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From: "Harvey Chao" 
Subject: Automatic Antennas that Don
To: "BMW" 
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  REGARDING           Automatic Antennas that Don't
Having recently seen some discussion on this topic on the net, I hope the
following will be of some interest (and maybe save somebody a few bucks).  By
the way this was originally written and formatted on a word processor program
and cut and pasted to here.  Hope the formatting doesn't get too badly messed

                                    Automatic Antennas that Don't

If the automatic antenna you have doesn't (fully extend/retract when the radio
powers on/off) this may help you.  The unit described herein is factory
original as installed on an '83 528e, and I expect typical/standard over many
years and models.
This is intended to address the situation where you can hear the antenna's
drive motor operate, but the mast fails to either fully extend or retract.  I
suspect that many "failures" can be simply fixed, w/o purchase of parts (new
mast $35 -40)  new antenna assembly ????.  The following describes how I fixed
mine at no cost.
The first concern is that the mast itself moves freely, and is not sticking
because of dirt or gummed up old lubricant.  Get the mast fully extended, or
remove it from the drive assembly, thoroughly clean, lube lightly and be sure
that the telescoping sections move freely, re-install and or fully retract. 
The Bentley manual describes how to remove and replace the mast.  If it fails
to fully retract, you can do this manually by CAREFULLY pushing it STRAIGHT
down.  Be careful that you don't bend it!  Note that there is a retaining
collar at the base of the antenna assembly on top of the fender that has two
flats (13mm if I remember) that unscrews to facilitate removal and replacement
of the mast.  The mast itself is a set of telescoping sections.  Attached to
the end of the topmost section is a long nylon(?) flexible "cable", about .125"
in dia. and maybe 3 ' long.  This is the actual part that is driven by the
motor/clutch assembly to raise or lower the mast.
Locate, remove the cover, expose and examine the drive assembly.  It is a box
about 4x6x1.5 " with a black plastic cover that is held in place by 4 Phillips
head self tapping machine screws into what appears to be a die cast aluminum or
pot metal body.  
 Inside the box you should see the following basic arrangement. [I will use the
"clock" scheme to describe where the various major pieces are, 12 o'clock = top
of the box, 3 o'clock =  middle right hand side, etc.]  
The drive motor is a metal cylinder, oriented long axis vertically located at
between 2 and 3 o'clock.  It has a worm gear on its shaft that drives a large
plastic gear at the very bottom (or back) of a stack of various disks, some of
which freewheel on the shaft of the gear driven by the motor, and some of which
are "keyed" to that driven shaft.
On the opposite side from the motor, about 10 o'clock, you will see a relay.
coil on top, contacts on the bottom that pivot up and down from one side.  With
the radio off and the mast retracted, if you use your finger or a wooden stick
to push the relay contacts closed (up), it will actuate the motor.  If you hold
it that way, the motor will run until the driven shaft described in the prior
paragraph rotates a fixed number of turns and the mast (if properly operating)
will have been driven to full extension. When you release the relay, the motor
again actuates in the opposite direction a fixed number of turns to retract the
The largest and bottom disk on the driven shaft, looks like a pulley, is just
above the gear driven by the motor, and FREEWHEELS on the shaft.  The "flanges"
of this pulley, along with a pinch roller located at about 7 o'clock grip the
nylon cable and drive it up or down.
The next piece outward from the pulley is a clutch plate that drives the pulley
but also freewheels on the shaft.  Next outward is the "pressure plate" that is
keyed to and driven by the shaft.  What I think usually happens, if the motor
operates and the mast is free to extend and retract, is that the clutch slips
and thus the pulley doesn't turn enough rotations to fully extend or retract
the mast.
Next two pieces are spring "washers, very roughly star shaped, and convex,
assembled so that they provide some constant pressure against the pressure
plate and clutch assembly.
   \\\\\\\\\\\    gear driven by motor worm gear
  _______________    "pulley"
/                              \
    _____________        clutch plate
    _____________        pressure plate
   \____/    washer #1
   /        \    washer # 2
                                      \  washer with lock tab
 I                                 I       Sheet metal nut.
Next outward from the second washer is a large flat washer with a tab that is
bent up to prevent "loosening or tightening" of the large sheet metal nut that
tops off this whole assembly.  This washer is also keyed to the shaft.
The way this whole thing works is that the driven shaft transmits torque
through the clutch assembly to drive the "pulley" which in turn drives the
nylon cable up or down.  If the clutch slips, the pulley doesn't turn the
required number of turns to drive the mast, but the counting mechanism on the
driven shaft does turn, and after the predetermined number of rotations, stops
the mechanism.
If you carefully use a screw driver to bend the locking tab back you can free
the large sheet metal nut (about 1.25 inches).  Before you start to move that
nut, mark it in such a way that if you remove it, you can get back to the
original starting point.  Then remove the nut, disassemble the layers down to
but NOT including the pulley.  Use some solvent to clean off the clutch face
and pressure plate dry well, and re-assemble.  
Starting with the mast fully retracted and the sheet metal nut in it's original
position, actuate the relay with your finger until the mechanism "auto stops",
release the relay and again let it run to "auto stop" so that the mechanism
runs through a full extend/retract cycle.  If there is still some slipping of
the clutch and the mast does not fully extend and or retract, push it down so
that it is fully retracted, try tightening the nut one flat more, and again
actuate the relay to run through a full extend/retract cycle.  Repeat until you
get proper cycling of the mast, bend the lock tab up to lock the tension
provided by the nut and button up the cover.

Hope this helps.