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rear defrost test? 200 1989

posted by  Yaleh on  Sun Nov 11 11:00 CST 2007
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last visit: Wed May 7 20:35 CST 2014
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Hi,

Just recently replaced the whole trunk door with a "less" rusty one (let's hope POR-15 is for real).. But I noticed the defrost light doesn't illuminate anymore (almost positive that bulb is okay)- there's a clicking noise by my feet? The relay I presume, and 12V to the black spade from the demist. Demist ground is good. Maybe I'm being paranoid, but before the weather finally turns really ugly (Toronto), I'd like to be sure that it works. I checked for continuity (100%) and looked closely for little breaks (I don't think I saw any). I'm not too knowledgeable in electricity, so does this all sound good? What other sorts of electrical tests can I make or other tests?? I have a basic digital multimeter. I guess the question is: how does one test an heat element for wear?

Thanks!

Yaleh.








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    rear defrost test? 200 1989

    uid 25912's pic
    posted by  Sven's Maintainer on  Sun Nov 11 14:50 CST 2007
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    last visit: Sun Sep 16 20:44 CST 2012
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    I don't think the bulb tells you much about the health of the rest of the circuit.

    Usually those little indicator bulbs are fed by the same output that powers the main device (wired in parallel) - but the bulb has its own ground nearby. If device is dead the bulb will still light, and if bulb is dead the device will still work if it's OK.

    I'd swap in another bulb to test. Likely the same bulb as the fog lamp switch, auto trans position indicator bulb, and seat heater indicator bulbs.

    When working on the tailgate wiring in my '89 245 I noticed that it appears there are two hot wires for the rear demister. I think maybe just one wire leading to the junction under the roof, but there it splits into two wires running thru the left side hinge to run the top and bottom half(s) of the grid. I can't swear to that but I think it's so.

    Then pop the tailgate inner panel and have a look at the two ground screws near the center of that panel. I had problems caused by one of those screws loosening.
    --
    Sven: '89 245 NA, 951 ECU, expanded air dam to 7' from ground, forward belly pan reaches oem belly pan, open-front airbox, E-fan, 205/65-15's at 44 psi, IPD sways, E-Codes, amber front corner reflectors, quad horns, tach, small clock. Wifemobile '89 245








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      rear defrost test? 200 1989

      posted by  Yaleh on  Mon Nov 12 04:39 CST 2007
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      last visit: Wed May 7 20:35 CST 2014
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      Thanks for the reply-

      I too noticed that there were 2 wires. I had to also rebuild the loom since of course the wires were all broken at the hinge. I swapped out the tiny bulb and it's illuminated now.

      Thanks!

      Yaleh.








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      rear defrost test? 200 1989

      posted by  Rule 308 on  Sun Nov 11 17:36 CST 2007
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      last visit: Tue May 8 09:20 CST 2012
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      If you have a mutlimeter then do a simple amp test on it. Open up the circuit and install the meter inline, this is to say that you have one lead going to the defrost circuit disconnected and going to your meter, have the other lead of your meter going to the defroster circuit and then turn it on. It will now read exactly how much current your grid is using. A simple test light or volt meter will tell you if you have current going to and through the defroster grid but the ampmeter will tell you how much current.

      Mark








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        rear defrost test? 200 1989

        posted by  Yaleh on  Mon Nov 12 04:50 CST 2007
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        last visit: Wed May 7 20:35 CST 2014
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        Hi, thanks for the response.

        So it looks to me that (from investigating the fuses) there's one circuit to drive a relay (8 amp?) and one for the defrost element (16 amp?). So you are saying I put my meter inline ahead of the element and take an amp reading? Is that how it works?? So the thing I'm a bit confused on in my meter. There's one setting that says 10A (10 amps range I assume). Then there's another area with three spots that says collectively DCA and mA. Each spot in this group then reads sequentially: 2m, 20m, 200m.

        So if I go with the 10A will my meter die? Assuming that this is a 16 amp circuit. Also I noticed that it looks like I have to move the probe over to a different spot for the 10 amp and it says "unfused, max 10 sec each 15 min".. Sorry if this is basic meter knowledge!!

        Thanks!!

        BTW, this is the meter I'm using:

        http://www.canadiantire.ca/browse/product_detail.jsp?FOLDER%3C%3Efolder_id=1408474396672448&bmUID=1194886153159&PRODUCT%3C%3Eprd_id=845524442132041&assortment=primary&fromSearch=true








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          rear defrost test? 200 1989

          posted by  jdsullivan on  Mon Nov 12 16:00 CST 2007
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          last visit: Fri Jun 22 13:20 CST 2012
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          Yaleh,

          If you want to know whether each trace of the grid is heating, get a spray bottle and spray a fine mist of water on the outside of the window. Then hit the switch. During the next few minutes, the mist will evaporate over the traces that work, but remain over those that don't. You may have to find the right angle to look at the window in order to see what is going on.

          If all the traces work, feel fortunate.

          If none do, you probably have a circuit problem.

          If some do and some don't, you need to find and fix the breaks in the bad traces. That is very doable, and there are lots of earlier posts on the subject, so use the search function to become expert before you start.
          --
          jds








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            rear defrost test? 200 1989

            posted by  Yaleh on  Tue Nov 13 16:42 CST 2007
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            last visit: Wed May 7 20:35 CST 2014
            [RELATED]

            Thanks jdsullivan,

            I'll try the mist trick. Good idea..

            I also went through and checked voltage at the start, middle and end of each line. Looks like 2 lines are broken at least, which doesn't sound too bad. Gonna go over to crappytire and see if they have a kit for the fix.

            Regards,

            Yaleh.







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