Trying to find coolant gauge problem

["Dave Ohlemacher" <dmo1205@ix.netcom.com>]

I have a 1990 80q. The coolant gauge will work when the car is cold and the
needle will rise to just past the hash marks and then fall all the way back
to nothing. A few times the gauge has started to work after driving for a
while, but it never lasts for more than a few minutes.

When I asked this before of the list I had a number of "It's the $50-64
sender, common problem" OK, so I go to the parts guy at the Audi dealer and
he says "There are 2, which one do you want the black 2 wire ($49) or the
brown 4 wire ($99)?" I have no clue because Bentleys says that the 2 wire is
used to send info to the warning light and the gauge and the 4 wire is for
the auto-diagnostic stuff (which I don't have, but the sender is on the car)
The parts guy says he stocks the $99 part because it's the one that goes out
all the time.

I tried one experiment in that I disconnected the 2 wire and thought, OK,
car cold, gauge sender disconnected, gauge shouldn't work, but I found out
the car would start, but not stay running for more than 30 sec. ?????

Any and all advice welcome.

Dave 0