what does the rear window defogger relay do?

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

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

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

Ant wrote:

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