What is this cable?

[edmayo]

The power is on one side of the window, other side of window is ground. There is no spec that I've ever found, but my occasional testing shows that if you have around 4-6- ohms from hot side to ground side the defrost should work. They are almost all higher, indicating the painted strips are worn off.

[tom1394racing]

I test the defrost rear windshields with a 12 volt power supply and a test light. Connect the positive output from the power supply (a 12V battery will work) to the positive wire to the windshield. Connect the negative to the test light. Use the probe end of the light to check each heating element on the windshield. I have checked a dozen or so windshields and found about 75% to be bad.

[gerakroo]

Please remind that the original engine fusepanel has 2 of these connectors, one for the (2 stage) rear window heater, and one for the (optional) Sportomatic airvalve. This connector can be recognized by a blue-yellow and brown-white wire. Each 1970-1973 911 has one.

[911MRP]

I seem to remember back in the day touch up paint for element if there was an obvious break? Still available? Anyone try it?

[NorthernThrux]

Before I tackle the defroster wires, I want to check the relay works and I get voltage at the mystery connector on both detents of the fog light switch and hence all that wiring is intact.

In the frunk is a harness with 4 wires. One of those is for the handbrake/seatbelt interface, and the other 3 are white/green, white/black and black. White/green is for the rear fog, so I’m guessing the black is for the low setting and white/black is for the high setting? Am I warm (ha, ha!)?

And yes, Permatex 09117 is a conductive paint made to repair breaks in painted defrost wires.

[rvdv]

Your defrost relay is in the rear electrical panel looks like this, I am looking for a schematic for this circuit if you can find one.

[NorthernThrux]

Your defrost relay is in the rear electrical panel looks like this, I am looking for a schematic for this circuit if you can find one.

Apparently valid back to 1970. Note in the thread a well known error in the schematic that goes back to the dawn of time. http://forums.pelicanparts.com/porsc...ch-wiring.html

I noticed at least 3 of us here looking for the same info in the past few weeks!

[NorthernThrux]

Well, a full day went into getting the rear defrost to work.

Wired up the 2-stage defroster switch. Note brown (ground) goes on the bulb contact, the bulb get’s its power from contacts 2 or 3 (which are joined with a jumper). Contact 3 gets a black line in the harness, and Contact 4 gets a white/black. These go to the plug for the 2-stage relay in the engine bay.



Then hours spent wiring up the 3 grounds on the passenger side with terminals that slide onto the spade connectors at the junction points on the windshield. Likewise on the driver’s side where the top and bottom connectors were wired together (2nd stage) and the middle one got its own wire. These hot wires went to the “mystery” connector that started this thread. Ground wire goes to the body ground in front of the fuel filter. Used a soldering iron to make the holes for the ground and 2 hot wires through the seal, which pass through a hole at the very back of the driver’s parcel shelf (come out by the hinge and if you jave a good sound pad, there will be a hole in it to feed the wires from the inside to the engine bay.



This was not fun. Very little room to insert the lugs and cramped quarters back there!

Then I had to diagnose the relay as the 2nd stage wasn’t working at the mystery connector. Relay was fine, just a bit of oxidation in one of the connectors in the plug. Cleaned that up and now I can switch 1st and 2nd stage. Hooray! White marks the red/white wire. The other side is the red/black (2nd stage).



BUT, resistances are super high (about 160k ohms). As a number of you pointed out, even if the wires are intact, the thing may not work. Doubt I’ll get usable heat given that the current in each stage is 12V/160,000ohms. I didn’t notice I was on the kilo ohm scale when I first measured these yesterday or I might have not done all this. But at least now I have all the wiring in place and working and could fix the defroster with copper tape strips or some other fix or kit.

[gled49]

The rear window, seals, trim and wiring should be assembled as a unit. That’s why the wiring is so long, so the wires can be run thru package tray hole with grommet, with the window still a ways away from the body. Unless the rear window is new, it probably won’t work and I’ve never seen any repair work either.

[NorthernThrux]

The rear window, seals, trim and wiring should be assembled as a unit. That’s why the wiring is so long, so the wires can be run thru package tray hole with grommet, with the window still a ways away from the body. Unless the rear window is new, it probably won’t work and I’ve never seen any repair work either.

You are no doubt right, but I guess it was worth a shot. If nothing else, I got the relay and switch working and learned something about my car.