Tech Topics

The New England M.G. ‘T’ Register has a long history of providing its members with advice and technical expertise on the restoration and maintenance of our special cars. We present some of our favorite technical articles and tips here for members and visitors alike.


The Ignition Warning Light- How does it work?

If you have often wondered why the red ignition light is bright when you turn the key on and fades away as the car is started and runs up to speed then this is the article for you! I have had a few questions about this subject (two actually) and thought it would be interesting to those members who have had charging problems in the past or a faintly glowing light with the headlight on etc.

In the prewar days, as I read it, the sole function of the red warning light was to remind you to turn the key off so the coil would not overheat and expire. As time progressed it also became part of the first on board diagnostic system for backyard mechanics. It has two functions. First to let you know that when the ignition is switched to the “on” position that power is where it should be and you have a good chance that the car will turn over. Second it is an indicator of the operation, or non-operation of the charging system.

The warning lamp bulb has always been red in T-series M.G.s and is located on the instrument cluster. It is not exactly easy to replace the unit but usually only the bulb may need to be changed. I have only changed one warning light bulb in my memory so they last a long time. A good description of the unit itself can be found in the June 2004 TSO.

One side of the lamp is wired to a 12v output at the ignition switch and the other side to the D terminal of the voltage regulator (control box). If you squeeze under the dash to look at the connections you will see a white wire from the ignition switch to one side of the lamp unit and a yellow wire on the other side. Moving out to the control box you will see two yellow wires connected to the D terminal. One
goes to the warning lamp and the other is connected to ground (earth) via the generator (dynamo) armature and brush-gear. The D terminal is a junction point for these two wires.

When you switch on the ignition but have not started the car, battery voltage is supplied to the lamp through the white wire and the circuit goes to ground through the generator. Once the car is started and the rpm increased, the generator begins producing it’s own voltage which will hopefully be greater than the battery voltage. This opposing voltage from the generator gradually equals the battery voltage and the red lamp fades out. At this moment of equal voltages the cut-out points in the control box close, usually at 13v, and short out the warning light keeping it off.

If the voltage drops down due to a generator problem or a poor connection in the charging circuit then the lamp may glow faintly or suddenly glow bright red indicating a major failure like a broken fan belt or battery failure.

I use a voltmeter to test charging systems and find the ignition warning lamp to be a crude guide as to the state of the system. It is only to alert you of a possible problem and more sophisticated instruments are needed to pinpoint the cause of the dreaded red light!

Drive safe!
by Donald M. Lawson #5687






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Disclaimer: All Tech Tips presented here are copyright of The New England M.G. ‘T’ Register. Be advised that every effort was made to verify the validity of each tip. But as in any free advice, it’s up to the user to judge the usefulness of each tip presented here.


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