The story so far:
I found the nipple for one of the transmission oil cooler lines sheared off at the radiator tank. I'd hit a wicked, unmarked narrow and tall speed bump a few weeks earlier. The day of the blowout was the hottest of the year, so far. (Note: stay clear of all side streets in San Pablo, California). The 2000 Sonata is an unusually low-clearance car. My car leaped into the air, then slammed down onto the bump.
I'm not able to work on the car here. My friend brought over his trusty backyard muscle car mechanic (a former dealership man like HT). The fix: we put on a nice transmission oil cooler, bypassing the radiator for this. There's a lot of room for the cooler between the front plastic grille and the radiator.
Here's the interesting part: everything's fine, except that the temperature gauge is reading higher than it ever has before under all conditions. I'm carefully testing the cooling system's prowess, putting the car under a little more load every day. We're having a heat wave here, so I gave the car its biggest city street loading so far: driving home uphill with the AC on. Although the temp gauge reading is higher than it's been for the last 7 years or so, it cycled over a small range and kept within that range and recovering nicely. The computer reveals that there are no stored codes and all the drive cycles have completed.
I've been a good mechanic, but mostly on stereo equipment, and tend to look for correlations. Here's an interesting one. I keep a little digital volt meter plugged into the cigarette lighter. This is showing unusually high system voltage, which has been ranging between around 14 and, today, briefly 15.5.
My theory: if the system voltage is high, the reading on the temp gauge is increased because the gauge, itself, is getting more juice. Is this correct? Any comments?