I just bought a new car and out of curiosity, during one of my trips, logged the data using OBD scanner. I noted the max engine coolant temperature was around 100 deg C. What is the redline here? What temperature should I be alarmed at?
Far enough below the coolant's boiling point that you don't need to worry about what happens when you shut the engine down (and you loose coolant flow, but still have a lot of residual heat).
The actual value will depend on the coolant that you're running, the concentration, and the setting of the pressure cap. But assuming a fairly standard coolant, diluted 50/50 with water, and a 15 PSIG cap you're looking at around 265° F or 130° C before things start to boil. The data sheet for the coolant should have a curve for different concentrations and pressures (although 50/50 and 15 PSIG is very standard).
Normal operating temperatures for a VW diesels are in the range of 170° F to 210° F with the thermostat set to open around 190° F. The actual gauge is controlled by the ECU and is filtered to read 190° F anywhere in that range (to avoid causing concern at the temperature varies in normal operation). The recommended coolant (G13), when diluted 50/50 has a boiling point of 228° F (109° C) at atmospheric pressure and 248° F (130° C) at 15 PSIG (roughly 2 bar or twice atmospheric pressure).
So it seems like what VW is saying is that with these engines somewhere around 40°F (22° C) below the boiling point is a reasonable margin.