Re: Trans temp gauge
Having the gauge in the "to cooler" line will show the hottest point in the system. It's that temperature that is the best indicator of transmission fluid life. Many of the electronically controlled transmissions of today have a sump (in the pan) temperature sensor. That sensor is intended as an input to the control system, and as a diagnostic indicator of the temperature of the transmission itself. However, to monitor the temperature the fluid is really exposed to, the "to cooler" point is still the best.
If at the "to cooler" port the fluid sees 300 deg F, but you're monitoring a sump temperature of 250 deg F, you'll never really know the fluid is breaking down.