Re: Seat Heater Repair 101

[nwlove <dloves@myhome.net>]

Any clue why the heat will not turn off? Had to remove the fuse even after
trying a new thermo. switch and by the way, the other side will not go on at
all.

tia
Tom
'85 4KQ

Jeff Goldberg wrote:

> Huw,
> I always hated homework assignments, but after you took the time to set me
> up with the required knowledge to diagnose my seat heater problem, you asked
> that I document my findings for the list and your webpage. Since I owe you
> and the list an awful lot of thanks, it is my pleasure to report I have
> fixed my driver's side heater and this is what I found.
>
> After removing the seat from the tracks and opening the wire binding around
> the bottom of the seat upholstery ( 86 4KCSQ - sport seats), I peeled back
> the upholstery from the seat bottom and twisted open the hog rings one by
> one with a wire cutter and screwdriver.( When I was done I closed them by
> squeezing with a pliers.) I did not approach the seat back because that
> circuit had continuity.
>
> Once peeled back, I located the wires to the seat heater at the rear center
> of the seat cushion. The relay clicked when I touched the wires which told
> me the problem is in the wire connections to the seat cushion. A black wire
> with a yellow stripe supplies the 12v power to the cushion heater. The
> ground for this circuit is brown with a blue stripe. Both wires are soldered
> to the pad at opposite ends of the element wire. There is also a black wire
> with a white stripe soldered to one side of a button sized "thermostat"
> (looks like a small hearing aid battery & is glued over a wire in the seat
> element). The other side of the thermostat has a brown wire with black
> stripe soldered to it. The black/white wire carries 8 volts from the relay.
> As the thermostat is heated (I tested with soldering iron) the relay reduces
> voltage to black/white and eventually opens, eliminating current to the main
> circuit (black/yellow). This black/yellow wire feeds current through the
> heater element and in series feeds the brown/blue which returns to the plug
> under the seat. The plug  has a connecting loop that powers the seat back
> element and returns to the plug which brings it to ground.
>
> With the wisdom I have been so fortunate to gain from this list, I inspected
> the entire heater pad (after cutting back the foam cushion backing) and
> found only 2 burn spots. One at the thermostat and one where the heater
> element terminates at the solder joint for the brown/black wire. I
> resoldered the element wire to the solder joint after stripping some
> insulation and I resoldered a loose connection to the thermostat. This
> repaired the circuit. If a wire is loose from the thermostat button, that
> will open the relay and the heater will not receive power, even if the
> element is in good shape, so this is a good thing to check first. The relay
> clicks when the wire is attached / removed from the thermostat.
>
> When checking voltage at the plug ( relay side) this is what I found when
> everything was working OK.
>
> Black / White  8v
> Black / Yellow  12v
> Brown / Blue (loop) 6v
> Brown (2)  Ground
>
> Thanks to all who have helped with this project as well as all the others.
> Hope to undertake the passenger seat soon.
>
> Jeff
> 86 4KCSQ Daily Driver
> 86 4KS      Spare