Chuck, you might be right, the sending unit is obviously variable resistance to ground, but it might not be based on battery voltage. The "instruments" schematic shows the temp gauge and the fuel gauge hooked to the same source, but I haven't figured out the source. The lead is marked "continued on following page", but it's the last page! An update on my problem: I just took a drive, about twenty minutes in 85 degree temps, stop and go, A/C OFF (stupid me, I hadn't tried leaving the A/C off for an extended amount of time), and the temp gauge registered a steady 90c. Turn on the air, and it begins its slow climb. Earlier today I took the same test drive, A/C on, and it never did peg (though I think continuing the drive it may have), but hovered around 110-115c. I wouldn't think that it should run so hot with the A/C on; based on this my attention turns to the radiator. Thanks for the input, guys, this site is great.

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