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    Rear Defrost Wiring

    Does anyone know how the wiring goes for the rear defrost (asking for my '89)?

    When I did the 3G swap, I no longer have rear defrost for some reason. I checked all the fuses under the dash and they're all still good.

    I'm thinking I might have blown something out on that original charge wire that goes to the starter solenoid (or somewhere down the line). Didn't realize that wire also (for some reason, unless it actually is a different one?) also powers some of the relays on the driver's side of the engine bay that feed power to the inside of the car.

    I saw a post in my search about one of the wires being loose at the starter solenoid causing the car to only start when the rear defrost was on. I don't think that's the case here as I feel like I would have noticed other things as well if it were. Looks like as far as I can tell just the rear defrost is out.

    Here's what my fuse box looks like if needed. Not sure what #14 is suppose to be, but there's no fuse there and hasn't been since I've owned the car (#7 and #15 are the same story but I see they only have the one terminal in the plug so I figure they must be purposefully blank):


    Here you can see that yellow/gray wire on the left I had issues with during the 3G swap that I had to connect to the fuse temporarily because my starter solenoid post stripped out. This is what I thought was just the alternator charge and sense cable but apparently it also powers the relays talked about above:


    Thanks!
    Last edited by slack; 12-02-2018, 04:31 AM.

    '78 LTD | '87 Grand Marquis | '89 Crown Vic (RIP) | '91 Grand Marquis (RIP) | '94 Town Car (RIP) | '97 Town Car (RIP)

    #2
    I just happen to have a 1987 LTD CV/MGM/TC EVTM in my locker on the ship, so I'll have an answer in about 12 hours, maybe.
    My memory is that the grey/yellow wire is the battery power to the rear window heater relay IN FAIRMONTS AND ZEPHYRS, though I suspect it's the same in all 70s and 80s Fords.
    It also goes to other relays on some cars/options (defogger fan--on some F/Z cars again--your wireage may vary).
    I also recall a fusible link in that wire (solenoid end) but I'm working from a known faulty memory here.

    Comment


      #3
      87 EVTM info here..

      Fuse link E feeds a constant 12v to your rear defrost control. Fuse link E is a 16 gauge black fuse link which then connects to a yellow wire feeding the rear defrost control.

      Fuse 6 located in your interior (and only) fuse box is for HOT IN RUN power to the rear defrost control.

      According to the EVTM you may find a short section of yellow wire connected directly to the starter solenoid that then splices into a number of fusible links (F,G,H,E and L)

      Again all info above is from an 87EVTM.
      ~David~

      My 1987 Crown Victoria Coupe: The Brown Blob
      My 2004 Mercedes Benz E320:The Benz

      Originally posted by ootdega
      My life is a long series of "nevermind" and "I guess not."

      Originally posted by DerekTheGreat
      But, that's just coming from me, this site's biggest pessimist. Best of luck

      Originally posted by gadget73
      my car starts and it has AC. Yours doesn't start and it has no AC. Seems obvious to me.




      Comment


        #4
        87gtVIC got it. I'm not sure of the forum rules of posting pictures of Ford manual pages, but that's all the circuit described.

        Comment


          #5
          Awesome thanks! So that wire i connected to the mega fuse does power it.

          So, if I check that I have 12v at the 6 fuse, the problem is most likely something happened inside the dash, if it has no voltage at the fuse box, I have to go and trace from the starter solenoid to the fuse box seeing if there's a break somewhere.

          Looks like tomorrow is going to be nice again so hopefully I'll be able to run some checks with my voltmeter after work.

          Thanks again. I'll keep you all posted with what I find.

          '78 LTD | '87 Grand Marquis | '89 Crown Vic (RIP) | '91 Grand Marquis (RIP) | '94 Town Car (RIP) | '97 Town Car (RIP)

          Comment


            #6
            Originally posted by bgreywolf View Post
            87gtVIC got it. I'm not sure of the forum rules of posting pictures of Ford manual pages, but that's all the circuit described.
            We've done it a bunch of times and to my knowledge nobody has had their pee pee slapped over it.
            ~David~

            My 1987 Crown Victoria Coupe: The Brown Blob
            My 2004 Mercedes Benz E320:The Benz

            Originally posted by ootdega
            My life is a long series of "nevermind" and "I guess not."

            Originally posted by DerekTheGreat
            But, that's just coming from me, this site's biggest pessimist. Best of luck

            Originally posted by gadget73
            my car starts and it has AC. Yours doesn't start and it has no AC. Seems obvious to me.




            Comment


              #7
              Does the trunk release work ? Thats on the same key-on circuit as the defrost.

              I'd pull the defrost switch and check for voltage. white/pink is key-on, yellow is constant power, gray/yellow is output to the glass.
              Attached Files
              86 Lincoln Town Car (Galactica).
              5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

              91 Lincoln Mark VII LSC grandpa spec white and cranberry

              1984 Lincoln Continental TurboDiesel - rolls coal

              Originally posted by phayzer5
              I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

              Comment


                #8
                Originally posted by gadget73 View Post
                Does the trunk release work ? Thats on the same key-on circuit as the defrost.

                I'd pull the defrost switch and check for voltage. white/pink is key-on, yellow is constant power, gray/yellow is output to the glass.
                Thanks!

                Hmmm. After seeing your attachment, it looks like it has to be the hot at all times line going to the defrost switch. The trunk release works so the fuse box line must still be good.

                Also interesting, I have a 30a fuse there but it looks like it should be a 20a. I wonder what other ones have the wrong value.

                '78 LTD | '87 Grand Marquis | '89 Crown Vic (RIP) | '91 Grand Marquis (RIP) | '94 Town Car (RIP) | '97 Town Car (RIP)

                Comment


                  #9
                  Later than I was expecting before getting a chance to check this out again. It was/is indeed the always 12v line that goes to the rear defrost that was the issue... or it's source at the starter solenoid.

                  My positive lug is stripped out so things weren't making as good as a connection over that as I thought with the smaller eyes further down the post. I moved the wires around (which moved because it's still loose I guess) and the engine revved up a bit for a few and then came back down. When I went back in the cabin, I was able to flip the defrost on and off like it's supposed to work. For years I've had to wiggle the switch to make it turn on... looks like it has always been a bad connection that went to complete crap.

                  Tonight I picked up another starter solenoid from AutoZone (had $20 loyalty cash... so it was like $2) to throw on and replace the stripped out one this weekend. Problem solved!

                  Btw, I think the yellow and black wire that I have on my mega fuse might not actually be needed... My issue with the car not starting could have just been a coincidence with the other wires at the solenoid making a crap connection. Will have to investigate further when I replace the thing.

                  '78 LTD | '87 Grand Marquis | '89 Crown Vic (RIP) | '91 Grand Marquis (RIP) | '94 Town Car (RIP) | '97 Town Car (RIP)

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