converting from dual to single stage defrost?
 

So, I just received a rear glass with single stage defrost, and it is in pretty good shape. I am doing a back date and this "new" replacement glass will be going in my '76, which had a two stage defrost. I've done a lot of searching and haven't found exactly the information I need.

This post is pretty informative: http://forums.pelicanparts.com/porsche-911-technical-forum/143220-where-do-rear-window-defrost-wires-unplug.html
I have confirmed the wiring for dual stage. The wires are still in my old seal, and I have the red/black going to the lower element/jumped with a black wire up to the upper element. Red/white wire going to the center element. Brown ground wires going to the appropriate elements on the passenger's side. It matches with both the graphics in the above post.

To convert, I would think I would just need to choose the red/white lead and one of the brown leads, and connect them to each side of the glass. But my switch is still a dual stage unit. Is this something I need to be concerned about? What about the relay? Is there a different relay for a single/dual stage defrost ( I searched for this too)? Is it necessary/possible to switch from one to the other if they are different?

Thanks,

~Chris

morning bump

I have found some additional info here: http://forums.pelicanparts.com/porsche-911-technical-forum/565756-rear-defrost.html

Still looking for input though.

~Chris

From what I have seen the single stage was eliminated before your year. There is no relay socket for single stage, only the 2 stage on later cars. I would think there would be no problem just using the R/W wire from the 1st stage to connect to the heating element and safeing off the R/bk for the second stage. The brown wires are just common grounds.

There is mention of the 2 stage relay in this thread. I do not know how accurate it is regarding the second stage.
http://forums.pelicanparts.com/porsche-911-technical-forum/740918-question-rear-window-heating-elemnt-wiring.html
Here is the diagram from 1973 showing the defroster hooked up as a single stage using the R/W.
http://forums.pelicanparts.com/uploa...1375459923.jpg

http://forums.pelicanparts.com/uploa...1375459689.jpg

Ed: thanks, I think you've cleared it up for me. I think I will go so far as to disconnect the white/black lead from the switch to the relay.

~Chris

I have the reverse issue - I have a '69 Coupe with a single rear window defrost relay, but the rear window is from a later car with the two stage wiring.

Can I just run the two window stages in parallel off the single voltage output of the relay? Or will I be drawing too much current for that relay? I'd rather not have to re-wire the car for the dual stage relay or add another relay in parallel with the one that's in the car.

Thanks for any help as always.

Bill

^^^^ Any thoughts?

I don't know the amperage rating of the relay for single stage heating, but there was a feature in the 2 stage relay that the single stage relay does not have. The 2 stage relay is designed so that the first stage is always on when the switch is on, but the 2 stage relay has a voltage switch built in so that when voltage drops to 11V or less the second stage does not operate, and comes back on when voltage rises to 11.2V and above.
This would prevent excess power draw when idling as the early alternators did not have a lot of amperage.

Interesting, I could not figure out why that circuit had that feature, but makes sense.

I still have an early alternator in my car, so I may need to find a two stage relay. I will measure the currents first to see what I'm dealing with.