Page 1 of 1

The coolant tank sensor - why it fails!

Posted: Tue May 10, 2016 11:13 pm
by Delanor
Finding myself with some surplus time on my hands I decided to have a look at a coolant tank and find out exactly what happens when it fails! (Thanks to BigPete for supplying the tank.)

Well after several hours playing about with it with an aim to trying some sort of economical repair it just cannot be done with the existing parts you just cannot get into the float as it sits in its own baffled compartment, no matter how I tried you just cannot get into the float section without cutting a hole and then I had to break the float off the magnetic collar to remove it.

Firstly how does it work? basically there is a float that encapsulates a magnetic collar and a NO (Normally open) Reed switch which is open when the coolant level is high, the switch sits in a tube which is accessible from the bottom of the tank, the float is clear of the Reed switch when the level is high then if the level falls to the low limit the float drops and the magnet closes the Reed switch which makes the circuit and switches the warning lamp.

It is all quite simple really and other than the Reed switch failing which it shouldn`t as it only ever works if the coolant is allowed to fall to the low limit then really it should last for the life of the vehicle.

So why does it fail you may ask well I have come to the conclusion the tank is designed with a specific life as the float definitely does lose buoyancy and just sinks in the coolant as has been discussed before, but why does it fail? well that is a strange one as the float itself appears to be made of some sort of hard cellular material a bit like pumice but I can confirm it is not "waterlogged" which causes it to sink, the float is very light and as I said it is not waterlogged but it just refuses to float and sinks like a stone, it maybe that the float originally had a coating of some sort to give it buoyancy and it wears off, if the float had been made of plastic or stainless it should continue to float and the tank should never fail but at £175 each as I suspect it has been made to fail – if not why make the float of a material that sooner or later will "fail to float"

If your warning light has come on I suppose it is entirely feasible that the Reed switch has failed but unlikely as it would have to be in closed state to illuminate the warning lamp (you can remove the switch to check it with a meter) I did notice on my project tank the inside was covered in "crud" ie lime scale etc and the float was stuck on the Reed switch tube until I freed it off again it`s unlikely it would stick in the low position unless the level had been kept low.

You just cannot make a good working tank out of a failed one, I thought I might be able to fit a separate float switch but you cannot do this without cutting a sizeable hole in the tank to fit it and then you have to re-seal it
Untitled 3.png
Untitled A.png
Untitled 1.png
Untitled 2.png

Re: The coolant tank sensor - why it fails!

Posted: Mon Jan 02, 2017 11:01 pm
by warpc0il
For those wondering why their coolant level sensor light comes on when the engine is working hard and then goes out when you lift-off, as mine has done for the last couple of years, just watch the video, with these thoughts in mind...

1. The float can become saturated to the point that it just about floats.
2. The float material is compressible.
3. The pressure in the tank increases with the temperature of the coolant.
4. The coolant temperature increases when the engine is working hard.
5. The light is controlled by the position of the float; up = off, down = on.

Now watch the video.

Re: The coolant tank sensor - why it fails!

Posted: Tue Dec 19, 2017 12:16 pm
by warpc0il
More details
Coolant Tank
Coolant Tank
coolant tank.png (138.25 KiB) Viewed 2634 times
Aftermarket alloy replacement tanks are unbaffled internally and therefore don't provide the degassing or air/water separation functions.