Help!!! [V8] RE: Hydraulics & Overheating

[Jeremy Ward]

Dear overheating in Vegas,
 (Ooh, I feel like dear Abbey now!) :-) 
 Zay, I found this writeup from Brother Keith in my archives; I hope he 
doesn't mind the reprint, because it is the best one I have ever seen. Good 
luck!
 Cheers,
 - Jeremy
 KT says:
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 


<<snip>>
Answers are appreciated for this desperate man.

Zay Campozano | ComputerEngineer | MGM MIRAGE(tm)
rcampozano at MGMMirage.net