One of life’s little mysteries

This is the strange tale of the mysterious ignition warning light saga that can afflict owners of the more mature MG. So, for the benefit of the younger generation that have taken up the challenge of pre 1956 MG ownership let me tell you a story.

Situated on the instrument panel of many of the older cars is a light that tells you when the ignition is switched on. When the engine is started and the speed increases the light goes out to tell you all is well with the electrical charging system. An ammeter is also fitted to reassure you that some electricity is coming out of the dynamo and that the battery will actually have some juice put back in it so that you can start engine again. In the words of the little furry meerkat friends, simples!

Horror, what happens when the little light does not come on when you turn the ignition key? This is where our mystery tale begins. As long as the battery is connected, the obvious starting point is to fit a new light bulb. This is where the MES becomes interesting. As with any other snag, the first port of call, for the less experienced, could be the Operations Manual or even the Workshop Manual. However, this is not as easy as you might think. Let me explain.

According to Operations Manual, if you have a TA the bulb is a Lucas 252 MES, if you have a TB, TC, TD, Y or YB the bulb is a Lucas 970 MES. The other panel bulbs are Lucas 987 MES. But they all look the same! Some Operation Manuals do not state a voltage so they must all be 12 volts surely? For reassurance you can consult the Maintenance Manual and Instruction Book for older models or the Workshop Manual for the newer types. This might not even be of much help. On page N-9 of the Y Type MM and IB the voltage or wattage of the ignition bulb is completely ignored, but the WM for the YB is a bit more specific as here on the same page number we can find a precise specification of the offending bulb.

Now for the science! For the logic of using a low voltage bulb, you need to consult the Bible of older MGs, the Blower Book. Here is an explanation of why and how the 2.5 volt bulb works. Of course such a low voltage bulb would soon be zapped if subjected to a burst of 12 volt + power. The low value filament is protected by a resistance that is wired in series. Look at the wiring diagrams and there it is shown as an additional curly line tucked into the bulb pictogram.

Quote from Blower’s tome page 262 ‘A resistance is always included in the warning light circuit to prevent the voltage rise of the dynamo burning out the bulb, and in the light under consideration the resistance is of sufficient value to permit the use of a small 2.5 volt bulb of considerably longer life than the earlier heavy consumption type.’ Sounds convincing, but is it really that important? Checking other cars from the same era, especially those without Lucas systems, a 12 volt bulb seemed to work OK.

On my 1936 Morris 8 the ignition warning bulb is also quoted as 2.5 volt C252A MES, but the system is 6 volts! Again Mr Lucas seems to know best. In the new 1947 Rover cars, the ignition warning bulb is quoted as Lucas 12volt 2.4 watt screw cap MES 1224M. However, in the 1953 E93A Anglia, Mr Ford leaves out the ignition warning bulb altogether and tells me to check the ammeter after starting. Very confusing!

So do I need this little light at all? The answer is probably yes as it gives me information. Firstly, after turning key on but before starting engine it confirms ignition is on, and then, when engine is running the absence of a flickering light tells me something good is happening to charging system and thirdly if it stays on I know there is a problem. If the warning light stays on when key is turned off, time for immediate action as there is possibly a short circuit in charging system that, if ignored, could burn out components. Does it matter if it is not working? – not really as the ammeter tells me the system is charging and the loss of bulb does not affect charging system (as it can with more modern alternator power supplies).

Now, to show how the resistance is applied to the bulb. The resistance comes in the form of a coil wrapped around the bulb holder (see photos of typical holder this one from MG YB).

Photo 1 – shows how the resistance is applied to the bulb. In the foreground (left) is a low voltage bulb and (right) the look-alike Lucas 987 MES.

The bulb is supplied with its own direct connection to ignition switch. The ignition switch has a parallel line that goes via a connector (A3 in either the control box or fuse holder, dependent on model) to coil and petrol pump but does not go through any fuses. Essential running needs are hard wired. If the warning bulb is open circuit, the ignition and fuel systems are unaffected.

Photo 2 – the ignition warning light at the back of the dash in Mick’s YB.

So the proper bulb is a Lucas 252, C252A or 970 MES. Of course these are all the same part, just Mr Lucas trying to confuse us. But where can you buy such a bulb these days? Most of the usual suppliers carry the look-alike Lucas 987 MES, but this is a 12 volt / 2.2watt bulb. Yes, it will fit and yes, it will work but can be difficult to see in good light. The 2.5 MES bulb was standard fit in old fashioned bicycle lamps (made by, you guessed it, Mr Eveready and Mr Lucas!). Never fear, just Google or Ebay ‘bulb 2.5 volt MES’ and the magic of the internet will supply your needs. Maybe not from a car accessory supplier, more likely from a vintage flashlight or cycle specialist!

The mystery is really why do we call it the ‘ignition warning’ light when it should be called the ‘I think the dynamo is charging up the battery’ light; long winded perhaps but no more than the old description of a rotor arm in my pre-war maintenance manual that referred to the ‘rotating electrode on a rotating moulded arm’, such is progress.

Safety Fast,
Mick Bath

Ed’s note: I thought that I would have a look at the various Instruction Manuals/Workshop Manuals/Operation Manuals to see what each had to say:

TA – Page 73 lists the Ignition Warning Light as 252MES 2.5-v (.2amp). Page 75 states that “A warning light is usually provided which gives a red light when the ignition is switched on and the car is running very slowly or is stationary, thus reminding you to switch off.” my underlining!

TC – Page 88 duplicates the wording of Page 75 above. Page 86 lists the bulb as Lucas No. 970 2.5-v, .5 watt.

TD/TF Workshop Manual – Section N (Electrical Equipment) lists the bulb as No. 987 12-v, 2.2 watt. A separate listing for the TF shows it as Lucas No, 985 12v, 2.2 watt.

TD Operation Manual – Page 61 shows Lucas No. 970 12v .5 watt.

TF/TF1500 Operation manual – Page 56 shows BMC part no. 2H4732 (12-v, 2.2 watt).

Only the TA & TC Instruction Manuals are correct!

2 thoughts on “One of life’s little mysteries

  1. Richard Michell says:

    I am sure that those with electrical knowledge will respond on this one but, first, the apparently easy one. I presume that it is called the ignition warning light because it warns you that you have left the ignition on. This will eventually drain the battery if not corrected.

    Re the resistance in series with the bulb, I had presumed that this was because this circuit – the one with the warning light in it – is also the one via which the generator charges the battery. It may be to limit current in a controlled way, not subject to the vagaries of which bulb the owner chooses. Note that the bulb does is not actually “off” when the generator is charging. It has a current flowing through it in the opposite direction but, because the voltage coming from the generator is not 12 volts above the battery, the level of current is too low for the bulb to actually glow (although I have had cars – not a T-series – where a very dull glow could be detected at night if you turned off the panel lights).

    Note also that, when the engine is running and you turn off the ignition switch, current from the generator can still get to the coil primary winding. If this is too high the engine will not stop. I have achieved this outcome on an Austin 7 when trying to jury rig an ignition warning light. Again, the resistance may be to limit this current and prevent this slight embarrassment.

    Over to the electrical engineers!

  2. Andrew says:

    How bloody wonderful to have a bloke go into such great and meaningful detail on such a component that I have spent hours searching for information on. Great stuff Mick in explaining what it does and how it works and why it’s made as it was. Old Joseph certainly must have had a wonderful mind – just a little south of the border!

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