Aug 03, 2022     Seat heaters - (12 hours)       Category: Interior Finish

Was gone to Oshkosh last week. It was great to catch up with old friends!

I'm planning a Lemke interior and based on the lead time, I paid for the switches and wiring harness and got it shipped with my fabric samples. Great timing, as it arrived the day after I got back.

My panel is pretty crowded, so I had planned for the smaller 2 position switches. I didn't realize at the time, that the wire harnesses that incorporates those switches also has two relays wired in. No big deal, but the way I had it laid out, I didn't have enough space in the spar pass through holes to get the connectors through.

I looked at the way this was laid out and it just didn't seem logical to put the relays behind the spar anyway, but that's where they had to go due to them being close to the connectors where the seats plug in. Keith Rea on VAF told me thats what he had done, but when I looked at it on the bench, it seemed like that would result in a whole bunch of wires running back and forth.

I ended up cutting this bundle up, so ultimately if I had the right end plugs it would have been easier to make my own.

Anyway, what I ended up with was switches in the panel, relays right behind them fastened to an instrument panel rib with nut plates, a short run to the breaker panel, and the ground going to my forest of tabs.

This seems like the most efficient way I can run these, and resulted in only 3 wires going aft to each seat. However, it resulted a bunch of butt splices up under the panel. I also spliced the connectors for the seat end back on after routing the wiring. there, I used E splices, but with heat shrink instead of the waterproof sleeve tat cones with them, in order to make them small enough to fit through the hole in the floor pan.

Aviation grade butt splices will usually last the life of the wire bundle, but I still hate to hack up a new bundle like this. It's just a luxury accessory, but still. unfortunately this system has all custom connectors and pins, so it was either that or reinvent the wheel as far as how everything plugs together. If anything in this bundle ever fails, I'm going to gut it and go with simple on/off switches.

I got guidance on where to terminate these plugs from a couple of guys on VAF. When I asked the question they posted pictures of what they did and approximate location. In case anybody is researching this for themselves in the future, what I did was install a -4 snap bushing 1 1/2" outboard of the aft most piano hinge in the floor pan. This is a nice tight fit for the lemke wiring in the chafe sleeve it comes in, but a I said, you do have to be creative with the way you connect/splice this in order to make it work.

When I modified this harness, I made sure that I didn't cross anything up, but I didn't have a schematic on how the darn thing actually works. So after all this cutting and splicing, I put a meter on this and had a heck of a time figuring out how to ops check it.

3 wires going into two plugs, one for the seat and one for the back. plug 1= black & yellow, plug 2= red & yellow. The yellow is a pigtail that goes to both plugs so is essentially the same wire. panel switches are simple SPDT rockers, and I don't see any resistors or rheostats or anything, so how the heck does this turn into HI/LO heat settings. And what's with the 3 wires running to each seat?

The wiring schematic on Lemkes website depicts the 5 position rotary switches and a much simpler wire run but they also don't have relays involved. I'm guessing that the switches themselves have resistors or whatever built in. As I said, my smaller switches are simply SPDT, but they are 20 amp switches, so they shouldn't need relays either. what the heck is going on here?

So, after much head scratching and online research I finally figured this out; When you switch HI current runs down the yellow wire, and the heating elements are wired in parallel so they are both able to gobble up a bunch of electrons and get hot. the path back to ground is through the black wire for one element, and through the red wire and the normally closed relay pin 30-87a for the other element.

When you select LO heat, current goes through pin 30-87 into one element, out through the yellow pigtail into the other element, then to ground through the black wire. This puts the two elements in series instead of parallel, thereby doubling the resistance in the circuit. In effect, the relays cause the healing elements to become their own circuit resistors, thereby allowing for a LO heat setting.

It's clever, but honestly, I don't know that I've ever used the low setting on any seat heater in any car I've ever owned. I'm pretty much a high or off kind of guy. If anything in the wire bundle ever fails, I will likely junk about half of it and just go back with a simple on/off setup.