In a message dated 4/13/98 7:58:14 AM Eastern Daylight Time, DUHART@symbol.com
> Let me explain where I am in the story. I have replaced the Rectifier pack
> and the Voltage regulator with a 4-wire unit from TRF. This weekend I
> charged the battery with my battery charger. I start the car up and the IGN
> light dims a bit but never extinguishes. If I rev the engine to about 1300
> RPM and measure the voltage cross the battery I now get a steady 13.88V (
> digital volt meter). No matter how high i rev the engine the IGN light
> goes out.
There are two sets of diodes in the alternator: one set provides the main
charging current to the battery, and the other supplies the field current to
the field windings of the alternator and the supply voltage for the regulator
circuit. When the key is on, the indicating lamp is connected between the
battery and the second set of diodes (field current diodes). Normally, there
are only two conditions under which the light comes on: When the voltage from
the battery is higher than the alternator voltage (indicating a defective
alternator), or when the voltage from the alternator is higher than the
voltage From the battery (indicating a defective battery -- I'm assuming your
battery is good. If it were a bad battery, the light would get brighter as the
engine reved up). It takes a voltage differential of a few volts to get the
lamp to light -- the larger the difference, the brighter the lamp.
A good battery will produce 12.6 volts maximum; a properly functioning
alternator will produce *around* 14 volts. Since you are measuring 13.88 at
the battery (which is also the output of the alternator -- they are connected
directly with a piece of wire). The only way you could measure 13.88 volts
here is for the alternator to be producing it. That's the good news -- your
alternator is working!
Assuming you made no changes to the wiring around your ignition key, one side
of the lamp is connected to the battery, and should be seeing the same 13.88
volts that you measured. Since it requires a voltage difference across the
bulb for it to light, this means that the other side of the bulb, the
alternator side, must be at a higher or lower voltage. Given the internal
construction of the alternator, it is impossible for this set of diodes to
produce a higher voltage than the main output diodes (unless the main diodes
have failed, and you aren't getting the correct output voltage from them. We
know this is not the case, because you have measured it). Therefore, the
voltage at the alternator end must be low.
There are two ways for the voltage to be low:
1) the wire from the bulb to the alternator could have a "partial" ground --
not enough to make smoke, but enough to keep the bulb lit. It could be
shorted and still connected to the alternator, or it could be shorted and
disconnected from the alternator. Theoretically, if the bulb is disconnected
form the alternator, the alternator won't work because it receives its initial
field current -- required to produce the magnetic fields that make the
alternator work -- from the battery via this bulb. In the real world, though,
sometimes the iron core of the alternator will become magnetized, much as a
screwdriver does, and will have just enough magnetism to allow the alternator
to produce a weak current. This weak current will increase the magnetic
strength, increasing the current, and so on till the alternator is producing
2) a fault inside the alternator itself.
To test for No. 1, disconnect the alternator plug from the back of the
alternator, and turn on the key. If you have a short, the light will still
operate. If the light doesn't glow, then you don't have an external short.
For No. 2, reconnect the alternator plug and start the engine. Measure the
voltage at the brown/yellow wire in the plug. It should be equal (or at least
very close) to the voltage on the large brown wire in the same plug (which
should be very close to the 13.88 you measured before, with the engine at high
idle). If the voltage is low, and I'm guessing that it will be, then you have
a problem inside the alternator. If so, then follow the procedure outlined by
Charlie Brown on the VTR page.
If these two tests don't pinpoint the problem, then the problem is on the
other side of the lamp, on the ignition key side. Unfortunately, there is no
quick and dirty test for this. You will pretty much have to physically trace
the wires looking for the fault.
As usual when talking about electrical problems, it is possible that there is
a seemingly unrelated gremlin somewhere else that is causing the problem. If
the above doesn't solve the problem, let me know and we will try something
Hope this has been of some benefit.
BTW, it's good to be back on the list!
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74