Appendix “J”: Adding heat to hard-back 993 sport seats.

Disclaimer: The content of this document relates to the Porsche 993 (911 1995 to1998) I am not a professional automotive upholstery heated seat installation technician so use my procedure and guidance in the document at your own risk. There are points in the process where I deviate from the standard installation instructions you may consider using my approach at your own risk. I am not a professional automotive technician and some of the opinions expressed in this document are just that, opinions, therefore, please feel free to use this document at your own risk. Many times a lot can be learned by simply watching qualified professionals work on your 993

Appendix “J”

Recently I recovered the hard-back sport seats in my 993. At the same time, I decided to add the heated seat option. During the process, I learned how shockingly easy it is to add heating to the seats in their 993. For this reason, I am recounting the process below for those who may want to add this feature to their existing hard-back seats.
These instructions are not for the standard softback seats.
Also, read this document from beginning to end before proceeding with the procedures contained herein. Contact the publisher with any questions you may have before proceeding: Contact Page.
I ordered two heated seat kit on eBay for $20. It included two heating pads (grids) each for the seat bottoms and backs, the wiring harness, a relay and a round, two heating level switch. The heating grids can be cut down to fit each seatback center cushion. It can only be cut along the one side where the internal edge electrical connections and the wiring pigtail are not present.

Removing the seat: See "how to remove front seats without drama"

Installing the Seat Bottom Grid:

• To install the seat bottom grid remove the front plastic skirt that covers the seat motors and unhook the tooth-like leather retainers along the front and sides of the bottom seat cover.
• Pull up the front edge of the cover and slide one of the uncut grids between the leather and the foam seat base facing the narrow side with the electrical harness facing the front of the seat. This is contrary to most install instructions however the cross seam tie down near the back third of the seat limits options as to pad and wiring placement
• Peel off the grids adhesive backing's release sheet and place the grid in position. Some might find it easier to position the grid first and then peel off the adhesive backing release sheet. If your seats are original you may need to run your hand under the leather to release it from the foam base as there is a thin canvas liner that may be partially sticking to the foam.
• Position the heating grid so it stops just short of the furthest back seam that transverses the seat bottom cover and so the electrical connections inside the grid are far enough forward that they wrap around the front of the seat bottom pad to an area where you do not sit on them. My feeling is that this approach avoids bumps forming in the leather and protects the wires from fatigue failure over time.
• • Be aware that one and probably more of the hog rings holding the bottom seat cover to the foam and or seam tie-down rods has probably come loose over time. Feel around for any loose hog rings clipped to the foam and remove them to avoid them damaging the heating grid as this may result in an unsafe condition.
• Reattach the seat bottom leather to button things up and route the grid wire under the seat.
• While you have the motor skirt cover off, drill a hole in the center of it clearing the motors to mount the power/heating level switch,
  
  I used a stepped sheet metal drill bit from Harbor freight to do this. Notch one side of the hole so when threading the switch housing through the hole its positioning key in its housing will keep it from rotating in the hole over time.
• Thread the wire between the motors under the seat and reattach the motor skirt/cover.

For the seatback:

• Remove the center seatback center cushion and release the bottom edge of its cover.
• Cut the grid to match the curve of the top of the cushion. It can only be cut (shaped) along the one side opposite the edge where the electrical connections and the wiring pigtail are present.
• Install the grid under the leather of the cushion so the wire protrudes from a small hole on the back far left or right side along the bottom edge. 
• Peel back the adhesive backing to the grid to hold the grid in place as done with the seat bottom.
• To finish the cushion I tensioned the leather cover on the back of the cushion using waxed thread and a curved needle threaded through the leather tails tacking it to the foam core as I do not have the metal staples they used at the factory to do this. If you prefer a completely covered finished look to the rear-facing side of the center cushion consider covering it with Velcro-compatible upholstery fabric stitching it neatly around the edge of the cushion covering the leather tails and needlework that tensions the front face of the cushion in place. Not sure it matters much as the back of the cushions is not visible
• Thread the power wire along the seat hinge in such a way that it is not subject to stretching or being pinched.

Wiring under the seat:

• Under the seat attach the grid wires and the power/heat level switch by plugging them into the harness that came with the heated seat kit.
  
• Cut the length of the power leads so they are long enough to reach the plug that powers the seat adjustment motors.
• On the wiring harness in the car that powers the seat verify the power source is active. This is done by measuring the voltage in the socket on the seat's electrical pigtail wires with the ignition in the accessory position.

• Pin one’s socket and pin two’s socket should be showing 12 Volts across them with the ignition in the accessory position and unpowered with the ignition off.
• Pin two to chassis ground two should be showing 12 Volts with the ignition in the accessory position and unpowered with the ignition off.
• Pin one to chassis ground two should be showing no Volts with the ignition on or off.
• Prepare for attaching the heater to power by soldering an in-line fuse socket to the positive (red) wire from the seat heater harness that will eventually lead to power under the seat. Put a 10 amp fuse in the socket and insulate the connection. Position the fuse socket so it can be accessed without removing the seat and tie it in position with some tie-wraps.

• The originally installed plug attached to the seat is designed to be pried open and the pins within are designed to have wires attached to them by soldering. 
• Solder the black (-) wire from the heated seat harness to pin 1 (ground) and 
• Solder the remaining wire from the in-line fuse socket's wire to pin 2 (hot) in the existing electric seat plug. 
• Be sure to solder the wires to the pins while the pins are removed from the plug housing to avoid melting the housing. 
• The pin numbers are molded into the plug and/or socket's plastic housings.
• Use care to put any dislodged pins in the exact same position they were originally before closing up the plug housing, 
• For LHD (US) cars the wiring diagram specifies position 25 in the fuse box as the position feeding power to both l&R seat heaters.  
• Consider adding an inline fuse under each seat with an amp rating equal to or slightly greater than the heating grid but not equal to or greater than the preexisting fuse in the fuse box that powers both seat heaters.
• Use lots of tie-wraps to tie the heated seat wiring harness to the seat bottom. I would tie up the relay that is part of the harness and the inline fuse, if present, to the far left or right of the  seat bottom near the rear or front of the seat. This is so it does not interfere with existing or future under seat equipment but will allow for easier access to the relay & fuse in case of its need for replacement.  Make note of the location of the under-seat relays and added fuses in your owner’s manual for future servicing.
• The entire addition is now a self-contained part of the seat.
• Position the seat in the car and plug in the two harness connections under each seat.
• Turn the ignition on and then try switching the heated seat on. The control switch should illuminate verifying that the grids are powered.
• Reattach the seats using the information on this page.

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