Missing Interior Lights System diagram!

[CallawayTurbo]

The Bentley Manual for the 84-89 seems to be missing a page for the Interior Lights System. Its referenced in a few other drawings but mine doesn't have a page with it.
Does anyone have a schematic the shows the wiring for the interior lights + switches?
I have an odd electrical issue

[Cattler]

What system diagram are you looking for? I don't think there's a single "interior lighting" diagram.

[86 911 Targa]

If you can be more specific, I can have a look at my factory manual.

pm me as needed.

Gerry

[CallawayTurbo]

Well, this is what I was trouble shooting:

Fuse 1of the #1 block (Clock, interior lights ... ) had blown.
Now, I found that there was 8.5 v on the "other side" of the fuse.
Very odd.
Tracing it further I found that there was 8.5 v on the brown/white wire
that goes to the door switches (open / closed).
I removed every other fuse and the 8.5 v didn't go away.
I disconnected the battery ... it went away.
Reconnected the battery it came back as a fluctuating 10.5 - 11 v!
Hmmm ...
So now I just pulled me the relays behind the fuel gauge and it went
away, put them back in, hasn't come back ... flaky relay, bad contact?

[CallawayTurbo]

Whoops, spoke too soon.
Its still doing it, my ground clip had fallen out.

So if you look at page 970-86, Warning System 1988-1989.
The brwn/wht wire at both door contact switches (and back to the fuse box)
has a fluctuating 10.8 to 11.5 v on it!
This is with the power window relay out.

So I'm trying to trace that brown/white wire back to see if I can isolate this.
Maybe the anti-theft unit is flaking out?

[steely]

I had the same question some time back - and I actually think it may have been Gerry that helped me. He has factory schematics.

The Brown-Wht is a Daisey-chained return path for these lights - I forget from where it originates. I drew the figure below for my own use - and I can't recall how accurate it is, but maybe it'll help.

Keep in mind that the Brown-Wht will always be HOT when the Black wire is HOT - only when the applicable switch is closed, will the BROWN-WHT will be zero Volts in order to turn the light on. This is ok but cab fry something if it ever gets exposed and touches something. If it shorts to the body, the light will go on by itself.

BROWN is ground - and I think the White means it is switched-ground.
Somebody can correct please if I am wrong.
Sorry for the typos - switched in Light means that there is a switch inside of the light (like a tagra roof light or glove box light).

[CallawayTurbo]

Ok, well most of that circuitry is shown on various schematics in the Bentley.
I guess I was looking for a circuit track / bundle diagram thing so I could
see what might be shorting.

Anyway, my strategy is to disconnect everything that goes to the brown / white wire.
Working on the factory alarm. Which they conveniently put behind this mass of
black plastic!

So how does this thing come off? The cowling that hold the blower ...

[CallawayTurbo]

Ok, I got that off.
Disconnected the relays back their.
Now I have 2.1 v stable (drifting down) on all my test points.
WTF!
I have a feeling I'm doing something stupid here (chasing my tail).
But I really don't see how their could be any voltage on that wire
when the fuse is out.

[steely]

My bad - I missed the anti-theft part of your problem - yes, interior light are shown on that page of the anti-theft ckt - but no where else.
The door switches have 2 purposes - one for lights, one for the anti-theft.

I don't have the anti-theft, so the schematic I would need is missing.

Brown white is switched return - it is in the middle of ckts - neither a pwr ckt nor a constant gnd.

The fluctuating voltage means that you may be reading parasitic V thru some solid state stuff (like your anti-theft), it may be as expected and thus I would discount the reading.

I would also recommend that you PM Gerry - he may be able to help you. He knows his stuff, and has factory schematics.

[CallawayTurbo]

Ok ...

So I shorted the other side of the fuse (brown / white wire) to ground and the
2 volts went down to 0.

But then I went and reattached the anti-theft "relay" and the fluctuating 10.8 +
voltage came back. Hmmm, that seems like a bit too much to be stray capacitance.
Time to test the anti-theft unit I guess.
I mean it would have to send some current down that line to detect closure but
it would be constant not fluctuating.

[CallawayTurbo]

Ok,

So if I disconnect the non-powering connector to the anti-theft control unit
I get 12v stable at the "T-" pin.
Reading the corresponding pin in the female connector which goes to the brown / white
wiring I get a decaying small voltage (1.9 v) that if I short to ground stays there.

If I reconnect the anti-theft control unit I read a fluctuating 10-11+ volts on the brown / white
wire.

So is the unit bad?
Can't supply enough current?
Is this normal operation? (Really can't see that).

Anyone care to measure the brown / white wire on their car with a good DMM and see if its bouncing around?

My only other course of action is to get another unit and swap it in but I'm not so sure about that.

[steely]

Your fuse #1 (block #1) is blown and your problem should be on that ckt.
Pin T- doesn't furnish current anywhere, it is an input looking for an open or close from the doors.

If you think T- pin zapped your fuse 1 thru the interior light ckt, I would think one of them would have been burnt in the process.

As a test, and if your A-theft device is on fuse 7, I would hook everything back up minus fuse 1 and test the anti-theft device.

[steely]

monitor the brn wht wire with both doors open and then closed to see if there is a change.
I would rule out that fuse 1 ckt as a culprit just yet.
I think the 12V at T- was a clue, but not one that faults the alarm system, just the opposite, the wiring associated with Fuse 1, but I can't prove it.

[steely]

or, with the connector to T- removed, replace Fuse 1 and see if it blows.
if it does, the issue isn't the alarm system. (more work to do)
if it does not, hook the T- connector back and see what happens for the acid test.

[CallawayTurbo]

Ok, I went with fuse ...

Disconnected the alarm unit, put fuse back in.
Voltage stable on that leg, lights work.
Connect the alarm unit ... fuse doesn't burn out. Slap forehead.

But ... I have a ton of stuff disconnected at this point AND I did clean the
contacts on the alarm unit (boy were they brown).
So I guess I'll start putting stuff back together until something smokes the fuse.

I guess the voltage reading was normal.
There would have to be some voltage at T- to have a current flow to detect door switch closure as that connects to ground. It just seemed like 12 was a lot.

[steely]

Glad you got to this point.

If it goes again, I think your clock and interior light ckt would be to blame.

Pointless discussion about T- follows :-)

Originally Posted by CallawayTurbo

There would have to be some voltage at T- to have a current flow to detect door switch closure as that connects to ground. It just seemed like 12 was a lot.

No, no current of any significance. And any voltage needed would come from the light ckt, unless the T- only needed to see a ground (0V) from the door/light switch ckt.

I do not know anything about the anti-theft device, but I could tell from the schematic and the readings you took that the T- pin is an input to the device. It is not meant to furnish (out) or draw current (in) of the device to/from the lights or anywhere. It is the input to the device to sense a 12V or 0V input from the door switches. It only senses or sees 0 or 12 and compares it to a known internal reading. When disconnected, the input would float up to some undetermined amount. Undetermined because that is how 'floating' inputs to analog devices work, and because I don't have the schematic. Any current is probably low, like micro-amps. You could short it to ground and wouldn't kill it.

The lights always have 12v on them because they are ungrounded (no current in the light although you read 12V at the light to ground) until one of their switches takes the brown-white leg to ground. brown-wht side of light is 0V, black wire side of light s 12v, 12v across light and current flows - the light goes on.

Somewhere in the door / light ckt, the brown white line is ALSO fed to the theft device to pin T- to tell it if someone opened the door . The light current never gets into the device at T- . T- only probably ever gets the 12V from the door (and light) switch, or 0v from the same switch.

This is why I asked you to monitor the Brn-wht line with both doors opened and then closed. Because I can't tell from the schematic when the door switch is closed. For example, in the bentley, is that drawing of the door switch indicative of an open or closed door? You can't tell because the interior light schematic isn't completely shown in Bentley.

[CallawayTurbo]

Well, I did measure the T- pin of the anti-theft module when it was powered but disconnected from the brown / white wire and got 12 v. Granted that could be some sort of float there but like you said, without the schematic its hard to tell.

Anyway, I reconnected all the relays and the fuses and the lights still work.
Perhaps there was high resistance at the terminals of the anti-theft module that caused a problem. The terminals were very ... I guess dirty is the correct terminology. I cleaned them up a bit. I doubt it though.

I was putting the car back together after a re-spray.
The last things I was doing were connecting up the passenger side power window switch and attaching the ground straps to the engine lid. That's when the lights went bad. It wasn't a one off fuse blowing either. When I put the replacement in it blew in a dead short fashion.

What I was trying to do originally was just measure the resistance between the fuse terminal and ground to get a gauge on things and noticed that voltage on "the other side". Anyway, unless I find something useful to share with the group I'll just chalk this up to some heavy drinking one night in Stuttgart.