Audi cooling fan-stages

[rdh@sli.com (Robert D. Houk)]

   > Also, i've never heard what is being referred
   > to as "stage 3" (The DC10/Husqy stage :).

   I'll second these questions.  My (her, whatever) 84 5000S has never done any
   stage other than what I've heard discussed as Stage II, and NEVER has run after
   the ignition was turned off.  And the car has been driven REALLY hot before,
   because the radiator has a leak and runs out of coolant occasionally (why can't
   women remember to check these things?) which significantly raises the needle on
   the dash gauge.

OK, here's my take. This is directly based on my '83 Urq, which got many
of its components from the "5000" series parts bins, and at the least ap-
plies to many of the mid-'80s Audis, especially the 5000/turbos.

The main fan circuit feeds unswitched (i.e., battery => fuse => fan) +12
through the fan into the radiator-mounted (bottom left, viewed from the
driver's seat) thermo-switch thence to ground. [I think, but won't swear
that is the way; the switch might electrically be between +12 and fan, 
but I don't remember it that way]. This is the simple "single stage" mo-
del.

The "two stage" model adds a "dropping" resistor in series with the fan
motor. This provides a "low speed" (resistor in series with motor) air
flow with the ignition off (remember, unswitched +12) and a hot radiator.
With the ignition on, a relay engages which bypasses ("shorts") the drop-
ping resistor, applying full +12V ("second stage") to the radiator fan.
This is still a two-wire-to-the-fan circuit, and applies to the, er, low-
life Audis (sorry, couldn't resist...) of the time (like models without
A/C, turbos, and those other Amenities of Life).

Finally, UrQ's, 5000's (well, Turbos, anyways), etc. add a "third stage"
(and a third-wire-to-the-fan). I'm not sure if this third wire is a dis-
tinct winding on the motor, a different tap into "the" winding, or simply
a bypass for an "internal-to-the-motor" resistor (I've heard all put forth
as the way it is). Whatever, it is a "gale force" stage, noticeably stronger
than the normal radiator fan cooling stage (and a *severe* challenge to
the wimpy little 25A fuse that Audi routes it through [which is an entirely
different story that I won't re-delve-into]). This stage is relay-controlled
(what else?). The relay gets a feed from either the A/C master power relay
or the overtemp sensor mounted inline in the coolant-reservoir-to-engine
feed line. 

Earlier cars use a ~250 (?) watt fan motor (kinda long and skinny), while
sometime in the mid/late-80's Audi switched over to a (shorter and fatter,
or at least fatter...) 375 (?) watt "hurricance force" motor; dunno what
they did about fusing that sucker. At least they upped the wire gauge
(from inadequate-for-25A, to hopefully-adequate-for-40A) . . .

					-RDH