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Re: 1986 4000CS Quattro cooling fan problems

Dustin Wolpoff <Melon@centuryinter.net> wrote:
> Hi, my cooling fan will not turn off on my quattro at all unless I
> take the relay out.  It is even on when the key is off.  How do I
> diagnose and fix this problem?  Where are the sensors that tell it to
> go on? 
> Thanks.  Dustin

Hi Dustin. I just went through this recently.  Are you talking about
when you turn off the car?  If you are having problems with the after
run feature, the only sensor that controls it is the switch on the rear
of the valve cover, on the passenger side.  When it's hot it closes and
that signals the relay to turn on the fan.  Also, there is a timer built
into this relay (that is under the dash) that is supposed to turn off
the fan after it has run for x minutes, regardless of the switch on the
valve cover.  For this timer to work, the relay needs to have a good
ground connection, so check that there is continuity between pin 31 and
ground.  If all else fails, you could have a bad relay.  Removing it is
a temporary fix.  Keep in mind that the fan may come on even 15 minutes
after you park the car, but it shouldn't be running for an hour either.

Now, if the fan stays on whenever the ignition is on, there is a
different relay that controls it (it's in the fuse panel on top of the
firewall).  There are two switches that signal the relay to turn on. 
One is the radiator temp. switch (that gets power from the wipers) in
the bottom of the radiator and the other is the A/C High Pressure switch
(that gets power from the load reduction relay/AC fuse).  Either one of
these two will turn on the fan.  Any one of them could have failed
closed, or the wiring could be shorted.  Still, even if these switches
have failed, the fan should turn off once you turn off the car (at which
point the after-run relay takes over).

As a last resource, you can fully disable the after-run relay by
unhooking the big 4mm red wire that supplies power to it from the bus
under the fuse panel and live without it.  You may also want to fuse the
fan both to protect the wiring and to have an easy way of turning off
the fan.  There is a very easy way to do this using the factory "spare"
fuse location #5.  I posted the procedure about two weeks ago - check
the archives under "radiator fan fuse retrofit".  Good luck and hope
this helps.

Luis Marques
'87 4kcsq