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MG MGB GT V8 Factory Originals Technical - Low coolant sensor

I would like to fit a low coolant sensor to my MGB V8 and would like to know if anyone has a circuit diagram and a list of the parts required.

I appreciate the kits can be bought, but I am keen to make my own which will have a light and buzzer as warnings.

Ian Buckley

When mine started pushing coolant out of the overflow I fitted one myself. I think it is common for cars with a separate expansion tank to monitor the level in the tank, but that is only any good to detect gradual loss over a long period for a lazy owner. To my mind it is more important to be aware of a sudden and potentially catastrophic loss whilst driving, so I fitted a probe in the (plastic) filler plug of my radiator. The 'probe' is nothing more than a long bolt, about 1/8" dia, going through a spade terminal on the top and secured with a washer and nut underneath, with a sealing compound in the hole in the plug. Originally this was connected to nothing more complicated than a small voltmeter resting on the centre console with the other side connected to a green (fused ignition) circuit. When the probe is in coolant and the ignition is on it registers about 12v, when it isn't it registers 0v. This was fine in the short term, and is obviously monitored along with the oil and temp gauges. In fact on a trip to Le Mans and back I was monitoring the coolant level and temp gauges so closely I ignored the fuel gauge, until I noticed with horror that it was on E, having done 340 miles on a tank-full, but that is another story.

Eventually and I found and fixed the problem, but was keen to keep the monitor but in a neater fashion. I empirically came up with a two-transistor bi-stable circuit that switched one way when the probe was in coolant, and switched the other when it wasn't, with a certain amount of hysteresis so that under marginal conditions it didn't keep switching back and fore (I actually developed this circuit for something else many years ago and called it the 'Hunt Trigger', but then discoverd a bloke called Schmidt had got there first). When the 'coolant detected' transistor is on it lights a green LED, and when the 'no coolant' transistor is on it lights three red LEDs - three to catch my attention. The cluster of four LEDs is mounted in the centre of the instrument panel immediately above the steering column and hence in clear view.

It shouldn't be difficult to add an audible warning to this, and IMHO a visual and audible warning of temp, oil and coolant problems is more user-friendly than those pesky gauges that everyone seems to worry about so much when nothing is wrong. The RV8 temp gauge is controlled through the ECU so it doesn't rise and fall as the electric fans cut in and out - completely pointless to have the gauge at all in my view, much better to replace it with a warning light and buzzer. Again many years ago I had a car with an oil light instead of a gauge and made an 'exclusive OR' gate to sound a buzzer if either the ignition or the oil light should light on its own i.e. not when both were on together when the ignition was first turned on.

If you are interested I can look out the (coolant level) circuit and mail it to you. As I say I came up with the resistor values empirically for two transistors which I happened to have kicking around, you may have to fiddle with the values to get the right trigger points.
Paul Hunt

Being "electronically" challanged, when my mid 80's VW Golf kept blowing radiator hoses, I built something similar (but not as elegant) as Paul did. VW had a recirculation tank with a level sensor in it for the diesel version. Got a tank and the sensor (literally just two prongs sticking into the coolant, but only one is necessary if you're sensing the ground connection) a relay and a panel light. The relay was normally closed (NC) so when the coolant made the connection between the two sensor probes, it tripped the relay open, so no light. It would be easy to add a buzzer too. I think cost for the relay and light was about $5-7 at Radio Shack.
Good luck,
Tom Sotomayor

Hello Paul,

I am also interested in the schema so i would apreciate it if you could e-mail it to me.
I removed a later Sdi expansion tank with the additional connector but the electronics were already removed from the scrap car.
Thanks in advance

I found a triumph expansion tank, same style as the one in later MGB's, that has a low coolant sensor built in and lights up a low coolant light on the dash. Problem is getting one, they were only used on a select few of the last bit of a run of one model triumph.


Thanks, I would like have a copy of the circuit it would be a nice safeguard to have.

I will email you with my details.


Ian Buckley
Ian Buckley

Duly mailed, Peter. But as I mention above I would recommend the sensor is in the radiator (or block) and not the expansion tank, I have known the former to be low but the latter full.
Paul Hunt

Hello Paul,
After following your learned writings for quite some time, I would appreciate a copy as well Please.
Kind regards
Richard Evans

On its way ...
Paul Hunt

This thread was discussed between 17/11/2004 and 24/11/2004

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