[QUOTE=Jeff;2280545]But yours could malfunction just as likely as the warning devices.
Wrong! Whether a protection system shuts down the motor or sets off warning devices, one which has to function properly for the motor to run is more fail safe than one that has to function to shut it down or set off an alarm. Bloke at instrument place where I first bought my setup around 30 years ago, which have had installed on tractors ( especially running unattended driving pumps from power take off) and headers (combine harvesters) as well as vehicles, pointed out that he felt guilty at having sold a bloke another protection system, which I previously had which failed, resulting in cooked motor. ie Murphy switchgage.
When temperature gauge pointer touches set fixed pointer ( after which it cannot indicate a higher temperature), it earths out wire coming out of gauge. This completes the circuit to operate shutdown or alarm. It some problem, often a simple one such as a corroded electrical connection occurs, there is no power to operate protection device. Result is no warning or shutdown if motor overheats. What is desirable is a system that has to function for the motor to run or stop alarm from going off. So that it stops or alarm (which should be tested regularly) activates if something goes wrong with the protection system.
On diesels with mechanical shutoff , have used a fuel solenoid in line between secondary fuel filter and injector pump. When this closes, motor stops slowly as if from a fuel block. For a vehicle, would be relatively simple to install a bypass with a restrictor to give a "limp mode" effect. Sudden reduced power should alert driver that something is wrong and to check why. This is if a bypass switch /button which can be immediately activated if motor stops is not deemed sufficient.
Re shutting down hot motors, is important that turbo has slowed down so it is not damaged when oil flow stops. So if motor stops suddenly when under load for whatever reason, is important to quickly restart it for at least a short time or at least wind it with starter. Note this is not possible with seized overheated motor.
95 or 105 degree temperature sensitive switches on the head will usually function before loss of water from overheating occurs unless loss of water for some other reason was the basic cause. In either case continuing to run motor to cool it is unlikely to achieve much. Best if motor is stopped or overheating problem rectified well before it heats to the stage special care is necessary to try and avoid damage cooling it. Some trying to knock system I have described on this point just highlight their lack of experience of it.
