How to troubleshoot Audi central power locks for < $10

[Tony Lum]

Hi gang,

I got tired of having 2 cars with the power door locks out of commission
and decided to fix them today.  All you need is a DC power cord (Radio
Shack #270-1590) that plugs into the cigarette lighter and a miniature SPDT
toggle switch.  I used a center off switch with momentary contact, i.e.,
the handle always returns to the center off position.  Spent a total of
$6.  You'll also need the 3 terminal connector used on the door lock
motor.  Get this from a junk car.

You first need to make the test cable out of the power cord.  Take the
minus (-) side of the cord and solder it to the center wire of the 3
terminal conductor (usually brown).  This is the ground wire.  Now solder
the plus (+) side of the cord to the center terminal of the switch.  Solder
the other 2 wires to either end of the switch.  Doesn't matter which way it
goes.  Insulate as necessary to prevent shorts.  I used yellow heat shrink
over the entire switch to make it sturdy and easy to read the markings on it.

Now the first thing to test is the power lock motor itself.  Plug the cable
into the cigarette lighter which will light the little LED when it gets
power.  Now take your power door lock motor and plug it into the 3 terminal
connector.  Flip the switch.  If the motor doesn't run, flip the switch in
the other direction.  If the motor won't run in either direction, then your
motor is probably bad.  Double check the wiring on the test cable to be
sure.  If the motor does run, feel with your finger and see if the air from
the motor outlet is going out (pressure=unlock) or in (vacuum=lock).  With
a permanent marker, mark the switch accordingly.  To test the motor, simply
place your finger over the outlet and cover it.  The motor should stop
almost instantly.  Flip the switch in the other direction and repeat the
test.  If the motor stops in both directions, then the motor itself is
good.  I attached an old turbo boost gauge and found the motor makes about
0.4 bar (pressure) and about 0.3 bar (vacuum) before shutting off.

Now you will test the individual power locks themselves using your lock
motor and test cable.  Attach a foot long piece of vacuum tubing to the
lock motor.  Remove the rear seat bottom.  Near the bottom of the back
cushion on the right side (actually left if you're facing the rear of the
car), you'll see the green tubing and the junction which feeds all the door
locks.  Disconnect each one at a time and connect it to the vacuum tubing
from the lock motor.  Use the switch to raise and lower the power lock.  If
the motor refuses to stop, you've got a bad door actuator or a leak in the
line.  Those that are leaking can temporarily be disconnected from the
circuit to enable the other locks to work normally.  If none of the doors
are leaking, then test the trunk lock actuator.

Using this method, I found the problem in my locks in about 5
minutes.  Left rear door lock on the 4kq and the passenger door in the urq
are both leaking.  Unfortunately, you'll have to remove the door panel to
fix the problem, but that's another story.

HTH,




-Tony
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Tony Lum                                                    1987 5000CS
Turbo Quattro
Berkeley, California, U.S.A.                              1987.5 Coupe GT
Audi Owner/Driver/Mechanic by Necessity ;^)      1985 4000CS Quattro
                                                                          1983
Ur-quattro
                                                                             1980
5000S
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