Amadeus

Or: "Significant disparity between temp gauge and actual temperature."

I just had my thermostat replaced because I believed that I was running cold. The temp needle was taking for ever to come up to normal, and sank to cold when on the highway - especially on a long down hill in cool outdoor temperatures.

Well the new thermostat did not change much. Before yelling at the mechanic to do his job correctly, I hooked up the Vag-Com and logged the coolant temperature while driving. It reads normal. Up to temp in about 5 minutes (95 deg C), and then stays there (+- a few). At the same time, the gauge on the dash was doing it's own thing.

I have a set of Bentleys in front of me, but I can't figure out what sensor the dash is using, or what I should be looking for.

HELP! Anyone have any insight on this?

Amadeus

Nevermind, I think I found the sensor. They call it a "switch". I hate that.

Rud67

The senosr is probably bad, it developes a "dead spot"...make sure the thermostat is fine as well.

RangeR BoB

Where is the sensor located? At my last cooling system service, I had them replace th fan switch, I guess I should have had them do this as well. I guess I can replace it if I am quick with my hands w/o losing too much coolant ;-)

Amadeus

1995 A6Q (AFC)
Symptoms:

Temp gauge indicating engine running cool on the highway, especially when going downhill for a few miles.

Almost, but not quite normal readings indicated while driving in traffic (secondary roads).

Replacing coolant thermostat did not change the situation.

Ran diagnostics (laptop and Vag-Com) while driving, and determined that the actual engine temperature was normal (steady) during periods of temp gauge fluctuation (measuring block 99, channel 3).

Determined (discovered) that there were two temperature sensors, one for the computer, and one for the temp gauge. Replaced the one for the gauge. Problem fixed.

Gauge part number : 034-919-369-C. Stealership parice: $116, on line price $53.

Located near the fire wall (passenger side), attached to a coolant pipe going from the head to somewhere else. Very simple to replace. With a cold engine, I did not loose any coolant. Repair took about 15 minutes. Only tool needed was an adjustable open end wrench.

Lessons learned:
Use Vag-Com before jumping to conclusions. That is why you spent all that money to get one.

Do not purchase parts from the dealership, unless you have to.

Watching a graph of engine RPMs can be fun while shifting.

RangeR BoB

Good writeup, and thanks for the answer. Where did you order the part?