One of the most common root causes of Head gasket failure is unknown coolant loss. Our engine objects strongly to being run with insufficient coolant, and usually responds by blowing the head gasket.
Because of the complicated nature of a mid engined car with associated poor natural (wind) cooling and add in a front mounted radiator with externally routed coolant pipes, you can see that coolant loss can occur without being readily seen.
Obviously, regular checking of the coolant level is helpful, and any losses investigated immediately. However, what if you spring a leak whilst travelling a couple of hundred miles along a motorway? The first you might know of it, could be too late!
A low-level alarm in the expansion tank is the obvious answer, indeed MGRover began to fit these as standard in 2005.
There are commercial kits available from the likes of Brown & Gammons, and Rimmer Bros.
Being keen to have the protection of an alarm, and also being strapped for cash, I decided to make up my own.
I purchased one of these float switches from Farnell.
This switch good up to 110c, and I have had ours in use for two years with no problem.
After dropping the coolant level, I removed the expansion tank from the car, and drilled a 16.5mm hole in the side, (I intended to fit the switch so that in use it was normally open, and that a drop in level would allow the switch to close, completing a simple circuit.), so I made the hole towards the bottom of the tank.
I fitted the switch using the supplied seal and tightened it up carefully. I checked for leaks at this stage, but all was well.
I then refitted the tank to the car, I refilled and bled the coolant. I then ran the car up to operating temperature and checked again for any leaks.
I simply took a feed from a switched live through a fuse to a 5mm blue flashing led, which I mounted into one of the spare switch blanks. I took the return wire through to the engine bay via the centre console,and under the parcel shelf.
I connected this wire to one side of the switch and the second switch wire to earth. With the float switch in this position.
In that way the light will begin to flash only when the switch is allowed to close by a low coolant level, allowing the circuit to earth.
I'm sure that an auto-electrician would do it differently, but that's how I have done it, and it has been working effectively for two years. The simplicity of the circuit makes it easy to achieve but the alarm system is prone to false readings caused by bumpy roads, fast cornering, or speed bumps.
It is possible to fit a delay circuit into the system, and that will eliminate false readings.
The complete system is now in place and working perfectly.
I adjusted the delay to about 10 seconds. In other words, the alarm needs to be activated for 10 seconds continuously before he warning light is triggered to illuminate.
I did experience a bit of consternation, because after I had fitted the circuit board into my original simple circuit, the alarm was working in reverse.
It was flashing all the time until I held the floating arm down for 6 seconds when it went off!
I eventually realized that the float switch needed to be inverted to get the system working properly with the delay circuit. Positioned like this in the tank.
So that the float switch is earthed until the coolant level drops sufficiently to allow the float to fall breaking the earth circuit, and triggering the alarm.