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Rear Defroster Problems on 1983 242

whitedavidp

Member
Joined
Oct 25, 2010
Location
Near Seattle, WA
Some time ago, the original rear window defroster stopped working and I replaced it with a Frost Fighter kit - described here. As I pointed out in the final post to that thread, after a while, the Frost Fighter stopped working and it appeared that parts of the grid had burned/scorched.

After the Frost Fighter died, I decided not to replace or try to "fix" it. Instead, I decided to try using one of these heater/fan units wired to the power supply lines that fed the wire grid(s) and therefore controlled by the normal defroster switch (which I again point out has no relay like later model years but does have a light). I felt this was possible and safe because the fuse for the defroster circuit is 16 amps and the heater/fan is listed as 150 watts (which at at 12 volts DC is equal to 12.5 amps). The unit had a 3-way switch (off, on low, on high). But on low it really didn't do anything much. So I had to run it on high.

I used this a few times and it worked better than nothing. But then I started to get no illumination on the switch and nothing from the fan/heater. I tested with the switch in the on position and got no voltage at the hot wire that originally fed the grid and was now used to power the fan. The fuse was OK. So I decided to suspect the switch itself.

I managed to remove the switch and found the pretty heavy duty connector looking like it has been seriously overheated.

IMG_20250204_131740501.jpgIMG_20250204_131748965.jpgIMG_20250204_132142474.jpg

With the switched removed, I was not able to get any continuity between any of the spade connectors with the switch in any position.

IMG_20250204_132958730_HDR.jpg

So I figured the switch itself was dead as well and I proceeded to open it up. That was not a good idea as there is a ball bearing, some tiny springs, etc that have to fit in there somehow - and I am not at all sure how. But it would seem the switch was also shot anyhow.

Sadly, I do not know at what point this damage all started to happen. And I don't know if this was exacerbated by the Frost Fighter or the heater/fan unit.

I am also not sure what to do now. So I would appreciate any suggestions. Thanks!
 
From what I can read/tell, this sort of melting results from too much load through the switch/connector. Is this correct?

I understand that the actual load should be only 80% of what the wire/fuse/etc can handle to ensure that melting/fire/arcing/etc does not occur. If this is true, then I would expect that the wiring/connectors/switch are designed for at least the 16amps as that is the fuse rating. And at 12v and 150 watt load of the heater/fan unit, the load of that unit is 12.8 amps which is about 80% of 16 amps.

Given this, I am not sure why this melting and problem would occur. But obviously it has. I have no idea what the actual volts/amps rating are for the connector/switch. But I wonder if I might be able to deal with this by getting a new switch with higher load specs and re-wiring... For example, I see here a single pole, single throw rocker switch that claims to support 15 amps at 28 volts. Since the fuse is 16 amps at only 12 volts I think this means that at 12 volts the switch should be able to handle 35 amps:
  • At 28V with 15 amps: Power = 28V x 15A = 420W
  • To find amps at 12V: Amps = Power / Voltage = 420W / 12V = 35A
But is this correct thinking/computing? And is this a reasonable thing to do/try?

Thanks for your help.
 
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