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Old 03-21-2021, 08:37 PM   #1
whitedavidp
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Default Rear Window Defroster (In name only)

I have checked around and am still not quite sure how to proceed. Maybe I missed something in another thread but...

1983 242. Rear window defroster doing nothing - no effect, no lines in the grid showing even partial joy. I see no relay in the wiring diagram for this year and the switch is not a momentary switch - just a simple on/off with a light that does light up when turned on.

In the back seat with my meter set to sound on continuity, power off, defroster switch off. I put the probes on both side tabs simultaneously and get a sound. Is this continuity from the grid? Or something else?

In the back seat with my meter set for volts DC, power on (but engine not started), defroster switch on. Dome light was on when I switched on the defroster and there was no dimming of the bulb as I have read might happen. With probes on both side tabs simultaneously, I get 11+ volts. So the switch seems to be working, power is being delivered, and it seems that there is good ground somewhere.

Should I conclude there is something wrong with the grid wires at this point?

A visual inspection from inside reveals nothing obvious - like breaks in the grid wires. If I understand properly, ones that are broken will not heat but should not affect unbroken ones. Yet none seem to provide any defrost.

A visual inspection from outside shows what might be oxidation - like small spots of discoloration on the wider strips to which the tabs connect. Nothing looks completely rotten or broken. I guess this might be expected for a car this old. I don't see this on the thin grid wires but it is much more difficult to see as they are much thinner. Are these conductors embedded inside the glass? I was under the impression they might be applied to the glass.

Can anyone suggest further tests/actions I can perform to try and isolate the problem and get defroster working again? I am pretty lame with electronics. But can follow instructions.

Thanks in advance.
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Old 03-21-2021, 10:07 PM   #2
nel621
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The wires are surface mounted. Inspect the wires closer. There is a break somewhere. It can be repaired with copper conducting paint. Also with adhesive copper tape. I'm sure there are other products.
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Old 03-22-2021, 03:48 PM   #3
whitedavidp
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Thanks indeed for your response. Here is a progress report... With regards these instructions given to me elsewhere:

o Connect one of your voltmeter test leads to a good chassis ground (you can use the right side defrost tab if that is convenient).

o With the defrost switch ON, try probing with the needle into the grid near the left (driver side) tab. If you don't see any voltage there (but still do at the tab connector itself) then that tab is your chief suspect.

o If you do see voltage there then try probing into the right side of the grid. If you still see a bit of voltage over there then that confirms (at least some of) the grid is okay and the problem is likely at the right tab.

I managed a good ground in one of the holes in the rear deck where the clip that fastens down the carpet is missing. I was able to confirm this ground with continuity sound to the right side tab on the window.

With engine running and defroster switch on and lit up, I get 13v at the left side tab and, with a sewing needle clipped into an alligator connector, I have probed up/down the majority of the left side strip under the tab getting voltage at pretty much every location. This makes me think the connection between the left tab and the window is ok.

When I look for voltage at the right side strip under the tab, I seem to find nothing. But I do note that in at least a few places more proximate to the left side, I get some voltage response when probing some of the grid wires directly. So I conclude that at least some of the grid wires still have a connection to the left side strip. But this suggests that virtually all of the grid wires are kaput somehow in their attempt to reach all the way across the window.

With regards this instruction:

o Try probing into the grid at the right side. If you see a hint of meter action then that suggests the right side tab is okay. Probe over on the left side of the grid and if you still see a bit of continuity then that confirms the grid is okay and it's likely at the left tab.

With engine stopped and defroster switch off and the same ground arrangement as above, I probe for continuity sound up/down the strip under the right side tab I get the sound pretty much every where I probes with the needle. This suggests that the connection between the right tab and the window is ok too. When probing the grid wires themselves proximate to the right side, I find virtually no continuity sound.

So now I am left trying to probe the grid wires themselves. This isn't easy so I first did a close visual inspection of all the wires, all the way across the window. I do NOT see any apparent breaks in the wires of any kind. But I do note that the width of the wires is uneven. Some wires appear to be thicker/wider than others. Some wires appear to be thicker/wider on the either the left or right sides (or both) than in the middle of the window. I am not sure if this is normal/expected or not. But I simply do not see any complete breaks through the wires at all.

I should be clear that when using the needle, I tried very hard to NOT scratch the wires - only to point and press in. And due to the quite thin appearance of the grid wires in some places, my aim may have been less than ideal. But I do think I found enough decent probings to feel that my report above is accurate enough.

Does any of this sound familiar or explicable?

Thanks!
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Old 03-22-2021, 05:00 PM   #4
bobxyz
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This used to be a very common failure with the early VW Beetles. They had the same direct switch setup with no disconnect timer. The defrost grid would be left on for days at a time and would eventually burn out. I'm not sure if they're still around, but try frostfighter.com for repair kits/grids.

Looking at an '82 wiring diagram, the bulb in the switch is connected across the tabs of the heater grid. I'm guessing that you're measuring the bulb resistance (with switch off) instead of the grid resistance. You could try removing the bulb and re-test the resistance across the grid.

It's been ages, but try turning on the defrost grid and then connecting a test lamp to one side of the grid. With a bit of aluminum foil in between, slide the tip of the test lamp along the grid wires and see when the lamp turns on or off (at a broken grid wire).
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Old 03-22-2021, 05:09 PM   #5
whitedavidp
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Quote:
Originally Posted by bobxyz View Post
This used to be a very common failure with the early VW Beetles. They had the same direct switch setup with no disconnect timer. The defrost grid would be left on for days at a time and would eventually burn out. I'm not sure if they're still around, but try frostfighter.com for repair kits/grids.

Looking at an '82 wiring diagram, the bulb in the switch is connected across the tabs of the heater grid. I'm guessing that you're measuring the bulb resistance (with switch off) instead of the grid resistance. You could try removing the bulb and re-test the resistance across the grid.

It's been ages, but try turning on the defrost grid and then connecting a test lamp to one side of the grid. With a bit of aluminum foil in between, slide the tip of the test lamp along the grid wires and see when the lamp turns on or off (at a broken grid wire).
Thanks. Your suggestion will be tried. Never would have thought of that test.

frostfighter.com does still exist and I will check it out further.

Since it seems, so far at least, that the wire grid is the source of the problem, is it possible to get/install a new one and attach the existing + - wires to it? I would hate to have to replace the entire window just to get a new grid.

Cheers
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Old 03-22-2021, 05:18 PM   #6
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For the VW bugs, there were overlay repair grids available, but IIRC they didn't have a good reputation. I'm not sure if more modern repair grids are available and work well. For my '69 bug, I had the windows out already for painting, so I just found a good rear glass instead of trying to repair the completely burnt out grid (as in every wire was toast).
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Old 03-22-2021, 05:43 PM   #7
whitedavidp
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Quote:
Originally Posted by bobxyz View Post
For the VW bugs, there were overlay repair grids available, but IIRC they didn't have a good reputation. I'm not sure if more modern repair grids are available and work well. For my '69 bug, I had the windows out already for painting, so I just found a good rear glass instead of trying to repair the completely burnt out grid (as in every wire was toast).
Hello again Bob and thanks! I had a 1967 bug and sold it while living in Boulder for grad school long ago! It had no rear window defroster. But I think that 12 volts only came to the bugs in 1967.

I really enjoyed performing your foil and test lamp test! It was downright entertaining. The results are these:

All along the left (powered) side I got pretty much continuous light as I moved up/down along the vertical strip that is under the wire connection.

However, as I moved away to the right an inch or two at a time and moved up/down across the individual grid wires I found that only a few got light at even 2 inches away from the left side strip. None of them got any light by half way across the window.

I cannot say 100% for sure, but at one point crossing one of the grid wires maybe 2-3 inches from the left side strip I thought I witnessed a small wisp of smoke rise from the foil and disappear. That was way cool but not likely an indicator of anything good :-)

It does seem that frostfighter.com offers just their stick-on kit. I MAY have to resort to something like that. But I suspect application with the window in place will be difficult. It might be better to just get a new rear window with a new defroster grid on it.

Thanks so much!
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