I see that VolvoKurt is experiencing some erratic temperature gauge readings, so I figured I'd add my experience as well.

Has anyone ever had a "good" temperature gauge bottom out while driving?

You see, while I was returning to school a few weeks ago, the temperature gauge needle dropped to nothing, where it normally lies when the car is off. Mind you, this reading came after at least an hour of interstate, 75-mph driving. A few minutes later, the temperature gauge went up to about midrange, perhaps a tad higher than 9 o'clock.

The only alternative to flaky wiring that I can guess is that the sensor did actually get cold. I don't know where it lies in the engine bay, but since I pulled the Aero headlights and put in the quad rounds, I don't think my front end is quite as tight as it used to be. I'm guessing there might be a little more airflow into the engine bay, as I did notice heat "waves" coming from the entire front edge of the hood while idling for a while at one point. With the old headlights, the waves would only come out through the grille area, not over the lights.

So, perhaps some good gusts of wind into the engine bay landed right on the sensor and chilled it for a while. Since then, there might have been one or two instances when the gauge was off, but it's been working fine otherwise, after it warms up in the cool weather we've finally started having. If I'm not right, though, I guess I might be bypassing the temp compensation board at some point.
--
'89 244 GL -- 108,700 miles (see profile for info on car)

Hi. I had that once, on my '84 240. I was inclined to blame my sender, as I considered my gauge trustworthy (because this '84 don't have the notorious compensation board that my two '93's have and both of which have been flaky -- and subsequently bypassed).

But, I also have a Raytek IR thermometer (the one that IPD sells), and I found that my coolant was really getting cold -- naturally, then I knew it was actually my thermostat, which was occasionally sticking open! I changed it, and it hasn't reoccurred.

It does happen! Consider this as a very real possibility!

I'm just grateful it was sticking open, and not closed!!!

Yup, sounds like a bad temp. compensation board.

--
90 244DL about 1/4 million miles - original engine/drivetrain :)

Surely a faullty Temperature Compensation Board... Very frequent on the bricks (mine too!): a little piece of (s--t) electronic components that is doing a bad job because of oxydation or play in connections!

Look for an old post... All is said about what to do, and very easy.

The temperature gauge sending unit measures the coolant. There's no way any gusts of wind are going to cool it down like that. It's possible your thermo is starting to stick, because that's the only other thing that will drop the gauge that quickly. If you've got an '89 and it still has the temp comp board, I think the writing is on the wall for you to do the bypass. And when that happens you can actually trust the gauge.