Schematic for rear defroster

[dshepp806]

Anyone have the schematic for the internals to the rear defogger relay (1989)....I do somewhere but not finding it today....

Again,..the schematic to the internals , not the wiring.

THANKS!
\

Doyle

[Walter_Middie]

This one mentions an 87 and 88 - but it's the same as my 75, so it should at least get you started for your 89.

[ossiblue]

Here's a close up of another two stage relay schematic that may help.

[dshepp806]

Close, but no bananas.....I have all of the above. Again, I'm talking SCHEMATICS (a CIRCUIT PRINT).

To cold to work outside on such a simple circuit...my bench pwr supply crapped out or we'dd be fixing this quickly. There are 3 transistors on this board and I'm suspecting one has failed. It looks (although I haven't yet drawn out the schematic---easy to do) that one of these transistors drives out a ground signal (probably a common emitter topo) for the opposite side of coil#2 , then I'd see 12 volts at 87a, as required (assuming relay is functional). No time to apply ground internally to the proper side of coil #2 to check coil functionality (or measure for ground there) but will do that, as well. Hopefully warmer this week and I can further the T/Shooting. WIll post accordingly.......

GERRY: 3 EXTRAS? One of those should be available real cheap, eh?

I'll keep looking for the schematic...I've got it somewhere on 3 servers............

Best,

Doyle

[dshepp806]

Thanks, Gerry....I'm an EE so I've got the protocol down. I prefer to keep my measurements at the board for now...as mentioned ,..too cold outside to T/S and my bench PS is inop,..so no arm bench work (inside) today..manually closing of contacts on relay should give me the 12 volts at 87a...if it doesn't work with 12 volts there,.THEN i'll chase outward towards the glass..haven't thought it to be a connection issue from the get-go....

VERY simple circuit that should be easily repaired. DId you find a source for the relays (just in case)?

,..about those 3 extras of yours'......(ahem?)

Best,

Doyle

[dshepp806]

,..standby for news!!!!!!!!

[dshepp806]

I'm not quite sure I'd change out everything EXCEPT the relay......one could isolate a bit better (MO)........

I'll have to acquire the schematic or draw the damned thing out to determine what they're doing,..but I assure you there's an answer.....once I get the picture (in my mind) , we'll see how the V/resistor places in the circuit. (whether it's voltage sensitive or time sensitive....maybe it's something to do with no cracking glass, for all I know!!!!!!)

Heck,..Looks like I'm gonna' have to sketch this one out,..I fully intend to repair this one, needing no mods.

Is your spare for sale?


I appreciate your input.

Best,

Doyle

Originally Posted by 86 911 Targa

Two thumbs up EE.

After harness continuity checks, from the relay connector ect.......

I made up this harness, which has helped other members.

When I fault isolated to the relay, I replaced every component with the exception of the the relays.

No joy!

Any thoughts as to why Porsche designed voltage sensitive second stage?

There is a relay on eBay, but I can test one of my spares,
and send you one as well.

porsche defogger relay items - Get great deals on eBay Motors items on eBay.com!

Good luck,

Gerry

[dshepp806]

Good info, bud,...thanks.....will know more once I've sketched the circuit.....

I was only joking about the "cracking"...

How did you determine that you need a minimum of 12.8 volts for the relay to function? Was it from the provided notes and no actual testing?

In terms of the mentioned 'trying different voltages",..would this possibly mean a variable resistor tweak? Now you've got me quite interested...when it warms around here (or I get my V/powr supply fixed) , Your mention of R1 adjusting the trip point is interesting...I noticed (today) that the setting on VR1 was ectched with green paint and in the originally set position...I swiped the resistor full left and right, eventually setting it a short distnce from it's original setting (near end stop, I would add)...I'll play with this...good stuff, bro. You'd think you'd see the lowered DC rail with a lowered trip point,..yet the vital test point is (straight up zero volts....) provides no engagement (duh?)

I'll press on this week.

Thanks for the participation!

Doyle

[dshepp806]

Did any adjustments, by yourself, make any differences on the stage two trigger point?

If you found relay failure,..was it coil related or contact related?

You mentioned earlier that you changed "all" out before the relay,...have yoiu the transistor numbers? (just for record?)

Thanks,


Doyle

[aigel]

Here another thread that may be useful it contains one schematic that hasn't been posted yet!

Where do rear window defrost wires unplug?

Thanks, George

[dshepp806]

BTW,..I would easily see a zener's use to establish a trip point.....can't wait to map this out.


Doyle

[dshepp806]

Originally Posted by aigel

Here another thread that may be useful it contains one schematic that hasn't been posted yet!

Where do rear window defrost wires unplug?

Thanks, George

It's still the same old stuff...nothing new......very block "diagramish".....

I'm thinking FIRE ALL OF THE ELEMENTS UP AT FIRST then one would decrease voltage on selected (outer) elements..? When done, cut the damned switch off....

We'll get to the bottom of this..

Doyle

[dshepp806]

Gerry,

What was the problem, in your case?

I'll get some time with her this week!

Thanks,

Doyle

[dshepp806]

Had a short visit with her today,..but the rain came and I had to stop. Qick glance at the ckt board provided the following (keeping in mind, I'm speaking for a 1989 S.A. Edition Coupe:

BC 337 NPN transistors (2) are used @ T2 & T3..... looks like the variable resistor drives the bases but I didn't have time to trace (but will) I had no time to get measurements through the range if this puppy (but will),,,the additional transistor @ T1 is a C547A ( adifferent fellow biased jsut right...no zeners in the circuit but a couple of diodes...cap was found to be a 47uF, 35 volt electolytic. All junction checks were good and tolerances ok. Relays working away. Adequate voltage (13.6 VDC battery-while idling= a healthy 12 plus volts off of both relays...still showing visually that the lower elements (ONLY) are not operational ...

I found the wiring feed-through grommet (rear left side), heading upwards towards the glass. This thing didn't want to come out (only 1/2 a tilt) and I didn't want to force anything just yet...(was wondering if there existing a connector in there)...left that alone....I planned on removing the rear panel today anyway to get these old blown speakers out...while in there I located the driver rear side wiring entry..had 2 brown wires, 1 black wire, 1 Red/black, 1 Red/White (power). No way messing with the rubber, as these pups disappeard quickly...

Will have to revisit maybe tomorrow...For the helluva it,..I shot some IR readings outside after having the system on (running for 1/2 hour......you can "see" the bottom elements drop in temp (just being funny here).. Still have to get in there one step further and meter some....

Off fottnote today (as to prints,....) We're all familiar with "86-89" defoggerr print, Bentley's page 970-55...particularly note the black ground jumper connecting the outer elements ' gounds (ELEMENTS #1 and #3)..looks like maybe a connection point AT the jumper???....I would remind you my #1 works fine with my #2......THAT tells me I'm getting what's needed on both sides of those elements ACROSS those black jumpers...so, either the connection point AT the element maybe bad ,..or the element itself is damaged...some further measurements are in order.


I happened to be looking at Bentley's ground distribution 88/89, section 970-25..and noted the black jumper to provide ground to the high mounted stop light used on a COUPE (of course this is working fine)...just an observation...

I just wanted to throuw some pics up,..hope you all don't mind...

[dshepp806]

,..and the final(bottom) pic...with some other "remnants"






I'll close in saying that a reread of Gerry's gracious post of Hella's letter tells me (simply), it's (the voltage staging) to save an excessive current draw when some of the alt/battery/reg failures occur...not too strange after all....( I guess they're trying to keep the system voltage up).



Best ,

Doyle

[dshepp806]

Josef's reference to trying different voltages speaks to the variable resistor....

I don't (at all) think this to be linked to my issue,..just a footnote to the Hella Letter post.

Best,

Doyle

[dshepp806]

If anybody has some really good pics of where these wires run (under the rubber,..and how they attache to "busses",..I'd like to see 'em... No way am I taking this window out for any reason just to check connections for the lower elements. It's quite hard to lift the rubber seal to see underneath..looks like the wires may run in a "channel",.of sorts...

Will further me endeavors today..I'm betting on the lower element (s) being the problem..however, maybe the ohmeter will bare something different. Thinking about recovering the damned rear shelf while in there, as I was successfull in generating a small tear (1 inch) while removing a blown speaker (DAMMIT!!!!!!!!!!!!!!)

Another day (shortly)..

Best,

Doyle

[dshepp806]

Loren chimes on the other side...THANKS, LOREN!

schematic for rear defroster-1989? - Page 2 - Rennlist Discussion Forums


I think the circuit to be "time-based"...but we'll get to the bottom,..and very shortly, at that.

Best,

Doyle

[dshepp806]

Thanks to Loren, we now have the sketch we were first looking for...taken from Rennlist's further discussion thread....@

schematic for rear defroster-1989? - Page 2 - Rennlist Discussion Forums

Thanks, again, Loren for taking the time to share,..delivering the goods!

Best,

Doyle

[dshepp806]

It's there. Thanks Loren!

Thanks,

Doyle