Changing 5KT afterrun fan time limit

[Aleksander Mierzwa <alex@matrix.pl>]

Hi List,

It seems that many 5KT(Q)s are plagued with a charging/battery problems.
The common explanation is that those cars, while having many
current-consuming accessories, have only 90amp alternator. What I noticed
is that actually the biggest battery-eater is the afterrun fan, which
usually runs for 10-15min every time the engine is switched off at
operating temperature. It does not cause problems if the car is used for
long highway trips (the alternator has plenty of time to recharge the
battery), but if one uses the car mainly for short trips around the city
(like I do), particularly with some accesories on, the alternator doesn't
have time to recharge the battery, and since battery is usually left in
discharged state for most of the time, it's lifespan is severly reduced.
Apparently, Audi knew about the problem. They changed the alternator to a
stronger 110amp version on late MC engines and changed the afterrun fan
thermoswitch to one operating at higher temp on 3B engines. After buying
3rd battery in 4 years, rebuilding the alternator, putting new voltage
regulator, replacing the fan thermoswitch, having found the wiring to be OK
and being *really* tired of having to constantly worry if the car will
start, I came to the conclusion that this indeed is a desing flaw. While I
understand the importance of cooling the turbo after shutoff, I think the
10-15min of cooling action *every* time the engine is at operating
temperature (not only after "spirited" driving) is just way too long. There
is a time limiting device incorporated into the electric coolant pump, but
it's set from factory to about 20min. My fix reduces this time limit to
lower value.
The electric coolant pump/afterrun fan relay is located on an additional
relay panel under the acces flap on left side of the dash. See the
schematics on the access plate for exact location. After removing and
opening the relay you'll se a circuit board with lot of transistors,
resistors and one 220µF/16V electrolytic capacitor. This capacitor is
responsible for the time limit. I replaced it with 47µF capacitor (NOTE
POLARITY before desoldering and use 16V or more capacitor) and now the fan
runs for about 3 minutes. I believe it's enough to cool the turbo and the
starter spins the engine happily at full speed every time now. If you feel
3 minutes is not enough, use larger capacitor.

Aleksander Mierzwa
Warsaw, Poland
mailto:alex@matrix.com.pl
87 Audi 5000CS turbo (mine)
88 Renault Medallion wagon (mom's)
91 mountain bike (just in case both cars broke at the same time :-)