I have fitted one of the $2 float sensors from ebay. After an initial hic-cup all is good and I would not be without it. Saved the engine once when the water pump gave up on a beach.
My 1st attempt was to put the whole sender in the expansion tank with just a hole for the wires. This worked for a few months but it started to loose insulation and I started to get false alarms. I then used a slightly different sender and fitted it onto the wall of the tank below the filler cap. This is still going strong with out problem. I like the fact that the warning goes off when very little coolant is lost as the sensor is at the highest point of the system and not in the top hose or there abouts. Total cost was about $10 for the sender, 2x flashing leds in housings and a 5 pin relay. Would be the very 1st job I did on a P38 if I was to get another.
http://www.aulro.com/afvb/p38a-range...ant-alarm.html

