low coolant message

[rescue7]

I am getting a ecu low coolant message and check engine light.
It only happens if your speed is over sixty fity MPH or quick stop. You can drive all day long at 62 and it will not come on. This is on a build date June 2006 Mercedes 900 engine. Coolant level is full. Had a freightliner service depot look at it but had no idea on problem.
Anyone from Freightliner have any ideas,.
Can someone tell me where the sensor is located.

Thanks

Don

[GaryKD]

Hi Don,
I do not know your engine. However, low coolant sensors can be found on the coolant expansion tank.