[V8] Overheating

Jeremy Ward jward at mti-interactive.com
Tue Jun 29 18:52:48 EDT 2004

Could be a number of things...  Is there voltage at the fan? (fusible
link under the hood could be shot)  Are both the 'in' and the 'out'
hoses warm on your radiator?  Might be a clogged radiator (shouldn't
affect the electric fan from coming on, though; might have more than one

You might see if there are any trouble codes.  There are transmission
error codes and engine codes which can be blinked using the instructions
found here:

There are also climate control codes.  Instructions found here:

When in doubt, refer to the trouble shooting wisdom of the master

"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
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

 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.

 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.

cool running 89 V8 # 527"


- Jeremy

-----Original Message-----
From: v8-bounces at audifans.com [mailto:v8-bounces at audifans.com] On Behalf
Of John Bysinger
Sent: Tuesday, June 29, 2004 3:40 PM
To: v8 at audifans.com
Subject: [V8] Overheating

We've had a nice streak of warmer weather these last couple of weeks up
in rainy washington, and it looks like the v8's not too happy with it.
got stuck in some stop and go traffic the other day and first noticed
the v8 didn't want to shift, then checked the gauges and the temp
high, right around the 100 mark on the gauge (still a new car to me so
not sure what the limits of normal and high are yet.)  I was able to get
of the traffic and drive home at a decent, constant speed and noticed
temp dropped back down to ~ 95 or so, but she still didn't want to
I'm guessing the shifting was a 'limp home mode' when the ECU notices
go awry, as it was in 4th and stayed that way until I shut it off and
started it back up that night and it shifted fine.

So now I've got a week off and time to put into figuring it out and
it up in the driveway, the overflow tank doesn't seem low, and no big
puddles under the car so I tried starting her and letting it idle until
operating temp.  It took close to 30 minutes for it to reach a full
operating temp, with a slow climb on the gauge.  However it passed the
degree mark and I noticed the overflow tank showing signs of a pressure
buildup and leaking a small ammount through the fill cap, so I shut it

Two things I think may be contibuting but wanted to bounce it off of you
guys first.  One, the drivers side electric fan on the radiator wasn't
running, is this a secondary fan?  and should it have been?  Two, I'm
thinking it may be a stuck thermostat not opening properly, and I'm
two possible thermostats available, a 180 degree and a 192/195 degree.
Which temp are you all running on yours?

And the last note, when pulling the airbox out of the way, I noticed the
previous owner had put one of those 'vortex' things in the openeing to
throttle body.  I had to laugh a bit as there's a laminar flow grate
behind it defeating the purpose.


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