Truck Repair: N 14 Temp problem, low oil pressure, coolant level

Question
Temp guage on 99 Volvo 770 quite showing operating temps
replaced sensor on thermostate housing, then replaced guage.  Temp guage only shows activity during hill climbs but does not register during normal flat grnd operation.
Told the engine  has other overheat protection. True??
Truck runs fine and operates normal just no guage to monitor.  Ideas?   Thanks for the service  

Answer
Make sure engine protection is turned ON in the ECM then no damage can occur from overheating,low oil pressure,low coolant level,and a few others.The engine has its own sensors for the ECM and Volvo hooks up to the data link of the engine to get the info for the gauges.If the temp sensor had a problem then a fault would be logged.