re 80q rad. fan won't work

["Smith, Kirby A" <kirby.a.smith@lmco.com>]

Andrew Steere asked:

My  1990 80q has decided its fan doesn't need to work anymore.
Unfortunately, the engine still requires said fan, especially if I drive in
city traffic or with the AC on.  Found Fuse #5 was blown (30 amp), but fan
still not functional.  Mechanically, the fan seems fine, spins ok, ran out
of light before I could rig a lead to it.  No fan with AC, no fan with temp
rise.  Any common failure modes here?  Where is the temp sensor on this car,
anyway?
######

Fan has three pins, a common, a low speed, and a high speed.  Between the
low and high pins is a nicrome-like internal resistor.  If the motor were
burned out, or a wire disconnected within the motor, both speeds could fail.
However, before assuming this, remove your belly pan (if you still have it)
and pull the connector on the thermoswitch at the bottom of the radiator.  I
don't have the wire layout here at work, but shorting between certain pairs
should lead to high speed and low speed operation.  No operation in the low
speed circuit, check that there is voltage on the afterrun resistor mounted
by the fan motor.  If there, the fan should run.  If not there, a circuit
problem.  If the fan operates, your thermoswitch is bad.

Kirby A. Smith   New Hampshire USA
1988 90q Titanium gray, 182 kmi
1988 90q Stone gray, 187 kmi 
1988 90q Stone gray, 180 kmi - parts car
1995 S6 Pearl effect, 82 kmi