1971 MGB Ignition light

[Davidwill]

Hi, I have a problem with the ignition light wont come on at all. I've proved the bulb to be OK by taking the holder out from behind the rev counter and putting an earth to the holder and it comes on. But then it stays on when the engine is running. I’ve put a volt meter to the centre of the bulb holder and to the outside and I get about 12v but when the bulb is put in, nothing happens. The engine starts & runs OK, but the battery will run down in a few days. Could it be an alternator fault. Any ideas?

[nta16]

yes it could but first check the plug connection to, and spade connection on, the alternator are all clean, secure and protected

modern made plugs for this are very poorly made, I've tried two but both were the same, they'll do the job but are not very sturdy

once clean you could put a very small amount of electrical contact lube on the connects to help keep moisture and crud away

be care with what you do to and with the ignition light as it goes back to the alternator

[kev b]

Correct me if I am wrong but as I understand it the bulb should not stay on if you earth it as it should be connected to a switched live at one side and the alternator at the other. This way the bulb will earth through the alternator and illuminate until the alternator is producing current when it will extinguish.

IE the bulb circuit goes to earth through the alternator when it is not spinning.

[nta16]

from Paul Hun'ts excellent mgb-stuff website

some to chose from - http://www.mgb-stuff.org.uk/electricstext1.htm#sch...

[Geordie MGmike]

kev b said:

Correct me if I am wrong but as I understand it the bulb should not stay on if you earth it as it should be connected to a switched live at one side and the alternator at the other. This way the bulb will earth through the alternator and illuminate until the alternator is producing current when it will extinguish.

IE the bulb circuit goes to earth through the alternator when it is not spinning.

Partially correct
The bulb will stay on if the holder is earthed as the supply is from a switched 12v. When the engine is running (and the alternator working) the alternator outputs 12 (and bit) volts and therefore there is no voltage drop across the bulb and it goes out.

Plus if the OP leaves a cable connected to earth from the bulb and then switches the engine on he's effectively shorting out the alternator warning light circuit Which if memory serves is only a diode away from the main 12v output. All this = a dead alternator.

OP, if you are lucky it's just a broken wire from the warning light to the alternator (most likely at the alternator end), if not you'll need a new alternator. But don't run it with an earth on the warning lamp!!

Best of

MGmike

[Davidwill]

Thanks everyone, I took off the alternator and found the brushes were stuck. I had to break one of them to get it out. With a narrow piece of fine sand paper cleaned out the hole until it was running free. I bought a new set of brushes from e-bay (£3.00 inc. postage) put it all back together and all is now fine. The battery is charging as it should.

[nta16]

well done and thanks for reporting back

for future reference when dealing with electric items glass paper seems to be preferred to sand paper