I’ve fitted the https://engineguard.com.au/ dual sensor engine temperature alarm. The sensors are like an electrical lug so can easily bolt them anywhere.
I mounted first sensor to the cooling water impeller housing using existing cover screw.
I would have fitted the second sensor into a spare cylinder head thread, but I don't have a suitable bolt, so I mounted it via the existing transmission heat exchanger bolt.
I mounted the display down near my right knee - it's clearly visible but not obtrusive and easy to reach.
To set it up, I ran the engine normally around lake for approx 10 minutes and then pressed buttons to recall the peak temperatures and set the audible alarm parameters slightly above that. It showed that the normal temperature for the impeller housing was 22°C (lake water temperature) and the cylinder head was at 72°C. Initially, I set alarm thresholds to 30°C and 75°C.
It works great and has immediately proven useful. During our three-week Christmas holiday break, it never operated then one day on the first start of the day the pump didn't prime. The housing began to heat up, and the alarm went off. A quick blip of throttle primed the pump. Without the alarm, it could have resulted in a failed impeller and possible engine damage, burnt exhaust hoses, etc.
There is one minor glitch though. When the engine is turned off for several minutes, heat soak warms up the impeller housing to 40-50°C, and of course, it alarms on engine restart. It's not a major glitch though since a simple press of 'SET' button mutes the alarm for 30 seconds. The temperature falls to 22-25°C within 10 seconds or so, indicating that the cooling pump has primed and all is good. I'm really liking the peace of mind it provides by giving almost immediate feedback on cooling water flow.
The instructions mentioned that on boats to install a sensor near the outlet of water-cooled exhaust. Personally, I think having it on the impeller housing at the beginning of the cooling system is better than at the outlet, but maybe someday I'll move the cylinder head sensor to exhaust outlet so that both my inlet and outlet are monitored.