12.5/12.6 is definitely not charging. 13v plus shows some charge, but it
should be more like 14.5v. The warning light glowing shows that the
brown/yellow at least is connected to something, to make sure that it is
connected to the alternator and not grounding somewhere remove the plug from
the alt, turn on the ignition, and check that the warning light no longer
glows.
Check that you have 12.5v or so on the browns at the alt, ignition off and
engine stopped. Start the engine and rev it to at least 1000rpm, and check
the voltages on the brown(s) and brown/yellow at the alt. You should have
about 14.5v on both. If still 12 point something rev it to about 3500 rpm
and check again. If now 14.5v it does indicate that there is a problem
inside the alt. If still at 12 point something volts check the brown and
brown/yellow wires in the wiring plug are connected to the correct spades on
the alt, there are connection differences with some alts.
If you have 14.5v at the alt but only 13v or so at the fusebox then you have
some bad connections. Check at the solenoid (each spade or lug individually
while they are still all bolted/connected together) and the battery. You
may find different voltages on each brown at the solenoid which indicates
bad connections either there or at the alt.
Other than that the warning light glowing shows it has more voltage one side
than the other. With the ignition off you have 0v both sides so it doesn't
glow. When you first turn on the ignition you have 12v on the white side
and 0v (ground) on the brown yellow so it glows. When the engine is running
and alt charging you should have about 14v both sides so again it doesn't
glow. When you turn off the ignition and while the engine is still spinning
down you have 0v on the white and 14v on the brown/yellow so it glows again
but flickers and dies as the engine stops. The warning light glowing while
the ignition is on and the engine spinning at over 1000 rpm indicates that
either the voltage from the alt on the brown/yellow has dropped below or
risen above 14v, or the voltage on the white has dropped below 14v. You
need to have a test-meter hooked up to first one then the other to see just
what is causing the light to glow while driving before proceeding further.
PaulH.
----- Original Message -----
From: "Tim Economu" <teconomu@whidbey.com>
To: <paul.hunt1@blueyonder.co.uk>
Cc: "MG List" <mgs@autox.team.net>
Sent: Wednesday, October 08, 2003 6:41 AM
Subject: MGB alt question
Hi Paul and all:
Wondering if you would have a moment to help me with my charging problem. My
ignition light sometimes comes on, and sometimes goes off when the engine is
running. My great MG electrical experience told me it was the alt, so I
changed it out with a tested rebuilt unit (tested good to 38 amps), and same
symptoms.
Here's what happens...
1. Turn on switch, red ignition light on. Vbat = 12.6
2. Start motor. Red light still on. Vbat 12.5
3. Rev motor, light still on until the motor... Vbat 12.5
4. Decelerates, light goes out. Vbat 13.6
5. Motor decelerates to idle. Light still out , vbat 13.0
6. Motor accelerates, light comes on, Vbat 12.4, then deceleratees and light
goes back out, vbat 13.5.
I am considering a loose connection in proximity of the engine, so that it
flexes with each accel/decel cycle. Is there a specific wire I should be
checking, or should I consider anything else??
Kind Regards,
Tim Economu
1969 MGBGT mona
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