[V8] Overheating problem - A tribute to Audi engineering and my own stupidity

[Intemann, Paul (GEAE)]

Think this is the post that Bob is referring to ...

Keith, I hope you don't mind my re-posting your words-of-wisdom below:

I figure I better explain the circuit fully to eliminate any confusion from
something I stated in an earlier post.

The following description applies to 3.6 and 4.2 cars.

 There are two sensors, G82 on the left head, and G83 on the radiator about
1
inch from the outlet. Both are used by the fan control circuit of the
instrument cluster.

 G82 is used by the analog temp gauge circuit and for secondary control of
the electric fan.

 G83's only function is as the primary sensor for control of the electric
fan

 Due to the relative locations of these sensors in the system it is not only
possible but virtually guaranteed that G83 will not read as hot as G82. The
situation this brings on is the gauge starts climbing into the high end of
its range and the outlet temp of the radiator may not be high enough to make
the instrument cluster trigger the electric fan. (explaining the sudden rash
of calls I'm getting from frantic owners)

 Geeky stuff........
 The instrument cluster fan control circuit completely ignores any reading
from G82 as long as G83 is in the loop, that is to say providing a current
path. Unplugging G83 results in the fan control circuit defaulting to G82
for
fan control. Unplug them both and the fan control circuit assumes an
overtemp
condition, takes control of the gauge making it read on the high side, and
engages the fan in 3rd speed.

 IMO the test procedure in the manual is a perfect example how incomplete
information can cause mechs and DIYers to develop bald spots. Since I
despise
"Easter egg" troubleshooting and "shotgun" repairs I've come up with my own
tests to fully exercise the system, someone may find them helpful. If anyone
decides to use these tests please read through them and notes a) thru d)
before starting.

 1) After the car has been running does the temp gauge have a reading? If so
G82 is working and the wiring to the cluster is good. If the gauge reads low
all the time the thermostat has probably failed, a defective G82 is possible
but not likely. If the gauge doesnt read at all then do test 2, 3, and 4. If
they pass but the gauge doesnt do anything in test 4 the gauge is defective.

 2) With the car running place the climate control temp setting in LO and
turn on the A/C. The fan should now cycle from low speed to second speed and
then stay in 2nd speed. Obviously this test requires a fully operational A/C
system. An alternate test is to jumper each speed by removing the respective
relays at the fuse panel to verify fan operation.

 3) Unplug G83 and insert a 100ohm resistor into the connector. Start the
car. The fan should now cycle through all three speeds. If this doesnt
happen
proceed to test #4, if test #4 fails the chip in the cluster has failed.

 4) Shut the car off and remove the resistor, leave the connector loose,
unplug G82. Start the car. The temp gauge should now read high and the fan
should cycle through all three speeds.

 5) Shut off the car. Reconnect G82 and leave G83 unplugged. Take a short
drive to ensure the engine is warm. Place a large section of cardboard in
front of the radiator ensuring the entire radiator face is covered (it
doesnt
have to be perfect). Start the car. The temp should rise fairly rapidly past
the 100c mark on the gauge. At about halfway between 100c and the next
higher
mark the electric fan should kick on in low speed.

 6) Reconnect G83 and redo test 5 with this one change. The fan should come
on at about the same temp as it occurred before. If it doesnt then G83 is
either defective or the radiator is clogged.

 Notes:
 a) The tests must be done in the order they are written for the results
I've
supplied to be valid.

 b) The A/C should be off for all test with the exception of #2

 c) The three speeds of the fan are pretty easy to recognize. Low speed
should be almost silent but moving a pretty fair amount of air (much more
than the engine fan)
 Second speed is easily heard and moves a lot of air. High speed sounds like
a blow dryer on high and will make you think you have a small hurricane
generator under the hood. For situations where the fan doesnt operate at all
or fails to operate in one or two modes please refer to the manual for
troubleshooting.

 d) If you have an overtemp condition and the entire electric fan system is
operational, there are a few other possibilities. Listed by likelihood IME:
Failed mechanical fan clutch, clogged radiator, blown head gasket/cracked
head, or stuck thermostat.

 The really quick minded may have seized on the fact that if you leave G83
unplugged an otherwise fully functional system will operate the way most
would assume, with the fan coming on by engine temp and not radiator outlet
temp. I dont recommend anyone intentionally bypass a sensor Audi installed
but ..........

 I know of at least two V8s operating this way, and one will soon have two
other mods to ensure proper cooling.


Keith
cool running 89 V8 # 527



-----Original Message-----
From: bob moy [mailto:mmodels at hotmail.com]
Sent: Monday, April 26, 2004 10:33 AM
To: V8 at audifans.com
Subject: [V8] Overheating problem - A tribute to Audi engineering and my
own stupidity


Sound as though you had a clogged radiator which prevented your electric fan

from coming on.

Keith from Audi Connection did a great writeup on this subject a couple of 
years ago.  Perhaps someone from this forum still have a copy.

Considering that this car is approaching 14 years, it would be wise to 
replace the radiator instead of trying to get it boil out.

Is there white smoke coming from the exhaust?  If the answer is yes then you

most likely have a blown head gasket.  Try running the car with the heater 
on full heat, this way if the thermostat is stuck the coolant will bypass 
the thermostat.

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