I installed an aftermarket water temperature gauge. I put the sensor in the hose that comes out of the top of the rad. The water temperature at that point get to just over 105 degrees Celsius. When the gauge shows 80 degrees the cars gauge shows as being normal (middle of display). Obviously it take the temp from a different place.
One other weird thing is that when I turn on the max AC the temperature on the aftermarket gauge -measuring the temperature in the pipe off the top of the rad drops to 80 degrees and slowly goes back up to 105 after the Max AC is switched off.
Anyone understand why this happens?

 

When you select max AC the fan runs harder/faster to create more air flow through the AC condenser and hence the radiator.
According to my ScanGauge II the average operating temp for my car is 220-227ºF. Not sure where the OBD II takes temp measurement from.

 

Got you. So because the to radiators piggyback on top of each other they share the same fan. Makes perfect sense.
Good to know that our temperature gauges give a similar reading. Your ScanGauge is saying 105 Celsius too.
Thanks for the explanation.

 

fyi,

 

fyI, at least with the 1st Gen cars the stock Temp gauge is not a linear meter. Is is more or less an "idiot light" temperature gauge. It will ramp up to normal operating temperature which would be just above the center tick mark and stay there and will not fluctuate even if the temperature goes up to 225 or down to 195. It will peg max when the engine overheats and exceed the 250 mark I believe. I use a scan gauge to monitor the "real" operating temperature.

 

BMW "feature" since at least the E36 days. Temp gauge reports mid-scale and steady, right up until threshold of overheating. On my E36, with an aftermarket sensor in the head, right next to the stock sensor, I watched the aftermarket gauge rise smoothly all the way to thermostat temp, whereas the stock dash gauge rose to mid-scale until 160F, and stayed there as the aftermarket gauge proceeded through 190-210F in track conditions.