We have 8 running 740s and 940s all with low coolant switches in the coolant reservoir tank. The first thing I do is replace the coolant reservoir with one having the low coolant switch and running a wire to the instrument panel. The instrument panel indicator for all the 940s is already in the panel and just needs a wire, ground wire, and connector. I used the parking lamp for the 740 since the parking brake is never used on that car. I started adding these low coolant switches after an 89 740 had it's first indication of low coolant when the "check engine" light came on and then my daughter saw that the temp. gauge was pegged, then the heater core blew anti-freeze all over the interior and the head gasket went. That car went to the crusher.
The faqs tells how to install the tank with the sensor.
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Mine: 4-940s running, 2-740 and 1-940 parts cars, and 3 John Deere 630s, dtr1:3-940s, dtr2:1-740