Thread: I almost cooked my diesel due to a faulty temp guage. I need an audible alarm.

Originally Posted by mox

Originally Posted by Jeff

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.

Jeff is entirely correct. You are relying on a temperature sender in a normal part of it's operation and a fuel cut solenoid. If the wire falls off the temp sender, the gauge goes to zero and your method won't work.
Futher, if your modifications to the fuel cut wire have issues you'll have an engine cut-out when you shouldn't.

What some other bloke did 30 years ago doesn't matter.

Originally Posted by mox

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.

Murphy switch type devices are only ever used in industrial settings. They are never, ever used in passenger vehicles as leaving the driver suddenly without power is incredibly dangerous.

There are also two settings on modern industrial engine controllers in the event of oil pressure loss or water over-heat. One is shut-down and the other is "run to destruction".

Because even in purely industrial settings there are many times when you absolutely cannot shut down an engine without warning. Engines can be rebuilt/replaced far more easily and cheaply than people can.

Originally Posted by mox

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.

Turbo slow down is not the main problem. Turbo cool-down is.
If you shut off oil to a turbo operating near full load (700C exhaust temp) then your remaining oil in the bearing housing turns to tar.

Winding it over with the starter won't help this at all.

Originally Posted by mox

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.

95-105C is normal operating temperature with no danger to anything.

With a 15psi radiator cap plain water won't boil until 120C. Glycol raises this boiling point even further.

The last point you seem to have missed entirely. The best way to cool down a hot diesel is to keep running it with reduced load. Even with all safety issues aside, stopping it dead is the worst thing you can do.
Stopping it dead because it got to 95-105C is especially dangerous and pointless.

Best to cool an overheating motor by continuing to run it with reduced or no load if cooling system is still functioning adequately. However, situations such as loss of drive to water pump and/or fan, eg due to belt failure or major loss of coolant so it can no longer circulate, to minimise damage motor should be stopped ASAP.


Dougal obviously does not understand setup I explained. based on a temperature sensitive SWITCH, not sender unit. Switch opens when temperature gets to its specified level. so power to whatever allows motor to keep going or stops warning system from operating is cut off. So a failure in the circuit causes the protection system to operate. Much better than the other way around, with it operated by power being switched on. Like air brake systems on trucks etc. Problem with old "air on" type is that if there is some problem causing lack of air, brakes cannot be applied. Can be disastrous if this is just discovered when brake application is needed immediately. The "air off" type are much safer. Loss of air holding brakes off results in them immediately being applied. Loss of compressed air supply or reserve means that if brakes are applied, there is no air to release them. So brake failures largely result in not being able to move vehicle rather than not being able to stop it.

Originally Posted by mox

Best to cool an overheating motor by continuing to run it with reduced or no load if cooling system is still functioning adequately. However, situations such as loss of drive to water pump and/or fan, eg due to belt failure or major loss of coolant so it can no longer circulate, to minimise damage motor should be stopped ASAP.

Your system has no warning device for those situations. Only the downstream effect of water temp.

Where the alternator light coming on is a dead giveaway.

Originally Posted by mox

Dougal obviously does not understand setup I explained. based on a temperature sensitive SWITCH, not sender unit.

Semantics. Using a NO vs NC switch just increases the likelyhood of your system killing the engine on someone in a dangerous situation.

So have I got this right?

You have a switch which must be functioning correctly for your engine to run.

Since switches can fail, doesn't that mean that your engine could suddenly stop in an embarrassing, inconvenient or dangerous situation even though there is absolutely nothing wrong with the coolant level, the cooling system or the whole engine?

EDIT:
Didn't VW get into all sorts of strife because they had a car that was capable of suddenly stopping for no reason? Didn't a woman die when that happened to her VW?

Originally Posted by Dougal

Your system has no warning device for those situations. Only the downstream effect of water temp.

Where the alternator light coming on is a dead giveaway.


Semantics. Using a NO vs NC switch just increases the likelyhood of your system killing the engine on someone in a dangerous situation.

Setup I explained is a cheap and simple way of protecting motor from overheating from a range of causes. One possibility is large quick loss of water from eg split bottom radiator hose. Overheating may then not even register on water temperature gauge but temperature sensitive switch will respond. Alternator light coming on just indicates alternator is not charging. One possible reason is failure of belt which also drives water pump. If this occurs, driver may notice it and may or may not stop before temperature sensitive switch which in this case could be backup protection opens. In this situation some drivers are likely to initially think it would be an alternator malfunction problem to have looked at later, Not an urgent overheating one.


Remember motors can also stop in dangerous situations from fatal overheating. With an override switch, probably push button next to ignition switch, can often push it well before motor stops completely. Could then keep going even under full load for a short time without cooking motor. I have had switch open on header (ie combine harvester) several times. Largely due to radiator or radiator screen getting blocked with dust while under heavy load. In this situation, it simply involved disengaging separator and driving back to bins for cleaning job. Water temp will quickly come back on lightly loaded motor but needs to cool a lot more to reset switch. Meanwhile, usually not even any water loss from pressurised radiator cap.

I see what you are saying about having a quick way to get your engine going again, but how long do you really think that will take. You might have trained yourself to react quick and hit that button if need be, but what if someone else is driving your car? If a motor stops, the usual reaction is WTF, followed by fumbling too find the right way of getting it going, by which stage 5 seconds could be enough for the truck to clean you up
On a tractor or piece of industrial machinery it would be a great system. No danger if the thing just stops. If my plane engine has a problem it doesn't shut down to protect itself, that could kill me. In some situations you can shut it down, in others you run it to destruction. Remember, you can replace an engine, you can't replace a life