Baffling Headlight Wiring Problem SOLVED!! (and After-run Serendipity)

["Fred Munro" <munrof@isys.ca>]

    Finally solved the problem with the headlights. As per earlier post,
with the dimmer switch in "low", both the high and low beams were on. With
the dimmer in "high", only the high beams were on. I noticed the situation
when installing a high beam relay circuit - the stock headlight connectors
were burnt around the ground pins from the high current to ground.
    Thanks to Huw and Fred who replied to my earlier post with ideas, but
this one is really weird, guys. And strange that the list is discussing
after-run cooling fans...
    I pulled the high and low beam fuses and found 12V at the high and low
beam "hot" contacts with the dimmer in "low" and 12V at the high beam only
with the dimmer in "high". Turned off the power and found a 60 ohm
resistance between the high and low beam contacts with dimmer in "low",
infinite resistance with dimmer in "high". Aha, a short in the dimmer switch
or harness!
    Pulled the connector off the dimmer switch. The short dissappeared. Aha,
a bad dimmer switch! Tested 56a & 56b sockets in harness - infinite
resistance. Tested 56a & 56b pins on dimmer switch. Infinite resistance.
What the...!
    Reconnected switch. Short is back. Disconnected switch. Short is gone.
Tested switch. No short. Repeated several times in confusion. See the lights
of Bucksnort on the horizon, aren't they pretty.
    Resolved it must be the switch. Drive to CTC to get a T8 Torx
screwdriver to dissassemble switch. Try to ignore small, still voice saying
"error,error,error - all data does not match hypothesis". Due to long bitter
experience, small still voice wins out.
    Return and re-test the harness. Sockets 56a & 56b - no short. But there
is also power to socket 56 to feed the rear foglight. Test socket 56 & 56a -
bingo, 60 ohm short. Oh please no, not the harness, anything but the
harness...
    Consult Bentley. Go to fuse panel to pull rear foglight jumper block at
socket #1 to re-check harness. Why is there a 214 relay where the jumper
block should be? And why is there no relay in #3, the 1st stage cooling fan
and after-run position? Aargh, the mystery tech has struck again!
    Checked #1 socket. Wonders of wonders, a high beam lead is run to this
socket. The relay coil was jumpering the high beam lead and the power feed
for the rear foglights. Interesting, could Audi have been considering a high
beam relay system at one time? BTW, the resistance of a 214 relay coil is 60
ohms.
    It looks like I have been driving for years on the 2nd & 3rd stage
cooling fan only. No cooling fan when the A/C was on unless the engine got
hot enough to activate 2nd stage. No after-run cooling fan. High & low beams
on simulaneously with the DRL's and low beams on - the stock headlights are
so pathetic that not only didn't I notice it, no one ever flashed me when my
low beams were on! Duh, can't believe I never noticed the cooling fan.
Always kind of wondered why it was so noisy when it did come on - it was the
high speed 2nd stage that was running.
        Gee, solved 2 problems when looking for one. Not bad for an
afternoon's work! Cheap fix, too. And the high beam relay circuit now works
great, not that I expect to see much better at night.


Fred Munro
'91 200q  251k km