Steve,

The idea is very simple and easy. First you have to understand how windshield washer level sensor works. The design is based on a "read" switch and a magnet the "round float". Inside the tube that float is moving up/down (it depends on the water level) in the bottom part there is a small glass bulb, it is called a read switch. When the float goes down because the water in the container goes down it closes the circuit (grounds it) and the light on the dashboard goes ON.

Now, take only the float out of that sensor and buy a "read" switch, normally-Open, in any radio shack store (it cost very little). Find a piece of antenna mast or a tube, on Volvo those antenna masts go down often so you'll not have a problem finding one.

Unscrew the coolant expansion tank cover and take the black insert out. Drill out the a small hole in the back isert, the size of the antenna diameter mast (or a bit smaller) it mast fit perfectly so there will not be any leak as there is pressure inside the tank.

Cut the antenna mast long enough so after closing there will be space between the end of the mast and the bottom of the expansion tank, about 0.5cm.

Solder two wires to the read switch and position it close to the end (not at the bottom). You can test it with the float and voltmeter (the buzzer) when you connect the voltmeter to the wires the float will close the circuit and you'll hear the buzz.

Seal the end of the antenna mast so it will withstand the pressure from inside the tank and coolant solvent, inset the mast into the black inset from the cap and seal it.

I used 5min. epoxy from the hardware store but it doesn't work very good. I have to find something else to re-seal it.

If you'll find something that will work let me know.

You don't have to drill a hole to run wires though the outside part of the cap. the insert is design in the way that you'll be able to run those wires out.

Run one wire to the ground, the most convenient location is by the battery, there are ground connectors.

Second end run to your dashboard.

WARNING: Next section apply only to 740 '86. It might apply to other models but you have to look up the manual. For example I have 740 '86 and 740 '88. The panel warning lights from 740 '88 is different and there is no coolant window light on '88 model.

The 740 '86 has a coolant warning window light. It is the last light to the right on you dashboard. You just have to supply a light bulb as there wasn't one from the factory.

Run the second wire to 12-pole connector (there is only one 12 pole connector YOU CAN NOT MISS IT). Connect that wire to PIN NUMBER 7 (seven) on the 12-pole connector. Those pins are numbered on the connector itself.

In Volvo Wiring Diagrams (manual #TP 30893/1) page #78 & 79. The 12-pole connector C7 has a NO description and under heading "Instrument panel lamps" number 1 says "Not Connected". This is for coolant level Sensor on 740 '86.

The 12-pole connector pin number seven has no metal clip that clips to the wire. You have to buy it from a dealer. Once you open that 12-pole connector, you'll know what I'm talking about.

You ARE DONE.

I would like to take a coolant level sensor from 850 and fit it to the expansion tank on 740 '88 but must find out how it works and a way to seal it as that sensor is positioned on the side of the expansion tank.

Regards,

Joseph

@Edmonton