V8 radiator fan, cooling problem. Long Post

["Scott M." <scottmo@teleport.com>]

Hello gang,

I was checking out the cooling system problem in John K's
1990 V8 Audi and found a "few" interesting tidbits of info
in the repair manual. The V8 uses a engine driven fan (with
temperature controlled fan clutch) as well as an additional
3 speed electric fan. The fan clutch is similar to what has
been used on many US vehicles (ford, chevy etc) for many
years, it has a thermostatically controlled valve which
directs fluid from the storage chamber into the coupling
chamber to allow the fan to be disconnected from the engine
driven pulley at cold engine temps and to being directly
coupled under hot engine temps. The two temp. sensors that
control the electric fan speed are located as follows:  one
is in the engine block near the front left where the second
leg of the top radiator hose connects, the second radiator
temp sensor is in the radiator lower right side underneath
the lower radiator hose connection.

The funny part is that the electric fan speed is controlled
by the computer chip in the instrument cluster! The test
procedure involves unplugging the connectors from both temp
sensors and then putting a 400 ohm resistance across the
connector. After starting the engine, the electric fan
should NOT  run under this condition with these 400 ohm
loads across the temp sensor connectors.( i.e. engine is
cold) Then you remove the 400 ohm load and connect a 100
ohm load across both of the temp sensor connectors (one for
engine block temp and the other for the radiator temp).
After doing this the electric fan should first run at the
low speed, then switch to the medium speed and finally
switch to the high speed. It takes a few seconds for this
low-med-High speed operation to occur. If you disconnect
the sensor connector completely for a few seconds the
computer will think the wiring is defective/open circuit
and will step through the low/medium speeds and then turn
on the high speed fan. The two temp sensors are spec'd to
have 360 ohms at 72F and 70 ohms at 266F (hot!), my
resistance tests with a DMM and boiling water indicate a
sensor resistance of aprox 135 ohms at 212 F (100C).

According to the Bentley the low speed fan is supposed to
turn on at 207 degrees F, off at 203, the medium speed on
at 212 and then off at 208 F and the high speed is on at
224 and off at 217F. 

This particular V8 had a sticky/lazy thermostat that would
only open under higher water temps/high water flow which
caused the temp to rise to 120C at idle even though at
higher engine speeds and more flow the T/Stat would open
and bring the engine temp below 100C. The thermostat is
supposed to begin opening at 87C and be fully open at 100C.
The thermostat housing uses two Torx screws (T30  I think?)
to mount it too the engine block in case you are out in the
wilderness and need to change the T/stat. The OEM T/Stat
has an air vent/check valve at the top of the T/stat to
help get air/water flow through the T/Stat during fill
up/run conditions. The Bentley mentions that during the
filling of coolant through the overflow tank  that you
should squeeze the upper/lower rad. hoses to get any excess
air out.

Oh BTW, The medium speed fan relay is located in position
#6 and the high speed fan relay is in # 12 which is next to
the two circuit breakers. (under the right kick panel)

The V8 has a sophisticated fan control.................
Scott M.