QuestionQUESTION: How do I replace the lightbulb in my 1978 MGB ignition warning light?
ANSWER: Hi Doug,
That is the second light down in the center idiot light cluster and as I remember the bulbs are inserted from the back and requires that you remove the speedometer and the tach to gain access.
The speedometer and tach are removed by pushing in hard on the rim and rotate the gauge clockwise about 30 degrees. With one or both instruments out you may be able to remove the center idiot cluster or access the bulb. I don't remember if the bulbs are inserted from the front of the cluster or the rear.
You might try putting a thin piece of wood or such up against the dash at the front of the light cluster to see if it can be pried out from the front before doing all that. It has been too many years since I did any dash work on a 78 to remember exactly how it came out.
The ignition light is the alternator charge light and is the exciter for the alternator.
Howard
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QUESTION: Your final statement about the alternator charge light being the exciter for the alternator has me asking if the bulb is burned out and disrupts the circuit could that cause the alternator not to charge the system.
ANSWER: Yes, it can cause the charging system not to work.
It would be a good idea to run a test before attempting to access the bulb.
Go to the back of the alternator and remove the small brown w/yellow tracer wire from the back of the alternator. If it is in a large formed plug with three wires, unplug the large plug and take a jumper wire and ground one end of it and turn on the ignition switch to "ON" (not Start). Then touch the end of the brown w/yellow wire with the ground wire and have someone watch the "IGN" (charge light) to see if it lights up. If it does you have a failed alternator, either a failed regulator or diode pack.
If that don't light the charge light then the bulb or wiring is at fault. While you have the plug off of the alternator you should test the other two wires in the plug. The large plain brown wire and the small plain brown wire should be powered all the time, key on or off.
Some alternators will self excite when the RPM is raised but as soon as you idle down it may drop it again.
A temporary fix is to wire a small 12v bulb into a wire and attach it to the terminal where the brown w/yellow wire was and attach the other end of the wire to the fuse box on a terminal that is only powered with the key in the "ON" position. This acts as the exciter and works until you get to the bulb and or wiring to or from the bulb.
When you turn on the ignition switch before starting, do the other idiot lights light up? If none do you may have lost power to the bulb and you need a wiring diagram to trace where it is lost power. If you don't have a diagram let me know and I will post one on my web site for you to copy.
Howard
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QUESTION: Hello Howard,
I followed your advice about going directly from the brown/yellow wire to ground. I am happy to say that the ignition light iluminated. I now know that I need to replace the alternator to solve my charging problem. Also verified that I had 12v on the other wires for that alternator connector. I really appreciate you taking the time out of your day to help me out.
Doug
AnswerYour welcome, glad I could help.
It is still possible that the problem is the internal regulator and not the alternator itself.
With a volt meter and a jumper wire you can test the alternator by bypassing the regulator in a test. (If it is a Lucas alternator)
Remove the rear plastic cover of the alternator and plug the three wires back in. (if the three wires are separate just plug in the two plain brown wires)
Put the red lead of the volt meter on the large brown wire and the black lead to ground. You should see 12v + on the meter.
Connect the jumper wire on the large brown wire also and hold the end so as not to touch anything with it yet.
Note if the small metal box mounted on the back of the alternator is mounted at a slight angle. This confirms that it is the regulator and is a Lucas alternator.
Have someone start the engine and set the RPM at about 1500 RPM (fast idle).
Watch the voltmeter as you take your jumper wire and touch the metal case of the regulator and you should see about 15v. Don't leave the jumper on long.
If you see about 15v then the alternator is probably ok and it is just a bad regulator.
If you decide to purchase a regulator, before you remove the old regulator make a drawing of exactly where each wire of the regulator attached and their color. Often the replacement regulator will have a different number of wires but they usually supply a sheet that shows how the new one is wired.
If you choose to have a alternator/generator shop run the test you can print this procedure out and show it to them as some newer alt/gen shops may not have seen many of the older Lucas alternators before.
Howard
