small pilot hole in the center of the facia ring of the unused switch position, then slowly and carefully, using a variable speed drill on low speed, just drill through the pilot hole with the stepped drill/reamer that has a maximum diameter of 1-1/8"(28.5mm)
  The most important part of the whole job is to use a stepped drill/reamer where its absolute maximum diameter is 1-1/8" exactly the size of the new socket's barrel and not a larger drill where 1-1/8" is just one of the drill's intermediate size steps. This is key in not over-sizing the facia as the drill penetrates further into the dash to drill the sheet metal behind it.
• Continued drilling very slowly at low RPM until the entire 1-1/8" bit passed all the way through the steel sheet metal. 
• Run a vacuum cleaner hose below the hole to capture the debris during drilling.

The result should be very clean by avoiding damaging the very thin and delicate facia by having a fresh very sharp drill/reamer and drilling very slowly.
Once the bit engages the steel hole, you may find it not perfectly centered behind the circle on the facia. Just keep on drilling letting the drill center itself in the steel hole. This should not create a problem later when the socket is installed.

Once the outlet in the dash is installed a volt meter

can be kept in it and now my 993 has a nice clean-looking volt meter gauge our cars should have come with, in the first place.

Powering the new outlet: 

The simple way is to attach the outlet to a circuit that is controlled by the ignition key referred to as a "switched" circuit.  The alternate and more complex method is to use an unswitched circuit in the car and convert it to a switched source using a simple relay. The relay method is generally used when one wants to add a new circuit directly attached to the positive terminal on the battery as its source and still have it switched. This is a common application when high-draw items such as high-power amplifiers are added to the audio system.

The Simple Way:

Wiring a high amperage switched power source from a switched circuit:

Supplies needed:

For the electrical connections, devices, and supplies:

• A suitable 1-1/8" outer diameter plastic threaded barrel accessory outlet with a retainer nut;
• An inline-fuse holder, generally come prewired with pigtails; 
• Two colors of 14 Gauge copper stranded automotive primary wire, 
• Insulated crimp-on female spade connectors, 3M Scotchlock wire splices, and crimp-on butt connectors (Splices) all made to work with 14 gauge wire and the new outlet's male connection spades; 
  
• A wire stripper/crimper pliers; 
• A 75-watt trigger-on soldering iron and small gauge rosin core solder if you plan to solder electrical connections. A 10 to 30-Watt pencil iron won't produce enough heat to solder the 14 Gauge wire connections. 
• 3M type 33 or 88 electrical tape and tie-wraps. The 3M tape goes on very cleanly and doesn't unravel or get nasty with age as inexpensive tape does.
• A $7 Volt/Ohm meter from Harbor Freight can be useful.
  
• All the above can be sourced from eBay, Amazon, or really any automotive supply house that sells electrical accessory items.
• Use the copper stranded 14 Gauge automotive grade primary wire because it can generally withstand sustained loads between 10 & 15 amps, a lot of amperage load, and more than most anything you would plug into an outlet and stranded wire withstands vibration and flexing something solid core wire is not very good at. 
• Use the two colors of wire one for connections to positive the other for connections to negative/body ground. Porsche used brown wire for negative/ground connections in the 993 and combinations of lighter colors for positive ones.

Here is how to wire the connections where a high-amperage switched circuit is used to power the outlet and no relay is required:

New Accessory Outlet Positive (+) terminal goes through an inline-fuse and then tapped into the switched high-amperage circuit.

New Accessory Outlet Negative (-) terminal goes to a steel body ground on the car.

Here is a page on protecting wires when added to the car:

https://993servicerepair.blogspot.com/2020/06/squeaks-rattles-and-rolls.html

See the section at the bottom of this page for guidance on how large an amperage-rated fuse to use.

On my 1996 USA Left-Hand Drive Carrera, the fuse box label lists the following high-amperage switched circuit that can be tapped into not requiring a relay: 

• Seat Heater both sides #25 rated at 25 Amps, switched NOT REQUIRING a relay,
•  Use a 5 to 10-amp fuse in the inline fuse socket you will be adding, 
• A nice connection point because the under-seat connection plug is designed to be disassembled and having an added wire soldered to Pin "2" which is the hot (+) pin for heated seats internal to the seat's connection plug. 
• If the car doesn't have heated seats, the wiring for it is still present, the fuse in the fuse box becomes dedicated to the new outlet. 
• If your 993 doesn't have heated seats and you chose to use this as your tap-in point for power without adding an inline fuse replace the fuse in position #25 in the fuse box with a 5 to 10-amp fuse. Also, add a note in the fuse box or owner's manual that you have done this and why.
• See the section at the bottom of this document to help in the correct sizing of the fuse added to this circuit.

WARNING the above information about fuse location in the fuse box may be different for Right Hand Drive cars and cars delivered to countries other than the USA so for cars delivered outside the US check the label in the fuse box on the specific car.

The alternate solution for the Engineers among us:

Wiring a high amperage switched power source directly from the car battery using a relay to provide an ignition switch controlled circuit:

The primary reason to lay a new wire directly to the battery for an accessory is if you plan to power high draw items such as a large power amp or inverter from the circuit. Running a new dedicated wire to the battery avoids unnecessary stress on the existing wiring harness. In most cases, these sorts of accessories exceeded the amp carrying capacity of readily available accessory outlets and should be hard-wired or a special high capacity connector should be used to connect them to the power-supplying circuit. 

Supplies needed in addition to those listed for the unswitched installation above:

• A Bosch style 12 Volt SPDT automotive relay with socket & wire pigtail if the source
  
  
  of power to the socket is an unswitched circuit such as the constant-on wire to the radio below or the existing constant-on cigarette lighter outlet. 
• The reason why a relay is needed is that the switched power from the ignition switch or the radio's switched power wire is not designed to support the high-amperage demands of the new outlet directly but can control a constantly powered high-amperage circuit supplying the outlet through a relay. 

• The relay can be sourced from eBay, Amazon, or really any automotive supply house that sells 12-volt electrical accessory items.
• Use the copper-stranded 14 Gauge automotive grade primary wire if the circuit will be fused at up to 15-amps or 12-gauge wire for a 20-amp fuse. The stranded wire withstands vibration and flexing something solid core wire is not very good at. 
• Use the two colors of wire one for connections to positive and the other for connections to negative/body ground. Porsche used solid brown jacketed wire for negative/ground connections in the 993 and combinations of lighter colors for positive ones.
• Here is a page on protecting wires when added to the car:

https://993servicerepair.blogspot.com/2020/06/squeaks-rattles-and-rolls.html

The below diagrams are for a Bosch style single-pull-double-throw relay meaning having a single switch (pull) that has two positions (throws) such as I used. It is one of the simplest, most common automotive relays. It is made by Bosch and any number of other makers.

Wire connections where a constant-on circuit controlled by a relay is used to power the outlet:

• Bosch Relay Pin 85: Goes to a wire from the ignition or the radio's switched connection wire.
• The radio harness switched wire goes to Block A, Pin 7 on the radio's plug, 
• See here for more detail.
• Bosch Relay Pin 86: Goes to a body ground somewhere in the car. A  good location is the body grounding point (nut) under the front edge of the passenger seat.
• Bosch Relay Pin 30: Goes to the plus "+" connector on the New Accessory Outlet. 
• Bosch Relay Pin 87: Goes through an inline fuse and then attached under the clamping bolt or nut on the battery's positive terminal connector using a suitable-sized ring terminal.

  

  Ring Terminal
  
  

  
  The fuse should be near the battery end of the wire to protect the entire length of the wire from the battery.
• New Accessory Outlet Negative (-) terminal goes to a steel body ground on the car.
• Bosch Relay Pin 87A is not used but if it is prewired the end of its wire should be taped over with a little electrical tape to avoid it contacting things it shouldn't.
• Run a wire of  large enough gauge to carry the amperage load expected;
• Install the in-line fuse on the battery end of the long wire run not to exceed the amperage capacity of the wire or the accessory socket used; See the section below for guidance on how large an amperage-rated fuse to use.
• Use care when running the wire so it is not subject to movement from vibration and is protected from abrasions and cuts. This is particularly important due to the typically high amperage capacity planned and because it is a battery-direct connected circuit.

Sizing a fuse to protect the outlet and circuit wiring:

• Picking the right size fuse for your newly added accessory socket: The purpose of the fuse is to protect the wiring in the car from a malfunction of the accessory plugged into the outlet. The new accessory outlet circuit should be fused with a fuse of a lower rating than the parent circuit you are attaching it to: 
• Ideally, the fuse should be the lowest amperage value available that is equal to or;
  higher than the amperage requirements of the accessories you anticipate using with the new outlet 
• For reference: 
  
  • A PC USB charger draws 1.2 Amps. 
  • An iPhone charger draws 2.4 Amps. 
  • A Retina iPad draws about 4.7 amps. 
  • A small150 Watt power inverter draws about 14.7 amps at 12 volts, a very large load, beyond the capacity of plastic accessory outlets, we are discussing here so a dedicated outlet and a battery circuit with adequate capacity wired through a fuse and directly to the car's battery terminals should be considered.
• Never use a fuse with a rating higher than the amperage rating of the outlet, 
• Never above the amperage capacity of any of the current-carrying existing wiring harness wires used or added to the wiring harness or the added relay if one is used. The current carrying capacity of various gauges of wire can be looked up on the Internet,
• Never the same or higher than the amperage rating of the fuse in the parent circuit you are tapping into. If the fuse is the same size or higher capacity than the parent circuit's it really offers no added electrical protection to the circuit from things plugged into the new accessory outlet that malfunction or draw excessive current. 
• Never ever put a bigger fuse in the fuse box to up the load capacity of the parent circuit you are tapping into as the existing wiring in the circuit is generally sized for a continuous load a little lower than the circuit is originally fused for and you don't want to melt the wiring harness.
• Look up the fuse size on the circuit you plan to tap into in the fuse box to get a feeling for how many amps it can support in total. 
• Accessory outlets for sale are generally marked as positive or negative on their connection spades. Wiring one inadvertently backward will damage what you plug into it so make sure it is attached properly plus(+) to positive power through a relay or directly and Negative (-) to Negative body ground. 
• Use tie-wraps, twist-ties, and/or electrical tape to tie the wires out of the way. This page contains some additional insights when adding new wiring to your 993.

Now for a little pure opinion:
I don't like using crimp connectors as I feel they can loosen up and the wire may dislodge especially in a high amp carrying circuit, say greater than 10 amps. For this reason, I solder all the wire splices and the crimp connectors for reliability that will outlast the car. Many folks don't go through this extra precaution. If you do use crimp connections without soldering make sure they are sized properly, use the proper crimping pliers to ensure maximum reliability and the wire doesn't come free and cause a short. Also, consider electrical taping over the crimp connections so if the wire is dislodged it is held captive where it can't cause a short to ground.

I give this job a 3 to 5 on a 1 to 10 scale, depending on if you go the battery direct route in your installation,  On the scale a one is replacing a tail light bulb and a ten is replacing the steering rack. A good beginner DIY project!

Andy ;-)

Update: 

Recently I added an additional USB outlet under the dash to the side of the center console to power a dashboard Camera.

 Since the wiring for the previously installed outlet was right there I just tapped into the wires leading to the switched cigarette lighter outlet and I was good-to-go. For the connection, I purchased a prefabricated 15-foot USB cable to tuck around the windshield, down the door jamb, and under the dash to plug into the added outlet.  The installation was low-effort, easily reversed if desired.

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