Coolant temperature alarm.

[PTB]

Hi all, my coolant temperature alarm light fails to illuminate when the key is turned on ( all other warning lights come on, and then off when engine starts). Then after about half an hours motoring the temperature alarm light and buzzer come on and stay on until the engine is shut down.
The engine is not overheating and the temperature gauge reads fine. All coolant levels are fine.
The alarm sender and gauge sender are separate fittings, both screwed into the thermostat housing so should be seeing same conditions, I think?
Engine is T95 Thornycroft (Mitsubishi base)
Would welcome any ideas on what's going on,
Thanks Phil

 

[prv]

The light not coming on at startup is normal - after all, the engine is cold, why would the overheat light come on?

I'll leave others to ponder why it does come on later.

Pete

 

[PTB]

All the warning lights usually come on when the key is turned and then go off when the engine is actually started. Obviously should only then come on again if a fault develops.

 

[VicS]

Hi all, my coolant temperature alarm light fails to illuminate when the key is turned on ( all other warning lights come on, and then off when engine starts). Then after about half an hours motoring the temperature alarm light and buzzer come on and stay on until the engine is shut down.
The engine is not overheating and the temperature gauge reads fine. All coolant levels are fine.
The alarm sender and gauge sender are separate fittings, both screwed into the thermostat housing so should be seeing same conditions, I think?
Engine is T95 Thornycroft (Mitsubishi base)
Would welcome any ideas on what's going on,
Thanks Phil

Should the temperature warning light come on when the key is switched on ?

If not I would simply suspect that the sender is faulty.

If it should come on then there is probably a fault in the control panel. Even then I suspect that the sensor is faulty anyway causing it to come on after half an hour

.

 

[PTB]

Thanks both, and, er, yes, my wife tells me the temp light never illuminated when key turned ... ( I've only had this engine 12 years !). Anyway have ordered new sensor so that should sort it. P

 

[prv]

All the warning lights usually come on when the key is turned

Are you sure about that? Maybe if it's a very new panel with electronics in it, but if it's the usual simple circuit then I can't see how it can. Battery -> lamp -> sensor -> block -> battery. Sensor only makes a connection when it's too hot. At switch on it is not hot, hence no connection, hence no light. Couldn't be simpler.

The other lights come on because there is no oil pressure and the alternator is not charging, again exactly what you would expect from an engine that isn't running.

Pete

 

[VicS]

I expect the OP's wife is correct but there is no reson why the panel should not contain the electronics necessary to illuminate the warninglight , to test the bulb, at initial switch on.

IIRC on modern Volvo panels the temp warning light comes on with the others.

The high temp warning light comes on in my car too, and goes out again when the engine starts.

 

[Billjratt]

You can measure the resistance of the sender. You will probably find the spec for it online. Let's hope it's not the other one that's at fault and you are ignoring a real problem.

 

[nimbusgb]

On 'modern' panels perhaps. On a 12 year old one? i doubt it. why complicate things?

 

[VicS]

On 'modern' panels perhaps. On a 12 year old one? i doubt it. why complicate things?

and perhaps not so modern.. The high temp warning light illuminates at switch-on on a VolvoPenta 2001 series ... and they date from 1984

So yes even on a panel as old as 1984 .....28 years old.

Why ? To test that the warning light bulb works !

 

[PTB]

I'll let you know if I or my wife are correct about light 'on or off' when key turned when, hopefully, we've got this fixed and next down at boat ! Phil

 

[single]

Never really noticed if the light comes on but when it did get hot a few weeks ago I noticed the gauge up near 100 but the warning beeper didn't sound so whats at fault there? Not got the wiring diagram with me.

 

[PTB]

Never really noticed if the light comes on but when it did get hot a few weeks ago I noticed the gauge up near 100 but the warning beeper didn't sound so whats at fault there? Not got the wiring diagram with me.

Forgot to let veryone know the outcome. Well, you were right, the warning light is not supposed to be illuminated unless the sender actually overheats, so my particular engine doesnt have any 'bulb testing' facility when turned on. I finally managed to replace the sender but I could not get one anywhere that triggered at 90ish which is the recommended point. So the temp alarm / light will not now activate until over 100 deg. !
So the bottom line is that I keep an eye on the gauge which typically runs at 85 and when the alarm goes off and the light comes on I need to panic.

As for singles query, sounds like he, also, has an over 100 sender. Phil

 

[single]

Forgot to let veryone know the outcome. Well, you were right, the warning light is not supposed to be illuminated unless the sender actually overheats, so my particular engine doesnt have any 'bulb testing' facility when turned on. I finally managed to replace the sender but I could not get one anywhere that triggered at 90ish which is the recommended point. So the temp alarm / light will not now activate until over 100 deg. !
So the bottom line is that I keep an eye on the gauge which typically runs at 85 and when the alarm goes off and the light comes on I need to panic.

As for singles query, sounds like he, also, has an over 100 sender. Phil

My panel is like this.Only way I noticed was the hot smell (enclosed wheelhouse)

View attachment 31688

 

[cryan]

On bigger engines there is always two alarms. one for the actual temperature and the second for the rate of rise. the rate of rise is activated if the temp rises so many degrees in a set time which could indicate a blown cylinder head gasket etc. I have seen this set up on smaller engines as well but in a commercial setting. It might be worth a quick look to see if there are any more sensors connected to the cooling circuit? I could not say one way or the other if your boat would be fitted with such an alarm but from experience when rate of rise sensors get old they become a real pain the backside.

 

[single]

After thinking about this post I decided to remove the instrument panel last night.I found the bulb that indicates temperature had blown and also the plastic hole where it fits was all melted! Why? Also discovered the spade connector that gives audible warning had come off.
Not replaced the bulb yet but with the spade connector back on, the panel beeps quickly when in glow plug position, but once started continues with a steady beeping.Does this mean the temperature warning sender is bust? Its not overheating.