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Thread: Rear Defrost Wiring

  1. #1
    GMN Regular slack's Avatar
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    Question 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 at 04:31 AM.

  2. #2

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    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.

  3. #3
    The Brown Blob 87gtVIC's Avatar
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    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.
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    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.

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    GMN Regular slack's Avatar
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    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.

  6. #6
    The Brown Blob 87gtVIC's Avatar
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    Quote 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.
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    But, that's just coming from me, this site's biggest pessimist. Best of luck

  7. #7
    I'm an air-conditioned gypsy gadget73's Avatar
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    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.
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    GMN Regular slack's Avatar
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    Quote 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.

  9. #9
    GMN Regular slack's Avatar
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    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.

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