Excellent deduction, cbdo, although I must admit I was hoping for a different answer. I'm fighting a high operating temperature (110c+), with an occasional pegging of the meter. The strange thing is, the cooling fan does not kick on. I manually grounded the fan switch, and it kicks on perfectly. I've replaced both the fan and gauge sending units, as both of them had the center probes loose (?). Manually kicking the fan on does not change the temperature reading. I should tell you this all started by replacing the thermostat, because on each start up the gauge would slowly increase it's reading to 110c+, at which point the fan would kick on, temp would go down and stay about 90c. I replaced the t-stat, trying a trick that DaimlerChrysler posted about drilling three 4mm holes in the frame of the t-stat, to fix the gauge reading problem. With this set-up, the gauge would slowly increase to peg, fan never kick on, and stay there as long as I was brave enough to let the engine run. I bought another t-stat, installed it un-modified, and same result. Thoroughly frustrated, I took the springs out of the first t-stat, installed it in a permanently open position, and it takes a lot longer (about 20 minutes of driving), but the gauge slowly pegs again. I checked the manual for testing of the temp sending unit or gauge, but couldn't find ohm figures per temp. BTW, fuel mixture was verified o.k. by a local shop with an exhaust anylizer, since I considered a lean mixture to be the culprit. Also, the engine runs perfectly through all of this, and doesn't seem to radiate much heat, as I've seen overheating engines do. Sorry for the long post.

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1983 380SL
1995 C280
1995 S500 coupe
1990 Mazda Miata
2004 Suzuki Hayabusa