1986 Carrera 3.2 - Rear Window Defogger/Defroster Relay Circuitry
02-22-2008, 06:56 PM
#1
Instructor
Thread Starter
1986 Carrera 3.2 - Rear Window Defogger/Defroster Relay Circuitry
Folks,
After 5 years (you can see where this went with regards to priority for me!) I'm back on to this.
Does anyone have electrical knowledge of the relay (911.615.115.00) used in this circuit? I'm confounded by its design. Bentley has a good representation of the circuit on page 970-55 in the wiring diagram section.
This 'relay' actually has TWO relays built in and here's what I've deduced so far. Power (+12V) for (and only for) the window elements themselves is provided through pin 30 on the unit and is always hot. The opened/closed state of the two relays dictates whether or not this voltage is appliled to the heating element segments.
Ground is provided to the unit on pin 31. One relay has its positive connection on pin 86 and when voltage is applied to this pin (as when the defog switch on the center console is depressed) the relay closes and provides voltage to ONE section of the three-section heating element via pin 87. This relay is very simple in that it should always have a ground (pin 31) so given voltage on pin 86 you should see pin 30 voltage immediatey appear on pin 87.
The second relay has its positive connection on pin 86a which is 'ganged' to pin 86 via the wiring harness so when 86 has voltage so should 86a. If the relay closes, pin 30 voltage is destined to appear on pin 87a which is routed to a SECOND section of the heating elements. This section is 'ganged' to the remaining THIRD section so in design this second internal relay actually controls TWO of the three heating elements.
What's different with this 'second' relay is that its access to ground is in no way as simple & direct as the first relay. THIS relay has additional circuitry (8 resistors, 2 diodes, 1 capacitor and three transistors) that appear (?) to determine whether or not this second relay gets the ground it needs to operate.
In my case I can get BOTH internal relays to close by providing +/- to their appropriate component pins directly on the circuit board they are mounted on, so I know the windings of the relays are good. Their contacts also pass voltage from pin 30 when doing so, so the relays themselves appear to be fine.
With regards to the second relay's additional circuitry, one resistor appears to be way out of spec and the two diodes are questionable but I don't know their composition or forward/reverse bias specs. The three transistors are another question and I don't know their pinouts (EBC) or design although two are the same with one having a different part#.
In short, the 86-87 relay always works but I can't get the 86a-87a relay to operate unless I bypass its additional circuitry. Why oh WHY is this second internal relay 'blessed' with this extra crap???
What oh WHAT is this 'extra crap' attempting to do with the two heating element segments it controls???
Why do I care? 1) The relay cost: US100.00+ 2) I haven't spent US100.00 in beer on this problem yet. I have a rule that states don't buy a new electical part to replace something that's 'repairable' until the cost of 'trouble-shooting beer' exceeds the cost of the new part. ;-) Anyway, after getting into this I JUST HAVE TO KNOW... why is this thing SO DAMN COMPLICATED???
Barry
Who could work on this for years 'cause Bud-Light's pretty cheap hereabouts...
After 5 years (you can see where this went with regards to priority for me!) I'm back on to this.
Does anyone have electrical knowledge of the relay (911.615.115.00) used in this circuit? I'm confounded by its design. Bentley has a good representation of the circuit on page 970-55 in the wiring diagram section.
This 'relay' actually has TWO relays built in and here's what I've deduced so far. Power (+12V) for (and only for) the window elements themselves is provided through pin 30 on the unit and is always hot. The opened/closed state of the two relays dictates whether or not this voltage is appliled to the heating element segments.
Ground is provided to the unit on pin 31. One relay has its positive connection on pin 86 and when voltage is applied to this pin (as when the defog switch on the center console is depressed) the relay closes and provides voltage to ONE section of the three-section heating element via pin 87. This relay is very simple in that it should always have a ground (pin 31) so given voltage on pin 86 you should see pin 30 voltage immediatey appear on pin 87.
The second relay has its positive connection on pin 86a which is 'ganged' to pin 86 via the wiring harness so when 86 has voltage so should 86a. If the relay closes, pin 30 voltage is destined to appear on pin 87a which is routed to a SECOND section of the heating elements. This section is 'ganged' to the remaining THIRD section so in design this second internal relay actually controls TWO of the three heating elements.
What's different with this 'second' relay is that its access to ground is in no way as simple & direct as the first relay. THIS relay has additional circuitry (8 resistors, 2 diodes, 1 capacitor and three transistors) that appear (?) to determine whether or not this second relay gets the ground it needs to operate.
In my case I can get BOTH internal relays to close by providing +/- to their appropriate component pins directly on the circuit board they are mounted on, so I know the windings of the relays are good. Their contacts also pass voltage from pin 30 when doing so, so the relays themselves appear to be fine.
With regards to the second relay's additional circuitry, one resistor appears to be way out of spec and the two diodes are questionable but I don't know their composition or forward/reverse bias specs. The three transistors are another question and I don't know their pinouts (EBC) or design although two are the same with one having a different part#.
In short, the 86-87 relay always works but I can't get the 86a-87a relay to operate unless I bypass its additional circuitry. Why oh WHY is this second internal relay 'blessed' with this extra crap???
What oh WHAT is this 'extra crap' attempting to do with the two heating element segments it controls???
Why do I care? 1) The relay cost: US100.00+ 2) I haven't spent US100.00 in beer on this problem yet. I have a rule that states don't buy a new electical part to replace something that's 'repairable' until the cost of 'trouble-shooting beer' exceeds the cost of the new part. ;-) Anyway, after getting into this I JUST HAVE TO KNOW... why is this thing SO DAMN COMPLICATED???
Barry
Who could work on this for years 'cause Bud-Light's pretty cheap hereabouts...
02-22-2008, 07:16 PM
#2
Addict
This is going to cost me my membership card....
What about Lorenfb?
Please forgive me folks, but it sounds like a perfect match.
https://rennlist.com/forums/members/14080-lorenfb.html
What about Lorenfb?
Please forgive me folks, but it sounds like a perfect match.
https://rennlist.com/forums/members/14080-lorenfb.html
02-22-2008, 10:33 PM
#3
Instructor
Thread Starter
Folks,
For the technically vicious here's a few more tidbits:
The GOOD news is that the 'Relay' is easily removed from its enclosure in order to reveal all of its interesting bits. In addition to THAT, all of the board's components are clearly marked on the board itself, to include diode orientation. What luck!
Two of the three transistors I don't know how to test (T2 and T3 as marked on the circuit board) are individually marked BC337-16 and I appear to have found them:
http://www.newark.com/jsp/search/pro...AFC-TL10000001
The remaining one (T1) is definitely marked with C547A and then has an additional indistinct marking of what appears to be PH49 (??). It could be the following:
http://www.chipdocs.com/pndecoder/da...GE/BC547A.html
The two questionable diodes are next. Both are axial in package design. D1 (as denoted on the circuit board itself) appears to be of glass construction but its exterior is colored pale yellow and has a single brown band on its cathode side. It DOES have a very prominent marking of PH BZX but I've had no joy searching for this on the web.
The second diode, D2 (again, as denoted on the circuit board) is unmarked but appears very similar to a 1N914 Type 50 (clear glass enclosed w/an 'orange colored' interior). The only visible difference is that it has TWO black bands on it's cathode side instead of the single black band on a 1N914 Type 50.
There's also one LARGE axial package resistor (R8), light brown in basic color with the bands of Orange, White, Brown and Gold. This (to me) should represent a 5% tolerance resistor with a resistance of 390 Ohms but it only shows 69 Ohms on the meter. There's another axial resistor on the board (R4) with the exact same basic color and markings (Orange, White, Brown and Gold) but it's only HALF of R8's diameter and length and it reads at 383 Ohms - close enough.
Why R8 is so large (higher voltage capacity?) and reads very low is a mystery to me. How would I determine what voltage R8 is spec'ed to handle?
All the remaining resistors and the capacitor appear to be well within specs, as are the solder connections and continuity between segments of the board traces, SO I'm taking the BIG LEAP of faith that the suspect list is now narrowed down to:
T1 - Don't know how to test it but hopefully pennys to replace
T2 - Don't know how to test it but it IS pennys to source & replace
T3 - Same as T2
D1 - Hopefully pennys to replace if I can figure out what it is!
D2 - Same as D1
R8 - Big resistor that way under-reads its correct Ohm Value
All of this, of course, is based on my failure to get the 86a-87a relay working and based on the understanding that I really don't know how it's supposed to work in the first place. What a 'Challenge'!
As always, any help would be much appreciated and my results shared with the board (for what's it worth!).
Barry
For the technically vicious here's a few more tidbits:
The GOOD news is that the 'Relay' is easily removed from its enclosure in order to reveal all of its interesting bits. In addition to THAT, all of the board's components are clearly marked on the board itself, to include diode orientation. What luck!
Two of the three transistors I don't know how to test (T2 and T3 as marked on the circuit board) are individually marked BC337-16 and I appear to have found them:
http://www.newark.com/jsp/search/pro...AFC-TL10000001
The remaining one (T1) is definitely marked with C547A and then has an additional indistinct marking of what appears to be PH49 (??). It could be the following:
http://www.chipdocs.com/pndecoder/da...GE/BC547A.html
The two questionable diodes are next. Both are axial in package design. D1 (as denoted on the circuit board itself) appears to be of glass construction but its exterior is colored pale yellow and has a single brown band on its cathode side. It DOES have a very prominent marking of PH BZX but I've had no joy searching for this on the web.
The second diode, D2 (again, as denoted on the circuit board) is unmarked but appears very similar to a 1N914 Type 50 (clear glass enclosed w/an 'orange colored' interior). The only visible difference is that it has TWO black bands on it's cathode side instead of the single black band on a 1N914 Type 50.
There's also one LARGE axial package resistor (R8), light brown in basic color with the bands of Orange, White, Brown and Gold. This (to me) should represent a 5% tolerance resistor with a resistance of 390 Ohms but it only shows 69 Ohms on the meter. There's another axial resistor on the board (R4) with the exact same basic color and markings (Orange, White, Brown and Gold) but it's only HALF of R8's diameter and length and it reads at 383 Ohms - close enough.
Why R8 is so large (higher voltage capacity?) and reads very low is a mystery to me. How would I determine what voltage R8 is spec'ed to handle?
All the remaining resistors and the capacitor appear to be well within specs, as are the solder connections and continuity between segments of the board traces, SO I'm taking the BIG LEAP of faith that the suspect list is now narrowed down to:
T1 - Don't know how to test it but hopefully pennys to replace
T2 - Don't know how to test it but it IS pennys to source & replace
T3 - Same as T2
D1 - Hopefully pennys to replace if I can figure out what it is!
D2 - Same as D1
R8 - Big resistor that way under-reads its correct Ohm Value
All of this, of course, is based on my failure to get the 86a-87a relay working and based on the understanding that I really don't know how it's supposed to work in the first place. What a 'Challenge'!
As always, any help would be much appreciated and my results shared with the board (for what's it worth!).
Barry
02-29-2008, 08:51 PM
#4
Instructor
Thread Starter
Folks,
Just a bump for 'Last Call' on this. Does anyone know what this extra circuitry on the second relay is attemping to do/controll? The switch on the console is an On/Off affair - no rotation or anything else; simply ON or OFF...
Thanks!
Barry
Just a bump for 'Last Call' on this. Does anyone know what this extra circuitry on the second relay is attemping to do/controll? The switch on the console is an On/Off affair - no rotation or anything else; simply ON or OFF...
Thanks!
Barry
