Seat Heater Repair 101

["Jeff Goldberg" <fourkguy@aol.com>]

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