pp000830

WARNING: 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.
Recently I recovered one of my 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 seat. For this reason I am recounting the process below for those who may want to add this feature to their existing hard back seats.
I ordered a heated seat kit on eBay for $20. It included two heating pads (grids), the wiring harness, a relay and a round two heating level switch. The heating grids can be cut down to fit the seat back center cushion. It can only be cut along the one side where the internal edge electrical connections and the wiring pigtail are not present.

• First put a drop of penetrating oil on each of the seat mounting allen head cap screws and give it a few days to soak in.
• Raise the seat both front and back to its highest position
• Remove the bolts that hold the seat down. Be sure to use the right size allen wrench and care to not strip the screws when releasing them. Note that the front screws have two washers under them as the screws have a little unthreaded neck that the two washers space for.
• Unplug the seats power seat and seatbelt warning light pigtails and lift the seat out of the vehicle. The seat is heavy and best removed with the back folded forward using some cardboard to protect the door threshold from scratches if you bump it.
• 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
• Peal the release tape over the adhesive edges of the grid and place the grid in position. 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 have 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.
• Remove the center seat back 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 along the one side where the internal edge electrical connections and the wiring pigtail are not 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.
• Button the cushion back up. I closed it up using waxed thread and a curved needle as I do not have the metal staples they used at the factory to do this.
• Thread the wire along the seat hinge in such a way that it is not subject to stretching or being pinched.
• 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 to 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 by measuring the voltage in the socket on the pigtail the seat wires attach to.

• The plug attached to the seat is designed to be pried open and the pins within are designed to be soldered to. Solder the black (-) wire from the headed seat harness to pin 1 (ground) and the red wire to pin 2 (hot) in the existing electric seat plug. Be sure to solder the wires with the pins 1 & 2 removed from the plug housing to avoid melting the plug.
• 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. The factory fuse is a 25A fuse . Seeing that the fuse that came with the kit is 10 amp and you added heating to only one seat, you can either install the additional 10 am fuse under the seat or consider lowering the amp rating of the fuse in the fuse box to 10A. If you add heating to both seats the safest approach is to just adding an additional 10A fuse under each seat. Ideally placing it where it can be checked without unbolting a seat. I measured the Ohm resistance and calculated. in my case, that both seats combined draw a little too much for a 10A fuse combined but less than 15A so for two seats in addition to the under seat fuses I would put a 15A fuse in position 25 in the fuse box.
• 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 to the far left or right of the seat so if one adds an audio component under the seat at a future date it will not interfere. Make note of the location of the relay in your owner’s manual in case the system need servicing in the future.
• 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 the driver's seat, the passenger seat only has one harness connection.
• Turn the ignition on and then try switch the heated seat on. The control switch should illuminate verifying that the grids are powered.
• Bolt the seat down using care not to mutilate the mounting bolt socket heads. Use two washers under the front cap screws as were there originally. Be sure to have each washer's side with the smooth edges face up as screwing the cap screws into a washer with the rough stamped face up can score the fillet where the shaft of the screw meets its head significantly weakening the cap screw and so the seat's safety; an important rule when installing fasteners with washers in general.

bcameron59

Nice writeup! Did you take any pics of the process by any chance?

Wondering if this would be basically the same thing for standard seats, or if you specify the hardbacks because they are different / easier?

Also, when you reach step 5 or 6, is the seat foam cushion fully exposed? My driver's side foam feels like it has compacted over the years and needs refreshing.

pp000830

Some pictures added;
Answers to questions;
Soft back seats are a little different I would think because the seat back cover is all in on piece. More of a roll it up like a shirt cuff, fit the grid and roll it down vs the hard back seat.
Yes the foam is fully exposed.
Replacement foam is a standard porsche part number or the one you have can probably be augmented to fill it out.
Andy

nile13

On a regular 993 seat the leather is actually sown to the padding. I investigated adding heaters some years ago but because of this design it was not trivial/possible.

It looks like the switch is attached to the front motor cover? I'd put it somewhere more visible. Many install it into the center console plastic piece surrounding e-brake.

pp000830

Wow, I looked in the OE seat catalog and you are correct there is no OE part number for separate foam seat forms. The approach would be to separate the two or buy aftermarket foam for the seat center sections and replace the glued in foam. Somehow I just do not see it as too difficult as unlike the sides of the seat, in the center sections where the grid goes the foam is not contoured. Basically flat with grooves at the stitch lines. I agree with you as a complete novice would be in for an experiment with a good learning curve to sort the job out.

The switch seems to wok out well as it is a three position rocker down is low heat. center is off and up is hi heat so the switch can be actuated by feel. My goal was to make the added heating self contained to the seat and avoid running a switch harness out to some other location. I do see your point and you approach could be easily done as well.

Andy

nile13

It's not glued it, it's actually sown in if my memory from 10+ years ago serves me.

Now, nothing's impossible. You can try to carefully cut the seams, though I believe that they are the same seams that are visible on top of the seat and you _really_ don't want to be cutting those. You can install the pads under the foam, not sure f much heating would get to your butt that way. I'm surprised that hardbacks are constructed differently in that respect.

The self-containment is a good idea. Heated/cooled Recaro Trends in my first 993 had switches in the side and that worked OK. The current solution, heated 996 seats, have switches that for now are tucked between the seat and center console. It gives me some weird confidence to see the Hi/Lo lights and also making visually sure that switch is OFF. not a real necessity, but just nice to see at a glance.