Diesel Temperature 300 SD
 

What is a safe reading on the dash temperature guage in the summer heat, with the ac running, sitting in traffic? At what point is my engine over heating? Is 100 degrees C safe? That is 10 o'clock on the guage.

So long as the temp drops back down when you start moving, this is normal. You should hear the aux fan running in high speed intermittantly, and the visco clutch should be locked up. Check that by shutting the engine down while the temp gauge is reading 100C or higher, the fan should stop within one revolution. If not, replace (or refill) the visco clutch.

Peter