W123's do not have a coolant level sensor in the expansion tanks. The W126's do and I know you can use a W126 tank in a W123 but that is not the direction I want to go. I actually bought a W126 tank for my 83 300D (sold recently) and 2 after market float switches for it and one of the float switches failed on me (reed switch failure).

For my 85, I am going in the direction of VW where the conductivity of the coolant is measured instead of float switches. IMO, VW's method is more reliable. I have a circuit built and running on the bench where the probes are 2 stainless screws 1/2" apart. Dipped in coolant the buzzer and LED is off. Pull it out of the coolant and the LED and buzzer go on. Works great on the bench. I need to install this into the 85 tank and run tests but don't know where to drill to mount the 2 screw probes. I need another tank to cut in half so I can look inside and find the best spot to put the probes. There are no pic-n-pulls around here. Does anyone have a leaky W123 coolant expansion tank that is junk who can send one to me for the cost of shipping?

If the testing goes well I may build a few kits and offer them up for sale, if there are any demands for it.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked