Cooling water temperature sender -- working?

[jaylo264]

I installed a new 18hp VP D1-20F last year; not a bad engine to replace the 2002. Its done about 300 hours now.

All ok till the overheat alarm went off just after [15 mins running slowish] raising the anchor at Talmine bay. Switched off, cooled a little [didn't seem especially hot], checked everything inc impeller, fluid, belt etc, all fine.

No problem again until returning up Loch Ness; similar thing after stopping at Urquhart Bay, same reaction from me and the engine on tickover. No problems since. Raw water always coming out of transom hole.

So I bought a new sender --volvo, ouch. But thought I would try something before fitting it : with a well-run, hot engine, at the berth, pulled off the single electrical connector from the sender :

alarm goes off, loud.

put spade connector back on

alarm stops.

Now, this obviously SHOULD happen -- but, [being a zoologist and not an electrical type], does it mean that the sender is functioning correctly so the fault must lie elsewhere?

Thanks, and regards, Jaylo

 

[sarabande]

My first reaction (after a similar alarm happened to me a couple of weeks ago) is to check if the belt is tight and 'grabbing' the pulley. Mine wasn't at medium revis, but was a low.

Any belt dust near the pulleys ?

 

[jaylo264]

Hmmm...there IS some blackish stuff below engine, just a wee bit; will be on board tomorrow so I'll check the belt. It seemed tight enough at low revs to me but managed to take my hand away before pulling on it whilst engine ticking over.
What do you think about the sender?
Thanks.
J

 

[philwebb]

Hi jaylo, can you remove the overheat sender? If so, borrow or buy a multimeter. Set it to OHMS range lowest range is usually 200ohms. Connect it between the body and the terminal, hopefully with croc clips. Put the sender in a pan of water and heat it up. At low temperature it should show continuity (low ohms reading) At higher temperature around when the water boils continuity should be lost. Equivalent to when you pulled off the terminal. Test complete. If the sender goes open circuit much before 100C I would think it was faulty. The exact switching temperature you may be able to find with a bit of research. Happy testing Phil

 

[jaylo264]

Thanks Phil -- I will practise on the new sender first, as I think I will have to drain coolant off before removing older sender?

Can this not be done in situ and heat up with engine??

 

[QBhoy]

Hi

Is it possible to feel the temp of water coming out the exhaust (if it’s relevant) by hand ?

 

[oldharry]

Thanks Phil -- I will practise on the new sender first, as I think I will have to drain coolant off before removing older sender?

Can this not be done in situ and heat up with engine??

you would have to make the engine overheat in order to prove whether the switch is working. One way would be to carry on using it, but have tne meter to hand,and a thermometer capable of measuring coolant tempdature sufficiently to show if it's hotter than normal. Cheap Infra red thermeters off eBay will give an indication if the engine is actually overheating when the alarm sounds. Get a reading from the engine operating normally first so you know if it has overheated. These cheap thermometers are not very accurate, but will tell you if the temprature has actually risen when the alarm sounds.

 

[VicS]

Hi

Is it possible to feel the temp of water coming out the exhaust (if it’s relevant) by hand ?

The engine has an indirect cooing system.
The temperature sensor is monitoring the temperature of the engine coolant not the raw water.

 

[VicS]

Hi jaylo, can you remove the overheat sender? If so, borrow or buy a multimeter. Set it to OHMS range lowest range is usually 200ohms. Connect it between the body and the terminal, hopefully with croc clips. Put the sender in a pan of water and heat it up. At low temperature it should show continuity (low ohms reading) At higher temperature around when the water boils continuity should be lost. Equivalent to when you pulled off the terminal. Test complete. If the sender goes open circuit much before 100C I would think it was faulty. The exact switching temperature you may be able to find with a bit of research. Happy testing Phil

Thanks Phil -- I will practise on the new sender first, as I think I will have to drain coolant off before removing older sender?

Can this not be done in situ and heat up with engine??

ITYWF that on this engine the temperature sensor is a thermistor, connected to the MDI system whose resistance varies with coolant temperature. When the temperature is low the resistance of the sensor is high but when the the temperature is high the resistance is low.

 

[jaylo264]

Thanks, all

I have ordered a cheap IR tester, for just the reasons you've reminded me of.

I'm still unclear why I can't test the sender with a clip on the nut/body/engine and a clip/probe on the terminal [the reasons I ask are that 1/ I'm not on the boat yet today [Mr Marr on the telly] and 2/ being a volvo, it's a contortionists nightmare to get to that sender so asking questions is easier.
Watching Julia Hartley Brewer in her ? nightclothes keeps me here, too.

 

[oldharry]

Thanks, all

I have ordered a cheap IR tester, for just the reasons you've reminded me of.

I'm still unclear why I can't test the sender with a clip on the nut/body/engine and a clip/probe on the terminal [the reasons I ask are that 1/ I'm not on the boat yet today [Mr Marr on the telly] and 2/ being a volvo, it's a contortionists nightmare to get to that sender so asking questions is easier.
Watching Julia Hartley Brewer in her ? nightclothes keeps me here, too.

you can connect the meter across it, but it will only tell you what it's doing now, which isn't very helpful. You need to be able to heat it up to overheat level to see what it does then, in order to prove it is working as it should or not. The IR thermo will tell you whether you have an overheat problem next time it triggers, without actually removing the sender. Volvo are designed to be serviced by contortionist midgets. I think they got the idea from Ford's design team....

 

[VicS]

And here is a table of resistance values vs temperature for the sensor