Is your W124 losing coolant rapidly?
Check for signs of leaks around the connecting rubber hoses to the radiator, thermostat housing and engine cylinder head. Check also for water stains at the coolant expansion tank cap itself.
If external leakage signs are not present (ie water stains), it may mean the coolant is leaking into the combustion chambers via a damaged engine gasket or worse a cracked cylinder head.
Signs to watch out for :