It might be a good idea to install a coolant level sensor.

I put one in my DL 1982 and 6 months after it saved me big troubles on the highway. The coolant pump had a big leak.

I installed a sensor also in my 240 1991. As a warning light I used the break fluid level circuit warning light and connected to one of the wires at the master cylinder. I had to add a relay to make it work.

Now both break fluid and coolant low level work on the same warning light in the cluster.

The sesors I used are not ideal. One is a winshield washer fluid level float from a BMW and the other is water level sensor from a house air humidifier.