On my 1994 400L the hot light comes on shortly after start up then it fill the overflow tank with antifreeze. Any ideas?

 

Most likely a bad thermostat or some other blockage. You can check thermo, by putting it in very hot water. Wouldn't hurt to flush the whole system, either.

Another possibility is a leak between the cylinder and a water passageway, this will super-heat the water and doesn't always show up as a leak.