How-To: R&R UrS4 Power Antenna

[Fred Munro]

    I just finished replacing the mast on the UrS4 power antenna and
cleaning drive mechanism. These units are a nifty piece of engineering and
are easy to service. Thought I'd put together a "How-To" for those facing a
similar job.
    My original mast had two sections seized. You can replace the mast by
removing the entire unit or by leaving the unit in the car and running the
old mast out and the new mast in. I recommend removing the unit - if it has
run long enough to need a new mast, the drive chamber and cable magazine
will be packed full of dirt and grit. In my case, the threaded bushing at
the top of the mast housing was seized to the housing and the top snapped
off when I tried to unscrew it, so I had to remove the unit anyway.

Removing the Power Antenna Unit:

The unit is pushed into the rubber grommet mounted in the fender and secured
at the base to a bracket. To remove it:

1. Remove the bolt and nut (8 mm) from the bracket at the bottom of the
antenna unit.
2. Pull the unit down and out of the rubber grommet in the fender.
3. Remove the antenna cable, the ground wire (Torx T25), and the wiring
connector.
4. Carry it into the clean room for surgery

Disassembling the Antenna Unit

    The antenna box has two covers - a "front" cover over the cable drive
chamber and a "back" cover over the cable magazine. To disassemble the unit:

1. Remove the screw (Torx T25) in the centre of the front cover. This will
also release the back cover. Remove the front cover, the back cover, and the
cable magazine.
2. Remove the cable guide at the bottom of the drive chamber ( 2 Torx T8
screws). There is a steel guide wheel under this plastic guide - remove it.
3. Remove the plastic drive gear and the cable drive gear. This is a 2-part
gear connected internally by a hairpin spring. The outer gear is driven by
the worm, the inner gear drives the cable.
4. Pull the cable out of the magazine chamber.
5. Unscrew the bushing at the top of the mast housing. Pull out the old
antenna mast and cable. In my case, this bushing was seized to the housing.
I extended the antenna as far as it would go, cut it off 1" above the
housing top, and pushed the remainder back into the housing. I slotted the
bushing with a Dremel carbide cutter and peeled it out.

Installing the New Mast & Re-assembly:

    Clean the unit well, paying particular attention to the inside of the
mast housing. I found WD-40 and a 410 shotgun cleaning brush and rod worked
well. My power unit was completely packed with dirt and grit and needed a
major cleaning. I lubed the unit with synthetic grease (Superlube) on
re-assembly. To install the new mast:

1. Install the cable magazine and back cover. Tape the cover in place.
2. Slide the cable down the mast housing ( with the teeth facing "in"),
around the base of the drive chamber, and into the magazine. Push the mast
home and tighten the securing bushing at the top of the mast housing. I
seriously recommend an antisieze compound on this bushing.
3. Install the steel guide wheel and the plastic cable guide.
4. Install the inner cable drive gear and engage it in the cable teeth.
5. Install the outer driven gear, remembering to insert the spring in
between the gears.
6. Remove the tape from the back cover and install the front cover and the
securing screw.
7. Stuff it back in the car and enjoy the smooth action of a fully extending
antenna.

Fred Munro
'94 S4  114k km