Rear Defroster Not Working

[7783911]

well, winter continues to provide time to address issues..aka retired with time on hands

The last item on my list is the non functional rear window defroster (77 911)

I have a new relay in place

What is the best way to identify the wires and test them to determine if power is even going to the window? (I really dont want to pull out the window). Simplest way is to start by a test in the engine bay at the relay?

How about testing at the switch itself (Fuses are good in the Classic fuse panel)

Previous owner went to manual windows and cut power window connections in the door..can that be a culprit since schematics show that outside window switch communicating to the defogger switch?

Some say did the LED come on but that assumes the light is good too :-)

I ve read many posts here on PP and on UK sites but no definitive testing method was described

any guidance and pictures greatly appreciated

bob

[PeteKz]

Yes, simplest way to troubleshoot is to start at the relay in the engine bay. IIRC, there also is a connector in/under the electrical panel for the wires to the defroster. Follow the wires coming from the rear package shelf to find it. It just takes time and and ohmmeter.

That said, if you have not renewed the rear window seal in 10 years or more, you should consider taking out the rear window anyway to replace the seal. They do shrink and harden with time, allowing leaks that go down along the rear shelf and pool in the rear seat buckets, staying under the rubber insulation where it won't evaporate, and causing rust in the bottoms of the buckets.

[7783911]

Thanks Pete

No leaks and the seal is still reasonably supple...I had the back interior out at one point and saw those pesky lines and being an idiot didnt do anything while in there...doh. They were intact if I recall, I actually believe my issue may be at the switch which is as painful given removing ventilation panel and blower to get to..unless some how I can get to from inside

Also will check connections at the rear relay

sigh, another small project that will take a week ...

[7783911]

Pete

I have a two stage defogger...the relay is 911.615.115.00

Looks like all wiring at the rear fuse panel is in place except for one that is not connected but doesnt appear to be part of this circuit (77 Targa, but have an 83 sc 3.0 installed..so left over?)

what I d like to do is test that relay since the wires that exit go up into the engine pad so to get to their ends is from the interior only

My bentley does not have schematic for 2 stage, only single stage

single stage has pins

30 RED/Black (from fuse 24/25a..always hot)
87 RED White (Hot)
85 BROWN (looks like to a common ground in rear window)
86 Black (from defogger switch) must be exciter circuit from switch?


the 2 stage relay..unsure what maps where?

30
87
87a
86
31
86a

how do I test this relay? and know what pins map to match single stage since I dont have a schematic?

I will also research on internet and whatever pdfs I can find in my files for various years

thanks

Bob

[deemsriddle]

I am currently restoring a 1971 911T coupe. We checked the continuity of the electrical lines in the rear window itself. There was no continuity. Being that I won't be driving in weather needing defrost, we're just taped up the wiring under the rear shelf and forgot it.

[PeteKz]

7783911: I don't have the wiring diagrams nearby, and I don't think I have the correct wiring diagrams for the 2-stage defroster, so you will need to find and reference that. If I recall correctly (and that's iffy), you should have continuity on all the wires that go up to the rear glass, with some small amount of resistance. If any are open circuit then suspect some window elements are broken. But if they all have continuity to ground, then you're back to checking the relay. If all the wires do not have continuity to ground, then check the ground connections. In my 1973T, the ground connections go to the ground stud in the upper forward left side of the engine compartment (again, relying on memory from the last time I had the window out 4 years ago)

[proporsche]

i have some extra relays if anybody needs one..
Ivan

[7783911]

And if you get caught in the rain..of course if you have a coupe less likely to get fogging?

[7783911]

what i really need is a test procedure to confirm the relay by grounding one pin and power to the other to hear the "click" as the relay closes contacts...which connection #s?

also, should the third fuse in the engine (lowest) be hot, not sure what circuit its for but other 2 are hot

[7783911]

Found this from a later year



so based on reverse logic, my lowest is actually #1 for an interior fan which my car doesnt have

the TOP (#3 by the picture) is for the defogger so that eliminates the power source issue?

[7783911]

Proporsche..sent you a PM

[7783911]

One more idea...you can tell I am going nuts on this one

Can I test the circuits by removing the relay and putting my multi-meter probe onto the various harness connections (the 6 port connector from the harness)

I d like to test that the switch in the dash is sending its signal to the relay, would that be a voltage test or ohm test? or both? and again, need to know which of the 6 pins to probe to

electrical gurus..please save me from myself

[7783911]

This was in another post but is not for my year (77) but may apply?

except that schematic doesnt match the actual relay, it doesnt have 15 or 85 and the relay has 31 and 86A

[EC900]

I’ve yet to test my ‘77 which is be inoperative, but here in the GreatLakes Midwest without a rear defogger and no RH mirror driving in damp weather becomes a hazard. I bookmarked this a while back…

Question on rear window heating elemnt wiring

[7783911]

Yup, got that one

my plan is. probably over thought but its a process of elimination

the first trick is to know which relay position (#) does what, very confusing diagrams that vary so hoping Timmy steps in the assist, but I have this schematic from 82 SC Porsche Manual




Make sure ground circuit is good!
Then create some jumpers to do relay and harness testing (harness output plug) to figure out if both sides of relay work or not
Lets say relay is good…, then likely the problem is at wires in rear window channel, painful! will need to figure out how to test them before pulling everything apart
If Relay doesn’t click with switch, manually test relay, if manual works, then determine if inbound wires are good, if yes, switch ok, if no then pull switch and confirm all wires, while out .. test each connection in case it’s the actual switch or the feeds to the switch? Make sure it is all doing as expected and proceed back to relay..repeat initial test to focus on relay
If Relay getting correct signals and putting out defogger power then its to the window we go

What have I missed?

[7783911]

Ok, ran through all kinds of testing

confirmed relay works: YES
confirmed interior switch activates relay : Yes
Confirmed relay sends power to both heater wires: Yes
Check ground connection. Its there and now cleaned up

SO

it must be at the actual rear window that the system fails, likely the wires are not connected or something

That will be a glass out event so will order a new (porsche brand) rear seal sometime when I am ready to tackle that or if I am getting the car painted

in the meantime..i will create a new post regarding the wiring of the switch and why no lighting works (even managed to get one of those long tube lamps and installed).

[7783911]

Deems, sent you a PM re continuity testing

[Walt Fricke]

The first time I had my rear window out of my Euro '77 coupe, I was able, with a lot of fussing, to get the defrost wires (two for single system) reconnected and the glass, trim, and seal back in, not without quite a bit of effort - seemed much harder than replacing the front windshield.

The next time I had to do this I eventually gave up on reconnecting (the connectors kept disconnecting as I jockeyed the glass) and on the trim. I thought I had learned how to get it to lay flat, but obviously hadn't. You can get a rubber piece which can take the place of the trim and be installed after the glass is in the seal and in the car.

So I'd do everything I can to be sure that your ohmmeter shows a resistance between the separate ground wires serving the two circuits in the glass, and each of the hot wires. Stage 1's +12 should be red/white from relay pin 87, and stage 2 red/black from 87a. Looks like you have figured out the relay, and where the relay gets its control voltage and power voltage from, and that the switch up front is sending what needs to be sent to the relay in the rear.

And see if there is any way to get at the contacts back on the glass itself. Are all four on the right side of the glass? Or is one circuit on one side, and the other on the other?

Is a new rear glass with a two stage defroster even available? Cost? Though I can see why this option would be more attractive in Canada.

[7783911]

Thanks Walt


I was hoping to test with glass in first, continuity between relay and glass shows stage 1 open (OL ohm ready) but stage 2 appears to be closed (almost 0 resistance). The trick to test the glass (heater lines) is worth experimenting on but your correct, the real way is with access to the wires with the glass out (test wires attached to glass and not attached). Having a Targa despite every effort to button her up tight makes for foggy window if caught in a summer rain. Winter, I put her away to sleep.

My anal side hates when things don't work so I tackle as time and desire dictate. Honestly this was the last thing on the list..

Appreciate your insights

Bob

77 Targa (No S)

[Schulisco]

Bob,

I suppose you need to know what that "relay" is exactly doing:
That rear defogger relay (some say controller or regulator) does nothing else than switching both power stages of the rear window defogger in relation to the voltage of the car battery! IIRC the first stage is 150W and powers the upper area of the rear window only to defog it to get clear view with the windscreen rearview mirror. It will powered directly by the rear defogger switch in the dash if ignition is on. Anytime!
The second stage with 300W powers the whole rear window! BUT: Only if the car's voltage is not lower than approx. 11,2V! If not, this stage won't be powered! This prevents discharge of the car battery / supply due to a bad battery or voltage regulator, e.g. while cranking the engine etc. If the voltage rises over approx. 13,5V again, then the 2nd stage will also powered again. All in all it's a load shedding relay! But only for stage #2!

You can verify the relay / regulator if you connect it to a voltage controlled supply and playing with the supply voltage.





Test stage #1:
Connect #31 with ground, #30 & #86 with +12V to power stage #1 and check if if #87 is powerred with +12V. Vary the voltage from 9 to 14volts, stage #1 remains on (#87 remains powered).

Test stage #2:
Connect #31 with ground, #30, #86 & #86a with +12V (to power stage #1) and check if #87 and #87a are now powered with +12V. Vary again the voltage, stage #1 remains on (#87), stage #2 won't be powered if voltage gets lower than 11,2volts and getting repowered when voltage is 13,5v or higher.

If both tests will work as expected, the relay is okay.

Then check if the voltage is coming on to the connection terminals of your rear window. Put the relay back in place, turn ignition on, and check the voltage at the windo terminals. To get the terminals there you probably have to remove the back wall lining carpet. If no voltage gets there there's sth wrong with the cables. If voltage is there, check if the wires on your window are in good shape and have no breaks. If there are breaks, you can fix them with liquid silver.

HTH.

Thomas