fan doesn't work at all, even when I switch A/C on or temperature reaches 110 C. I disconected G82 and G83 and fan runs nicely through all 3 stages. Then I connected back G82 and G83 left disconnected, but still the same: fan not working with A/C on or then temperature reaches 100 C. Any ideas ?? Is there any special connection on A/C control unit ?? I was busy around it, maybe disconnected something.......
P.S. and I checked out by this procedure:

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.

 

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