So, someone post a picture of the backside of the PCB so we can draw the actual
circuit schematic and finalize the thread.

Loren,

I'm with you on the analysis,..and have proposed so from the get-go....therein the need to get her sketched out. I'm into electronic circuits (and have been designing throughout my life) and always KNOW one HAS to have the circuit to get the answers (for me, at least). I'm not a fan (nor student) of "guessing" (just as, I'm certain, you aren't)

Gerry's notes from "HELLA"(?) DO denote a system voltage dependent action..very helpful, I would say.

It's so odd that earlier years have high or low setting, apparently independent of system voltage, possibly,..then later, they decide to pay attention to system voltages when car is electrically (heavily) loaded..mmmm.. (or some idiot decides to leave the car acc'y switch on with the defog switch activated....alas,..we have a battery saver, eh?)

As you stated Loren, there is a variable resistor in there that I'm sure has a preferred setting (marked, BTW),..so we'll get to the brass tacks here shortly,..certainly with your expertise. Quite simple circuit (as stated MORE than once),..far cry from the DME discussions.

Thanks again for the interest and participation.......standby for further news,..as we put this all to bed.

BEST!

Doyle

Thanks, Ed.

Doyle

I had hoped to be the one to provide the circuit drawing (since I started this investigation),..but will be working a few hours yet. 10:1 Loren beats me to the sketch!!!!!!!!!!

Here's the pics you requested, Loren. Had to resize very quickly,..apologize for the sloppiness and hope i got it all..

Now,..for a nice hand drawn pic,..don't need a manufacturer for a print, eh? Hard to get? Bullsheeit.

THANKS LOREN!!!!!!!

Doyle:cheers:

The final back shot.....

Done!

Doyle

mine: 1989.

Thanks for nice pics!

Doyle

Alright,..finally ended this miserable work week and am parked in the home office for a short while,..then early wine with tracing/sketching...soon, to come.

COrrection to earlier info from me (at least, I don't think I corrected it?):

Transistors T2 & T3 are BC 387...1G NPN's........

Transistor T1 is a C547A NPN

It does use a 47 uF, 35 volt electrolytic....

Just some specifics (corrections/additions to information...)

Now,..for the afternoon.

Best,

Doyle

Here's the schematic. Not the greatest looking, but the info is there.
There may be some errors in the component values as the color codes were difficult to read.

Thanks Loren,......looks good to me...noticed Mr. Zener there (D1)!!!!!!!! the 470 ohm fixed and Mr. Variable into T1. Gonna be looking at this set point today (I hope). Although NOT my problem (at present, as the board is fixed and driving out signal to T2 just fine), AM interested in where this set point (VR1) should be for proper (normal) operation..

Next, to look in the rear window (lower) wiring arrangement (somehow)...

Very nice,..thanks for your time Loren. Cerainly delivered the goods....

Doyle

In the dark of the A.M......final test/setting of the VR1.....

The "marked" factory position of VR1 was set close to fully CW. It didn't take much movement CCW to turn relay #2 off. ..adjusted VR1 back to almost fully CW and healthy system voltage appears!. ....all done with car running. (so I didn't conduct any battery only tests to see this voltage sensing in action....disabling the outer elements)

'tis voltage sensing for sure. Thanks to Gerry and Loren. She's back in (for good) and I'm off to visit the interior spaciousness of the rear window's lower elements.....

Best!

Doyle

I've always thought myself adequate with circuits. I have an engineering (civil) degree and was captivated by the physics in that curriculum so much that I continued on in the physics program (through electives). Physical optics, or the study of light/sound/electromagnetic "pressure" waves was exceedingly educational. Circuits was a part of the deal, but rudimentary compared to what you guys are describing. I admire the advanced discussion here.
To zoom out from the focus herein; The rear defrost was a peculiar system with both Porsche and Benz as I went through their factory training ( a dealer tech was my day job - how I paid for my evening engineering classes at The Citadel). The point with them was that this circuit draws a ton of amps - as much, if not more, than any other in the auto. Heated seats/ mirrors, and even windshields were huge consumers as well, all "timed" out, or self defeating as not to cook something. Fail safes were part of the design objective, but these defrost circuits are integral to safety as well. The final conclusion was that these systems, when active, will reveal weaknesses in the charging system - smoke a battery/alternator that is on the edge, system draws, etc. A power antenna on older cars is an easily overlooked draw-source as well. Prudent diagnosis of electrical matters started with the battery - Porsche said ALL repairs start at the battery, even if its a wheel bearing. Check the battery on ALL cars first - this was a more common source of seemingly unrelated problems than one may think. The days of power/ground, testlight diagnosis are long gone. With micro-voltage in ABS/SRS(airbag)/CAN communication all working in concert throughout the car, weird shiet can happen; elevated/variable grounds and such with the instrument cluster as the "nerve" epicenter. Automotive diagnosis has become more "analytical". Benz has been working with a 24volt concept for years; copper wires are expensive in manufacturing, and "live" wires are dangerous in an accident. The trend in automotives has advanced way beyond elemental. We, as technicians, were instructed to "trust" whats inside the various control units, in fact, there was a shroud of secrecy from the factory.
I admire the fortitude you guys are bringing to this thread. A good electrical diagnosis is a challenge in the field, and welcomed by most technicians who are on "cruise control" with brake pads and maintenance. I've done my fair share of soldering faulty DME relays and control units and this puts a smile on my and the customers mug, but in fact, it's a "mechanical" fix still. Most of us in this forum and in the field have no clue as to whats going on inside those little boxes. AND, those little boxes are decades old - that we can take a magnifying glass and "see" the problem.
Good job guys.

Doyle, Caravan effing ROCKS.

Excellent post, Whalebird. That just made my morning, it was so interesting. This forum does rawk.

Thanks rus. How's the clutch coming along?

Hahaha.....well. I just checked with the IR thermometer, and it's 41F outside. I'm looking for my knit cap, and wondering just how badly I want to work outside. I ordered LR hard brake line and the Grade 8 bolts for the guide tube from my friend Randy at the Porsche dealership. And I want to do paint, but it's way too cold. I only have a day and a half because I need to get to the office to prep for a meeting on Tuesday, so I guess it's time to go freeze my rear off outside. Clutch is on, transaxle can't be mated until Monday night, and I still have the cables and suspension to do. I also am going to change out the shifter back to the Wevo billet shifter. Since the seats have to come out to adjust the parking brake cable (balance) I'm doing interior cleanup and some paint.

Nice informative comments, as always, Bird!!!!!!!!!!

YES SIR, IT (CARAVAN) DOES!!!!!!!!!

BEST!

Doyle