Subject: Seat Heater Repair 101 

    To: "Huw Powell"  
    From: "Jeff Goldberg"  
    Date: Sat, 4 Sep 1999 17:31:34 -0400 
    Reply-To: "Jeff Goldberg"  



    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