Coolant Level Alarm

The early MGF's and TF's coolant expansion tank did not include a built in level sensor & alarm, though one has been recently introduced .

I purchased and fitted the new expansion tank which is labeled PCF000140 - perhaps it is now the standard. Just make sure when you get buy the tank that it has the internal float and 'hole' in the bottom for the sensor. The sensor is PCJ000040 and costs £9.60 from Rover. The tank was £ 26.08, again from Rover (both prices include VAT).

The new expansion tank is basically the same as the original, except that there is a vertical pillar inside the tank and a float that is free to ride up & down the pillar. On the bottom face of the tank is a hole that allows the sensor to fit inside the pillar.

The connector for the sensor is the same as that for the, I think, the Air Temperature Sensor, and is an AMP 347882 2-pin connector. This appears to be a special, since details cannot be found on the Tyco/AMP website.

The Buzzer I used is a SoundTech SEP-2232A - See below for the Data Sheet..

My initial circuit was extremly simple - too simple as it turned out! An in-line fused 12v supply was fed to a 12v red LED and parralled 12v buzzer. The buzzer needs to be able to output about 92 dB to ensure you can hear it at speed with the roof down! These were connected to one side of the level sensor, the other terminal going to ground. This worked fine, provided you could tolerate the "in-built auto circuit tester" every time you went round a sharp bend or down a bumpy road. Even with the expansion tank filled to it's maximum limit, there was too much 'slop' of the coolant.

This shows a place from which to take the 12v supply.



There obviously was the need to build in a delay circuit so that the alarm was only triggered after the float remained 'low' for several seconds. The time period really had to determined by trial and error, so it was essential to build in adjustment to the circuit, of something in the order of 2 to 10 seconds.

FINAL CIRCUIT:

The Overall Scheme reverses the layout, in so much that the fused 12v supply is fed initially to the float sensor, and the output from the latter is then fed to the Delay Circuit Diagram. Everything is still 12v based, cutting down on complexity, and the adjustable resistors allow for a time delay range of somewhere between 4 to 10 seconds.

Holes drilled in the sides of the enclosing box allow for cable entry and also for tweeking the variable resistor pots. The box itself is velcroed to the inside of the fuse panel, so as to allow for easy access/removal, and also to avoid drilling holes into the panel.

I am sure that if I were to rebuild it, the whole PCB size could be reduced with an improved component layout.


This system is now in my car and being road tested. Initial results seem fine - no false tripping, and a static test of forcing the float down for 4 seconds trips the alarm. This will now form part of my daily/weekly coolant level check, to ensure the system is still active and okay. More information when we return from the Pyrenees.


Wire routing:

It is possible to run a wire from the engine bay to the interior of the car without removing the T-bar and drilling holes. If you look at the bonnet release lever in the boot, and follow the cable forward along the inner wing, the cable runs through a gap in the body panels and runs underneath the carpet adjacent to the door seal. Using a wire-leader or narrow rod, it is possible to feed a new wire though this gap, and then forward to the fuse box, or across under the carpet to the central console.


UPDATE: 25 June 2005
We have just returned from our 2000+ mile road tour down to the Pyrenees in southern France and Spain (and what a great tour it was), so I thought I'd write an update on how my Coolant Level Alarm system worked.

During all normal road driving there was no problem, and it wasn't until we started driving in the actual Pryenees mountains, with lots of very tight hairpin climbs and descents that the alarm tripped, but only very, very briefly, and only the once. Driving speeds were relatively low, probably down to 10 / 20 mph on a lot of the corners.

With the timer set at approx. 4.5 seconds, the most the the float could have been down near the bottom of the tank was about 5 seconds.

However, as we started our return journey, heading north on the A75 from Beziers and up towards the new bridge at Millau, the experince was a lot different. This route is a relatively new motorway, climbing and dipping through a very rugid area, with long sweeping curves. At high speed (no - all I'm saying is 'high' so don't ask!) the alarm tripped regularly on the left hand bends, especially those that dipped down, swung left, and then climbed, and it stayed on much longer, to the point that I began to worry. Out of the bend, a quick flick of the wheel, and it shut up. I've tried to estimate how long the buzzer might have been on, and I guess somewhere around 6 - 8 seconds is near the mark. That means a total of about 10.5 - 12.5 seconds.

Rob Bell comments:
"The principle of the expansion tank is that it isn't actually part of the normal circulation of the coolant system - i.e. water isn't actually pumped through the tank. The purpose is for it to act as a resevoir - where when the coolant volume expands under increasing temperature, the excess drains into the tank. Then, after the ignition is switched off, the contracting coolant draws fluid in from the expansion tank.

The two jiggle valves constrain the fluid entry to the expansion tank. To my knowledge, there is no non-return valve in the expansion tank outlet. However, the system is under pressure, and therefore, there ought not to be any flow through the expansion tank outlet unless, for what ever reason, there is a loss of pressure in the main coolant circuit.

Therefore I would say that there isn't anything to worry about Tony, even if the expansion tank outlet is exposed to air for a period of seconds under hard cornering. Perhaps a much longer delay period needs to be built in (especially for those MGF/TF owners who want to take their MG on track)? 10 or 12 seconds perhaps?"

Feel free, but remember to do the usual visual checks and, if you copy it, test the circuit every now and then. I will accept no responsiblity.


PDF files

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  Circuit Overview
  Circuit Diagram
  SEP-2232A buzzer