Rear Window Defroster Current Question

[Superman]

The Bentley schematic is wrong, both for the '78 I have now and for the '83 I once had. The schematic shows a single wire that energizes the rear window defrost wires in the rear window. In reality, the system is two-stage. Nineteen conductors mounted into the glass, total. The middle 9 are in one circuit. The top 3 and the bottom 7 are in a different circuit. The fuses (one to supply the outside rear view mirrors and the other to supply the rear window) are 25-amp, which scares me because of what I want to do.

There is a spade conductor broken off from the window, making it impossible to energize both the top 3 conductors and the bottom 7 at the same time. I could make it all work if I can use a single wire to energize the top 12 conductors at once.

Bottom line: Each of the rear window defrost "+" wires (for the two circuits) energizes at least 9 of the window conductors (one energizes 9 conductors embedded in the glass, the other wire energizes 10 conductors). My plan is to repurpose one of those wires (part of the chassis electrical system feeding the rear defroster) to feel 12 defrost conductors embedded in the glass.

I don't think that is too much to ask of a wire....is it? The wire is at least 12 ga, it appears. I could measure the resistance of the combined 12 defrost conductors, if necessary. IN fact, I could calculate the resistance of each conductor. Again, the factory designed this wire to handle 9 of the defroster conductors in the glass. I think it could handle 12 conductors. Anybody have a concern about that?

[Superman]

For the electricians: I've got automotive voltage (12-14 V) going into a circuit with 1.8 Ohms. How much current is this? The fuse is 25A. Will this tax that fuse?

[Superman]

It looks like this might be 2.79 amps. If so, then I think I'm good.

[schoward]

Originally Posted by Superman

The Bentley schematic is wrong, both for the '78 I have now and for the '83 I once had. The schematic shows a single wire that energizes the rear window defrost wires in the rear window. In reality, the system is two-stage. Nineteen conductors mounted into the glass, total. The middle 9 are in one circuit. The top 3 and the bottom 7 are in a different circuit. The fuses (one to supply the outside rear view mirrors and the other to supply the rear window) are 25-amp, which scares me because of what I want to do.

There is a spade conductor broken off from the window, making it impossible to energize both the top 3 conductors and the bottom 7 at the same time. I could make it all work if I can use a single wire to energize the top 12 conductors at once.

Bottom line: Each of the rear window defrost "+" wires (for the two circuits) energizes at least 9 of the window conductors (one energizes 9 conductors embedded in the glass, the other wire energizes 10 conductors). My plan is to repurpose one of those wires (part of the chassis electrical system feeding the rear defroster) to feel 12 defrost conductors embedded in the glass.

I don't think that is too much to ask of a wire....is it? The wire is at least 12 ga, it appears. I could measure the resistance of the combined 12 defrost conductors, if necessary. IN fact, I could calculate the resistance of each conductor. Again, the factory designed this wire to handle 9 of the defroster conductors in the glass. I think it could handle 12 conductors. Anybody have a concern about that?

No but I wonder if the 3 pack will seem to get more juice and defog than the 9. My system works but I notice the bottom 7 section seems to be the weakest, paired with the top 3. I remember someone saying electricity takes the shortest path (or maybe least resistance), so both hooked up may result in the shorter grid getting more action. Nothing much you or I do if the case - just how it is. Go for it.

[RDM]

Snowflake,

I = V/R, so 14V/2.8 ohms = 5 amps. If that's all that's on the circuit, you are golden.

The Targa window has two lugs on each connection on each side of the window, which makes it possible to piggy-back two wires onto the one side of the lug if the other is broken. I assume (by your lack of bragging about having a Targa) that you have a Coupe. Are the lugs arranged the same way on your window?

[86 911 Targa]

^^^^^
You have what is known as a two stage rear window defogger.

The defogger elements are basically a short.

And, the top and bottom elements are a parallel circuit.

R1xR2/R1+R2= Total resistance.

pm me as needed.

Best,

Gerry