Why is there a glimmer from my ignition warning light?

Dave Hedger posted a query on the V8BB about a glimmer from the non charging light on the dashboard of his RV8 and sought fellow members' help. (Jan 08)

Dave Hedger was out in his RV8 on a run on Boxing Day (hood down and glorious) and for the first time for a long time it got dark before I got home. Apart from the almost total absence of instrument lighting - something that I have been told is to be expected - I noticed that the charge light in the dashboard had the mearest glow - to be seen whilst on the country lanes - can't be seen where there are street lights! I am not suffering from any lack of charge to the battery. Is this an impending sign of doom or is it possibly a slightly resistive earth connection somewhere? Any thoughts would be gratefully received.

Nigel Melbert responded: I had the same problem with my MGBGTV8 when the ignition light was seen to be on when through France at night but it did not cause a problem. The alternator and battery were all in good order, the circuits were checked with the correct current flow from the alternator and battery to the ignition light. The problem turned out to be the bulb in the ignition light having the incorrect resistance. I had to try two or three different bulbs to find the correct one. No problems since. Incidentally the dim instrument lights can be solved by fitting halogen bulbs.

Brian Moyse with an RV8 noted: On dim instrument lighting, an improvement can often be made just by replacing bulbs at say four to five year intervals. Instrument lighting bulbs rarely blow, particularly when operated at reduced voltage with a panel light dimmer. They do however, over the years, suffer a build up of a black deposit on the inside of the glass envelope, presumably an emission from the tungsten filament, which significantly reduces light output. The same applies to all tungsten filament bulbs, so it's worth having a periodic check of all the external lighting - for example side/tail lights as well to ensure maximum visibility.

Ignition warning light and dimmer on an MGBGTV8. (Source: MGBGTV8 Driver's Handbook AKD8423)

Chris Hunt Cook who is a regular participant with his MGB Roadster prepared in road rallies, needs to see his instruments at night added: I agree that the bulbs blacken with time but also the panel light dimmer seems to deteriorate and give much more resistance in any position, and it is expensive to replace. You could try just shorting it out with a short piece of wire - that made my instruments readable at night and even in that improved state the panel lights are hardly bright enough to need dimming anyway.

Another thing to check with the ignition light is that light from the panel lamp is not leaking across into it producing a slight red glow. Try turning the lights off (having parked of course!) and see if that makes any difference to the glow from the ignition light.

Brian Moyse added: As I recall, if you take out the panel light dimmer, you can see the wire wound rheostat/contact wiper. An increase in resistance is most likely due to a build up of dirt in the rheostat, which may be removed using an aerosol electrical contact cleaner or similar. Then work the dimmer to and fro so that the wiper contact cleans the surface contact patch of the rheostat winding.

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