Heated Rear Glass Dissertation

[porschedude996]

Primer:
My 1983 911SC with two stage Heated Rear Window/Glass. During reinstallation of my rear glass, I forgot to mark the wires of the connector that needs to be disassembled to get the wires from the cab to the electrical panel on the engine compartment. Two hot wires, and the ground is separate from the connector. I find no connector representation in any of my data sources.

So I had to put on my Systems Engineer Hat on and dissect or bisect the system.

Physical:
There are three sections to the heating grid on the inside of the window. The upper third and the lower third are tied together in parallel. The center third is by itself. The system is controlled by a switch and a relay.

Assumption:
The center section is Defog, with a lower current draw, and the top and bottom are the Defrost with a higher current draw. See current and resistance comments below.

Verbose:
The Owners Manual explains that the Control Knob by the cigarette lighter, both twists and pulls. Manual says turn to Defog and Pull for Defrost. I’m guessing Defrost condition is colder the a Defog condition. There are three sections in heaters on the back glass, separated horizontally. The upper third and the lower third of the glass are tied together, while the middle section is on its own. I believe the center section is for the Defog circuit, which is half the size of the Defrost. mode. The center section has a resistance value of 3.3 ohms, and the combined upper and lower section measure 3.0 ohms. There is a relay that senses the output of the alternator, voltage I believe. And would turn on and off the Defrost mode depending on the availability power. I believe the Defrost mode requires a higher amperage. So 3.9 amps for Defog, and 4.3 amps for Defrost??? Seems minor to address 4 amps to toggle half the total full power draw. A modern alternator would hardly notice.

Operating Modes: So really three modes.

Mode 1. Center section controlled by switch (Rotate) and relay. Knob turned Clockwise, middle Defog section is powered.
Mode 2. Upper and Lower third are controlled by a relay that would interrupt power should the Alternator not have enough output for the two sections. Relay controlled by Pulling out on the Knob.
Mode 3. Knob Rotated CW, and Knob Pull out, all three sections would heat.

So if someone in the future would ever find my ramblings, they could identify which conductor goes where in the very easy to disassemble connector. They to have chosen a connector that is so easily disassembled because it needs to be removed to get the glass in and out. The hole doesn’t accommodate the connector.

Conclusions:
The circuit is over designed for today, and maybe yesterday year as well.

The driver should know that there is a difference between Defog and Defrost? And that critical thinking must be accomplished to operate the system. (Sound German?) Should I run Mode 1, Mode 2, or Mode 3?

Summary:
Run in Mode 3 and “Forget-About-it” if the rear window is obscured. Run the wiper if that doesn’t work. Quit looking in the Rear View mirror and DRIVE!


Physical Wiring:
I believe this to be true. Looking at the Body Half of the Connector, with the Post Hole (key) on the top of your view, the Defog wire (Red/WHT) is on the Left, and Defrost wire (Red/BLK) is on the Right. Correspondingly, at the Window half of the connector, looking at it with the Post (Key) at the top, the (Red/BLK) wire for the Defrost is on the Left, and the (Red/WHT) Defog wire is on the Right.

Closing Thoughts:
Either way, it will probably will never know the difference between which way it is wired. Personally I don’t drive my car in inclement weather. I live in California…

[REVerend]

Interesting work through of the problem… until reading your post I wasn’t even sure how to turn the rear glass heat on in my ‘83 SC, never have needed it. But this is one of those great posts about this place because at some point I’m sure someone is gonna actually need this info.

But in conclusion… what’s behind me doesn’t matter

[evan9eleven]

Thanks for going in-depth on this one. It wasn't until I replaced my rear window seal that I became aware of the different modes of the rear defroster... great summary!

[porschedude996]

Thank you both. I like Systems Engineering, was one in my professional career. Aerospace Airplanes and Satellites. Especially Reverse Engineering, taking an existing system and dissecting what others have built.

[wazzz]

I can't find any connection between this relay and the alternator on schematics. Can you point to it?

This 2-stage relay appears to be the same part number as the one used on later models, like Carrera 3.2 up to 89. The electronic part inside seems more like a timer that controls both stages (defog and defrost). On later models (Carrera), the switch became a simple On/Off push button. When pushed out (active when pushed out, German logic), both stages go on and the three heating elements are powered, then after a while, outer elements are timed out and only the center element remains on.

[GH85Carrera]

Interesting difference on my 85 Carrera.

If I pull the switch, it defrosts or defogs the rear window. If I also turn the knob, it defrosts the outside mirrors. I have only needed to defrost my mirrors three times in 30 years of ownership.

When the rear window is frosted, it for sure melts in the middle, and then the top and bottom catch up just a litter later.



And that is when having the full original heat with footwell blowers is wonderful. The car will get HOT inside in short order, and melt the defrost or de-snow the windshield.

Great engineering on your part to figure the system out.

[porschedude996]

Originally Posted by wazzz

I can't find any connection between this relay and the alternator on schematics. Can you point to it?

This 2-stage relay appears to be the same part number as the one used on later models, like Carrera 3.2 up to 89. The electronic part inside seems more like a timer that controls both stages (defog and defrost). On later models (Carrera), the switch became a simple On/Off push button. When pushed out (active when pushed out, German logic), both stages go on and the three heating elements are powered, then after a while, outer elements are timed out and only the center element remains on.

You are correct. I got the notion that the generator supply entered into the operation by what the owners manual states. I have the factory manuals with electrical schematics, but in looking at them on this subject, I do not have an ‘83 schematic. It stated it is controlled by the availability of power. The ‘82 schematic has only the single control and single element shown. I can confirm I have three conductors, red/wht, red/blk, and ground (brn). The red/blk isn’t depicted in any of my data. But that I wrote it based on my available data and the statement in my ‘83 owners manual. There must be a later design that I’m missing. I do find single and two stage rear glass in the porsche parts manual (PET).

Here is the page from my owners manual:

[wazzz]

Description in the manual is indeed strange. I can't see where there would be a link between power (battery voltage?) available and the defrost/defog system. Here is the schematic for model year 81 where the relay (actually a double relay) appears with the overcomplicated switch. This is from the Workshop Manual.
The red/black lead is the one between relay pin 87a and the two heating elements connected in parallel. Red/white is the one between relay pin 87 and single heating element.

[TastyReuben]

thanks. This will be useful, part of my dashboard / delete and overhaul is removing the stock defrost switch and replacing with a simple toggle. Where to place the new switch is still up for debate.

[porschedude996]

Thanks Wazzz, that is the schematic I was looking for. Again yesterday I cracked my Section 9 and didn’t see this schematic. Do you have a copy of it that is a higher resolution?

[wazzz]

Here is the original pic, it's a screenshot I took yesterday from the Workshop manual, which is not much better.

But I also have this one in French which is of better quality.
You can download them from the hosting website. If you cannot access them, pm me your email and I'll send a copy.

[porschedude996]

Thanks to Gilles input, I think I figured out how the circuit turns the dual circuit window grid on and off. The transistor in the middle of the two relay coils opens and closes the ground side of the right coil. The transistor circuit within the box doesn’t show any resistors that would tailor the threshold value. Functionally, if the voltage drops below a certain value, the ground on the right hand coil would open. The schematic was never intended for micro circuit troubleshooting. It shows that it is only a PNP Type Transistor. The symbol just shows that there is some transistor control.