what does the rear window defogger relay do?

[Ant]

Hi

I'm fixing the rear window defogger in my 72'SB and I wonder what does the
rear window defogger relay do? do I really need it? what happens if I
remove it?

Ant

[Speedy Jim]

Post by Ant
Hi
I'm fixing the rear window defogger in my 72'SB and I wonder what does the
rear window defogger relay do? do I really need it? what happens if I
remove it?
Ant

You only need the relay if you plan to use the defogger.
The relay (Yay! Relays rule!) allows a very small current from the
switch on the dash to control a large current to the grid.
Without the relay, a heavy cable would have been needed from
the dash switch.

Speedy Jim
http://www.nls.net/mp/volks/

[Hal]

Post by Ant
Hi
I'm fixing the rear window defogger in my 72'SB and I wonder what does the
rear window defogger relay do? do I really need it? what happens if I
remove it?
Ant

I have a 74 super but I will assume the function of the relay is the
same in your 72. You have a switch on the dash. Instead of running a
fairly heavy gauge wire from the battery up to said switch, and then
back the grid, they run a smaller gauge wire from the switch to the
relay. The smaller wires have no problem carrying the small amount of
current needed to turn the relay ON and the heavier gauge wires only
have to make it from under the back seat to the rear window to power
the grid. On my car there was also an indicator light on the speedo
head to tell you when the defroster is on. If that bulb was run in
series with the switch(and I don't know since I haven't looked at the
schematic) then if the bulb were to burn out, the circuit wouldn't
work.

In short, if you remove the relay you won't have a rear window
defroster.

Although the system in my car isn't intact, I have seen pictures that
show a fuse holder identical to the one used for the reverse lights
installed on the circuit that goes to the window grid. I'm not sure of
the rating, 15 amp seems to sound about right.

Chris

[Ant]

Of course I'm not planning to remove the relay, it was just a theoretical
approach, but thanks for all the info. Now I know that the defogger doesn't
work, and since I'm dying so much to drive it I will forget about it and
wait until winter for the window to fog up and swear loudly about my
lazyness. :)

Anyway, do any of you know of a decent after-market defogger useable in a
SB?

Ant

Post by Ant
Hi
I'm fixing the rear window defogger in my 72'SB and I wonder what does the
rear window defogger relay do? do I really need it? what happens if I
remove it?
Ant

[Leo Yudysky]

I had the same problem with a rear defogger...
It too didn't work, relay worked , it clicked on fine, had voltage to the
grid.
On closer inspection with a ohmmeter I found all, every one of the grid
lines was open!
Looking at the grid with a good light and magnifying lines reveiled that the
copper conductive lines were dissolved or eroded. I since heard that
washing your window with any ammonia based cleaner will dissolve the grid.
I bought some splicing dope from a parts store and thought I could 'paint '
the lines back on, but quickly gave that up.
What I did was buy a whole new window! Wasn't cheap but I needed that
defogger to work badly...in Montana its winter about 9 months out of the
year so a clear back window is nice. Think I paid about $165 bucks for a
new window with grid.

Cheers...
Leo from Montana
p.s., my red '68 bug is for sale
--
Leo Yudysky
'68 bug
'69 bug

Post by Ant
Of course I'm not planning to remove the relay, it was just a theoretical
approach, but thanks for all the info. Now I know that the defogger doesn't
work, and since I'm dying so much to drive it I will forget about it and
wait until winter for the window to fog up and swear loudly about my
lazyness. :)
Anyway, do any of you know of a decent after-market defogger useable in a
SB?
Ant

Post by Ant
Hi
I'm fixing the rear window defogger in my 72'SB and I wonder what does the
rear window defogger relay do? do I really need it? what happens if I
remove it?
Ant