theiceman

..er Doyle I thought I might ask ..
do you actually have a problem wit the circuit or are you just REALLY bored ...
Once you draw it out throw it up here and lets have a look at it ..

BTW got behind the lighted stage foe xmas on blue ray .. might watch it tonight ..

dshepp806

er,..yes, I do.(outer elements=no go [no 12 volts)

er,..yes,..once I draw it out, we will certainly look at it (my point entirely).

GREAT DVD!!!!!!!!!!!!


Best,

Doyle

dshepp806

..a continuance on:


http://forums.pelicanparts.com/porsc...defroster.html

Best,

Doyle

dshepp806

Bad transistor in the relay assembly ,..drive voltage just fine,..now I've top elements active,..center elements active,..but no "5-down low"...think I've an issue within the seal connections but will have to further troubleshoot resistances and continuity...good old magnifying lens, to boot)...shouldn't be to bad from here, ...(aside from dickin' w/ the seal......if (eve) necc'y)....hard part's over. While searching for my needle MM leads to walk these elements, read a great thread on the damage that can be impaired by monky hands and will now be using (conservatively) TIN FOIL!!!!!!!!!

Will keep you (defrost types) updated........

Best,

Doyle

whalebird

Nice...thats awesome. Did you just graft in a new transistor? What a cool diagnosis. Most of us would be inclined to just get a new relay once we isolated that as a problem. I wish I had the know-how to look at all those printed circuits and add it all up.
Nice going.

dshepp806

No "graft",..a simple solder job on a substitution component with the same specs 2N2222 get 's the job done......no one seemed to have the schematic,..and I've yet to sketch the circuit,..just saw a bad PN junction on the transistor and knew this represented failure.

Ti's an interesting way of handling the firing of these 3 elements, (as driven' into the ground in some other threads.......been conversing with Gerry , a fellow PP's guy (PMs) on this.....).....

In any case,..we're off tho the real headache issues now.

Hope all is well with you and YOURS", BIRD!!!!!!!!!!

Best!

Doyle

dshepp806

To follow-up...I hope to get some time this weekend to durther these trials on my lower elements...

As I'm not familiar with the actual physical connections within the window seal (nor has anyone provided any layout of such), I'm quite catious of "how" these electrical connections can fail wijin the seal....I may be maintaining ground (and such) to the necc'y components for operation (such as the overhead brake light ground....whch works perfectly.

We'll see,..and I'l report back.

Best to ALL!

Doyle

Ed Hughes

Mine has wires with small spade connectors to the window.

dshepp806

"where", (exactly"?)

Ed Hughes

Left side on the lower horizontal section.

Ed Hughes

Obviously Targa vs Coupe, I'd think yours would be just forward of lower rear corner.

Lorenfb

Post a picture of the bottom & top of the circuit board,
and I'll draw a diagram. Or if it's easy, I'll take out the one
in my car, open it up, and use it to make a diagram.
So where is it located?

My factory diagrams for the rear defroster indicate:

1. The unit has 6 pins.
2. Pin 30 gets power from fuse 3 in engine compartment.
3. Pin 31 is ground.
4. Pins 86 & 86a get power from the defroster switch.
5. Pin 87 (via one relay) sends power to one of the heaters once the switch is 'on'.
6. Pin 87a (via other relay) is a timed power to provide more heat.

Hopefully, based on the above, we're on the same 'page'.
Well, based on what I see over on the 'Dark Side', we're on the same 'page'.

The three transistors most likely form the timer for extra heat
and energize one of the relays. The other relay gets energized immediately
once the defroster switch is 'on'.

Without seeing the actual schematic, the circuit probably works as follows;

1. T1 & T2 (both NPNs) form a one-shot (O/S) timer and T3 (NPN) just drives 'on' the 2nd relay
with a diode (1N4148 guess) clamp for the relay overshoot. The other diode is a zener
for the O/S. The capacitor is part of the timing circuit. When 86a receives power from
the defroster switch, the O/S causes T3 to energize the relay causing pin 87a to power
the other part of the window heater which lasts for the time determined by the capacitor
and its resistor.

dshepp806

Frikin' A Loren......thank you Sir for your input. I still plan to sketch it to KNOW exactly how she operates..I felt it had to be "time-related" and not so much "voltage-level" related, even though some information had passed that suffested this approach. Timing makes much more sense than sensing voltage drops...soon we'll know "how much" time!!!!!!!!!!!! The pinout functions are quite (self) explanatory and I DO appreciate your time to comment on it, Loren. (YES,..they were 4148's).

I thought I had posted a pic or two,..but will doublecheck and meet your request, bro.

Since I've got the PC board working,..I'm off to find out about the lower elements this weekend.... I just hate dicking with that rubber...resistances and voltage: here I camoe.

Good to hear from you.

Best,

Doyle

Ed Hughes

My 993 rear defrost times out from high in 15 minutes, not sure if they used that logic all along.

Lorenfb

"When the voltage, as measured at the engine compartment fuse dropped below the threshhold the
defogger second stage (upper and lower elements) shut down."

A voltage sense circuit and not a time delay circuit is the correct
assumption given the component values found on the 'Dark Side'
without testing its functionality:

1. The capacitor is only 47uf which with a 1 Meg (max practical with
bipolar TXNs) is only less than a minute.
2. So T1 & T2 most likely forms for a Schmitt trigger voltage sense with the
variable resistor setting it.
3. A zener is used as a voltage reference or part of a filter with the 47uf.
4. T3 does drive the second relay for additional heat.

Again more guessing without being able to test or see the back of the
defroster unit.

"When the voltage, as measured at the engine compartment fuse dropped below the threshhold the
defogger second stage (upper and lower elements) shut down."

So the center element is always powered with the defrost switch 'on'
and the heavier load elements (upper & lower) only get powered
(basically) with the engine running (the voltage sense part).


Bottom line: Always the best way to analyze a 'black box' is to first
understand its functionality which reduces the guessing options.