In a message dated 2/24/98 4:41:23 PM Eastern Standard Time,
mmason@lindenwood.edu writes:
> BUT, the other day on the Interstate the ignition light began to cycle on
> and off about every other second or so. I turned on the headlights and it
> stopped. When I turned them off again, it began to do it again. What the
> Heck Is That?
Mike,
I don't know, but lets see if we can figure it out. Referring to the diagrams
on the VTR web site http://www.vtr.org/maintain/alternator-overview.html We
see that the alternator has two sets of diodes -- one set provides the
charging current to the battery, and the other set provides the power to the
regulator. When the engine is running (ignition key on), and the alternator
is functioning, one side of the warning light is connected to the battery
diodes, and the other side is connected to the regulator power diodes.
Now, for that light to come on, the voltage on one side must be greater than
the voltage on the other. There are only two ways for this to happen: Either
one side is grounded, or the alternator is producing more voltage at one set
of diodes than the other. Since both sets of diodes are connected to the same
source, it is not possible for one set to have a higher than normal voltage
unless they both do, but it is possible, with some sort of internal failure,
for one set to have a lower voltage than the other. It is also possible that
one side of the lamp experiences an intermittent ground fault.
An intermittent ground fault is not likely, because that would, in all
probability, smoke some wires, and you would soon be very aware of it. This
would lead me to think that the most likely problem is a defective alternator,
most likely in the regulator circuit.
What does you ammeter do when this happens? If it swings toward the discharge
side and back, then the problem is most likely in the output diodes. If the
ammeter doesn't swing, then the problem should be in the regulator diodes.
Of course, none of this rules out other problems. It could be that there is a
bad connection at the key switch, for example, and just the physical movement
of turning on the lights could correct it, until they are turned off again.
As always, the first thing to do is check all of your connections to make sure
they are solid. If the problem is repeatable, you might want to measure the
voltage on both sides of the lamp to see what is happening. By seeing which
side has the higher voltage, you can get a good clue as to what is happening.
Please let us know what you find, as this is a puzzling problem indeed!
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition, slated for a V8 soon!
'68 MGBGT---organ donor for the '74
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