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MG MGF Technical - Behaviour of low level coolant alarms

A couple of questions on how low level coolant alarms behave.......

1. Does anybody who has a low level coolant alarm fiited, ever find it briefly triggers within the first 5 - 10 minutes of startinng the engine, even when the water level is correctly topped up, and then work normally.

2. If you open the boot, remove the coolant cap, turn the ignition on, and then push down the float to the bottom of the header tank, how many seconds does it take for the alarm to trigger ?

Tony Thompson

1. When I had an old battery yes but only with a new demand such as headlights being switched on. It would just do the self test double beep. Now almost never even when the headlights switch on, since a new battery fitted.
2.c.12 from memory.
B&G one. I would not own an F without it.


My alarm is my own design, mage just before B&G released theirs on to the market.

I built mine so that it was fairly easy to adjust the time delay, the idea being that high speed cornering is likely to push the coolant temporally to one side, and I know that this can happen on left hand bends. It's currently set to about 9 seconds, which I agree sounds a long time.

I'm curious about why it triggers within the first 5 - 10 minutes of startinng the engine, a time which I would also think that may coincide with the thermostat opening and thereby change the flow rate through the system, causing the level in reservoir to drop significantly.

Could you check the time on yours?

Tony Thompson

I have the B&G one which paid for it's self with in days of fitting it, the water pump went and I knew straight away with the beeper going of. I don't have the light fitted as the beeper is loud enough even with a crash helmet on at very high speeds. I would not be without one.
Andrew W Regens

I have no doubt of the benifits of fitting such an alarm - that's why I designed, built and fitted one before they were readily available.

What I'm trying to find out is how the B&G (or any others if there are) ones behave. Do the B&G ones react the same way as mine on a cold start, by having a tendancy to trip on minor bends in the first 5 minutes or so?

Timing is of the essence in such an alarm system, because you want to know as soon as possible if the coolant level has dropped, but niether do you want to get excessive number of false trips. It's fairly obvious that going around a long left hand corner at speed forces the coolant in the reservoir over to the right, even when the coolant level is at it's maximum.

At that point, the float is at the bottom of the reservoir, and the actual coolant surface will be angled up towards the right. Is the top of the coolant intake pipe still covered or is the engine sucking in air ? I don't know. But if it is, 9 seconds before it trips is a long time, plus reaction time - is this a false alarm or not? - can i pull over suddenly on this motorway bend, etc?

I have been on A75 south of Millau in France where the motorway is fast, full of long winding up & down curves, where the G forces are enough to keep the alarm going until you straighten up out of the curve.

So, hence my question. With the car static and the float pushed down, how long does it take before the alarm trips?


So there needs to

Tony Thompson

the engine does not run the main flow thru the header tank, the flow runs from the outlet pipe, thru the heater/bypass and radiator and then back into the thermostat.

The flow thru the header tank is the bleed from the coolant outlet to catch any bubbles before they get chance to travel to the rad, the coolant tank is above all the other pipework by about 4", so the tank we be completely empty for some time before the engine starts sucking air.

The engine then has a small amount of time (60 secs?) with which to boil the coolant that is still in the block (assuming the coolant is not totally dumped by a hose blowing, which is not the normal failure mode)
Will Munns

I have used my car on some very tight and twisty race tracks without ever having had a false alarm.
Andrew W Regens

I rechecked and it is as close to 12 seconds as makes no difference. There has yet to be a false alarm not associated with headlight switch on, or parking on a slope!

I've got the B&G system fitted (about 2 years now) and I've never had a false alarm with it. Just tested the float and it started beeping approx 11 secs after I pushed it down.
David Clelland

Thank you all for your comments and timings.

Maybe I should extend my timing a little.

Interesting to hear that quite a few on here and the MGF Register Forum are suffering with some form on interference !

Tony Thompson

I think it is only the HIDs taking a lump out of the 12v and I would'nt be without either brilliant mod - no suffering!

This thread was discussed between 29/07/2008 and 30/07/2008

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