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MG MGB Technical - Ignition Light Stays On

A couple of days after returning from the British car show in Dublin, Ohio, I went to start my C and the starter just clicked and would not turn over. I checked the battery and found it to be very low. I charged it overnight and the car started right away. I did notice that the ignition light was not working. I pulled the light bulb out and found it to be burned out. I hooked up a trouble light to the socket and the light came on when the ignition was turned on. When I started the car, the trouble light stayed on and did not go out as the revs rose. I replaced the alternator with a rebuilt one and the trouble light still stays on. Does anyone have any suggestions?
Thanks,
Tony
Tony Shoviak

First thing I would do would be to check the new - rebuilt - alternator. I think the archives has lots of stories about bad rebuild jobs. I ended up buying a NEW Lucas and my alternator problems went away.

Robert
Robert Browning

What year is your C? Different setups between 68 and 69. Same as the B, but 68 has an external regulator, the 69's are built in the alternator. If a 68 the regulator is the box mounted by the fuse block.

Safety Fast

Ron
Ron Smith

Tony. As the others have indicated, the fact that the light remains on indicates an imbalance in the system. As a quick check, fully charge your battery, with the cable clamps disconnected. Then, measure the battery voltage. Should be 12.5 to 13.5V with a good battery. Reconnect the battery clamps and start the engine. Check the voltage on the battery clamps (not the battery terminals) and you should see 13.5V to 14.5V with the higher reading being preferable. (Batteries are always charged at above their nominal voltage.) If you do not see above the system (battery powered) voltage, assume that the battery is powering the system and the charging circuit (alternator/voltage regulator) is not working correctly.

If the test show that the alternator is working "correctly", i.e. 14.5 V, you need to take the car into someone who can determine the output of the alternator. I had a similar problem with my 68 GT, with the Lucas 16 AC alternator and external voltage regulator. The altenator which was supposed to put our 34 amps was only capable of putting out 17 amps under load. This, in spite of the fact that the alternator had been rebuilt within the last year and the voltage regulartor was new.

I replaced the bad alternator with a Lucas 18ACR, wired around the external voltage regulator and have been driving the car for five years with no problems. This would be a problem with the show car set, but my cars are daily drivers.

Les
Les Bengtson

Tony-
"I hooked up a trouble light to the socket" How? If you hooked it up betwixt power and ground, it would light and not go out, even with the alt working. The bulb is in series between IGN power (White) and the alt IND terminal (browN/Yellow). With key ON and engine NOT running or alt NOT working, power goes from key through bulb to ground in the alt. With the alt working, there is power at the alt side of the bulb as well as the IGN side, thus the bulb doesn't light. Further, if the alt does NOT have the separate regulator, the circuit through the bulb supplies the field excitation current, and the alt won't charge without the bulb in place. The separate regulator type has a resistor in parallel with the bulb, that allows it to work without bulb. Blown bulb is almost always a result of failed diodes in the alt; warning sign is that bulb gets brighter as engine speed goes up and voltage increases. The same high voltage (and resulting high amp output) can cook the alt.
Long ago before I understood this, I had an Austin America with this problem (and no $$ to fix it). If I ran it up to about 4500rpm, it would blow the bulb, then the alt wouldn't charge, then the formerly great headlights would get real dim! Used to carry a box of bulbs with me.
FRM
FR Millmore

A used alt will probably charge without a working ignition warning light, with my two cars the engine has to be revved to about 3k for it to start, then it will charge as normal down to the usual 600rpm or so. This is with the later integral voltage regulator types. A brand-new alternator out of the box may not have this characteristic as there will be no residual magnetism in the iron-work. However *don't* try and polarise the alternator like you do with a new dynamo!

If the light (including a test-lamp connected to both the terminals in the bulb holder and not with one terminal connected to ground) stays on with the engine running then it implies the alternator is not charging, although the voltage on the brown is the real arbiter. Should be 14v or better when charging, probably only 12v or so when not. If the light still stays on with a replacement alternator then you must look a little more closely at how the brown/yellow is connected, although it is quite possible that a replacement unit is also faulty.
Paul Hunt 2

Just a question about that damned light. After I start my 1974.5 MGB the ignition light will usually stay on until I rev it up a bit. It will then go out and will not light up again even after the engine returns to idle. Is this normal or a warning of problems to come. The battery is new by the way and as far as I can tell the alternator may be original (not sure about that though).
Henry

Usually they have to be revved to about 900 to 1000 rpm to get the light to turn off. It is quite normal to be able to start the engine without revving to this point, and so the light does stay on, until it is revved above it. If it needs to be revved higher than 1000 rpm then that would be unusual, and possibly a voltage regulator defect.
Paul Hunt 2

Thanks Paul, yes it is usually around 900 to 1000 rpm when the light turns off.
Henry

Gentlemen,
Thanks for the sound advice. I'll be using it this weekend when I tackle the C.
Thanks again,Tony
Tony Shoviak

This thread was discussed between 26/05/2006 and 28/05/2006

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