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mwerks.com Technical Features Forget traction control, VANOS, and iDrive- I can live without them. In fact, my E30 318is didn’t come with any of them. Of course, it also didn’t come with heated seats, and to me that’s just not acceptable.
Anyone who has ever spent time healing a sore back on a heating pad or slept beneath an electric blanket will certainly understand the premise of electrical seat heaters. Simply put, a length of wire is wound through a foam pad, which is mounted between the seat cushion and its upholstery. When an electric current is passed through the wire, it encounters resistance, which results in the wire heating up. By carefully controlling the current passing through this “resistor” it’s possible to safely enjoy its cozy by-product. In the case of Check Corporation, they create custom kits specifically for aftermarket applications. Not only do they offer kits that are designed to fit your car’s specific seat design, they also include all the wiring, switches, and control units to make installation as “plug-and-play” simple as possible. Installation Process There are three basic phases to seat heater installation. The first is physically installing the heating elements into the seats. After that, switches must be installed for easy access. Finally, the wiring must be added to the car’s electrical system. Installing the Heating Elements Most seat heater kits will include two heating elements for each seat; one for the bottom cushion and another for the back (or upper) cushion. The elements get sandwiched between the seat cushions and the upholstery. This requires the removal of each seat, and may also require partial disassembly of the seat itself.
In the case of our Project E30, we were in the process of reupholstering the seats, so they were already removed from the car and disassembled. For many DIY-ers, this step may be unfamiliar territory, but it shouldn’t be a daunting task. Most seats are assembled using simple hand tools (screwdrivers, Torx wrenches, etc). The only unusual tool you will likely need is a pair of hogring pliers, which you may find at some hardware stores, or pick them up from a restoration source like Eastwood. The keys to successfully disassembling seats are to work slowly to avoid breaking any trim or hardware and to take lots of digital pictures to document each step of the process. It also helps to do one seat at a time so you always have an assembled seat for reference. The first time I attempted to disassemble a seat, I also had the help of the Automotive Upholstery Handbook, by Don Taylor. It’s available at most bookstores for about $20 and will prove a valuable addition to your shop library.
With the seats out and disassembled (including the removal of the upholstery), we began installing the heating pads. Check’s pads are adhesive-backed; simply remove the backing paper, position the heating element properly on the seat cushion, and press in place. Once the pad was in place, we reinstalled the seat cover and reassembled the seat. Caution was used to carefully position the wiring harness in such a way that it wouldn’t catch on anything when it was re-installed in the car. At this point, however, the seat remained out of the car until the wiring was completed. Installing the Switches Check has done an excellent job of sourcing high-quality switches that look like factory hardware. How and where you decide to mount these will determine how “factory original” the installation looks. Our kit included a pair of 1” circular rocker switches, and we decided to locate them on the rear corners of the shifter console. Depending on your vehicle, you may be able to install them in the dashboard, and if you need to get really tricky with mounting the switches, Check also offers 6’ extension harnesses.
Placing tape on the area where the switches would be, we marked the drilling locations symmetrically on each side. The instructions called for a 51/64” drill bit, but we used a 3/4” (48/64”) bit and trimmed off the locating tabs on the plastic switch with a utility blade. It made for a snug fit, which is what we wanted. It should be noted that we used a step-type drill bit, called Uni-bit, to drill these holes. Uni-bits are designed for drilling clean holes in sheet materials like plastic and metal. A conventional large-diameter drill bit would most likely gouge the console and make for a messy hole. Consider buying a Uni-bit to do this job and you’ll have an invaluable tool for less than $25. Once the holes had been cleanly drilled, we checked the switches for fitment. Now we were ready to wire them up.
Running the Wiring There are two wiring harnesses for each seat; the first runs supply power from a positive, ignition-switched source and the second runs from the switch. Both harnesses are joined at the heater control unit, which also gets connected to the heating pads. All of this comes together under each seat, so it makes sense to run the wiring under the carpet and exit through the same opening the factory uses. To do this in our E30, we had to remove the sill plates and the rear seatbelt anchor so the rear section of the carpet could be lifted.
The switch harnesses were routed under the carpet from the center console back to about the parking brake, then dropped to each side accordingly. The power supply harnesses were routed under the carpet, and for the sake of simplicity, the passenger’s side harness was routed over the driveshaft tunnel and joined with the driver’s side supply harness, then both harnesses were run together along the driver’s side door sill toward the firewall.
In order to get power to the seat heaters, they had to be tapped into the car’s existing electrical system. Not only that, but the seat heaters needed to be wired so that they only come on with the ignition. Even though they have a 1-hour safety shutoff, it’s best not to have them drawing battery power with the engine off. The safest way to tap into switched power is to piggy-back an existing switched circuit in the fuse box.
Before we could make any connections in the fuse box, we had to get the wiring harnesses to the fuse box. Since the fuse box in the E30 is under the hood, that meant passing the harness through the firewall. We followed factory harnesses and found a rubber plug with available pass-through space. We opened the hole in the rubber block and used a coat hanger to pull the wiring from the passenger compartment to the engine compartment. After drilling a small hole in the side of the fuse box, we plugged it with a rubber grommet and ran our two positive leads through the small aperture.
We used a device called Add-a-Fuse that allowed us to tap into a circuit by simply plugging it into the designated fuse location. The original fuse was then inserted into the lower slot on the Add-a-Fuse, and the new circuit’s fuse (in this case 7.5 amps) was installed in the upper slot. The new circuit was made by butt-splicing the wiring harness’s positive lead (red) to the Add-a-Fuse tail. One fuse was required for each seat heater. The circuits were completed by joining the two ground wires (black) at a ring terminal and then securing it to good ground location. We used the through-bolt on the hood latch plate next to the fuse panel. Once all of the wiring was complete, we double-checked our routing and made sure nothing was rubbing or interfering with any of the wiring harnesses. Next we reinstalled the carpet and interior trim. Before re-installing the seats, we thoroughly vacuumed the carpets. There were plenty of shavings from drilling the console, and you’d be amazed at what shakes loose when you start moving all that trim and carpet. With the carpet now clean, we connected the seat heater control units (already connected to the power supply and switch harnesses) to the seat heater harnesses. The last step was to bolt the seats back in and test the heaters.
Call me superstitious, but I like to test new electrical equipment outside of the garage. If something does wrong and I catch the car on fire, I’d rather not take the house down with it. In the case of the seat heaters, our brutally cold winter was reason enough to take the car for a spin and actually test the effectiveness of the heaters. I’m happy to report that not only is the car still intact, but the heaters work just like all the new cars I drive. Adding seat heaters to your older car is a fairly straightforward task for most able-bodied enthusiasts. More importantly, the reward is a much more enjoyable driving experience, something that’s always appreciated in an older car.
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