[Mgb-v8] Alternator Questions

[Paul Hunt]

All MGB alternators had the output and warning light connections as spades,
some upgrades to more recent and higher output alternators do have threaded
studs.

If the engine has just been switched off then the battery voltage will still
be registering close to alternator voltage, it will take a few moments to
settle to its 'normal' voltage.

A battery may show 12v plus but still be in need of charging or just worn out,
it is the voltage at the posts *when cranking* that is a good indication of
this if it is below 10v.  If a battery off-load shows less than 12v it is
either *very* discharged or has a bad cell.

Normally the engine will have to be revved to 900rpm or so to start the
alternator charging, and once started it should keep charging until the revs
drop below 600rpm or so.  Note this is with original alternator pulleys, if
you have a modification with a different sized pulley the rpms may be higher
or lower than this.

Original alternators should charge i.e. show greater than 13v at idle (once
revved over 900rpm), but not at their full rate of current or voltage.  More
modern and more powerful alternators will charge at closer to their full rate
at idle.  For alternators the maximum voltage should be in the range of 14.3v
to 14.7v. Dynamos could charge up to 15.5v depending on ambient temperature.
Alternators at the lower end of the range *may not* fully recharge a battery
that has been partially discharged, at one time Mercedes were having to select
voltage regulators that gave voltages at the higher end because of this.

The ignition or charge warning light should glow when you first turn on the
ignition, and when the engine has been started and revved over 900 rpm should
go out.  The current from the warning light acts as a 'pump primer' to the
alternator and is needed to start it charging.  Without the light the engine
may have to be revved to around 3000rpm before it starts charging, but then
will charge as normal down to about 600rpm.  If the light stays on when the
engine is revving the alternator isn't charging.  You will only see 12v on
this wire at the alternator with the ignition on *if it is disconnected* from
the IND terminal.  When connected it is close to earth or ground and will show
close to zero volts.

At various times MGBs had five, four, three or two wires at the alternator.
The four wire alternators were with the external regulator in 1968 only.  Five
wire were used from 69 to 71, but two of the wires were brown/yellow and just
linked two terminals on the alternator.  Of the other three one was a standard
gauge brown/yellow from the warning light, and two brown wires.  One of these
was a heavy gauge wire and was the output wire.  The other was standard gauge
and was wired to the solenoid the same as the output wire, but was the voltage
sensing wire for the voltage regulator.  In 1972 the alternators reverted to
internal sensing and hence only needed the heavy gauge output wire and the
standard gauge warning light wire, but the harnesses (except possibly in 1972)
still seem to have the extra standard gauge brown, which is now acting as an
additional output wire.  In 1977 two *heavy gauge* brown output wires were
used for reduced volt-drop with the higher output alternator and higher
current loads.

Originally chrome bumper MGBs ran the coils at 12v, rubber bumper at 6v.
However you will only see 6v at the +ve terminal if the points or trigger are
closed and the coil is drawing current, which won't be the case with some
types of electronic trigger with the engine stopped no matter where the crank
has been turned to.  At all other times (with the ignition on) you will see
battery/alternator voltage.  For some reason many people converting the
ignition or engine do away with this 6v system, which is a retrograde step as
it boosts coil voltage while cranking for easier starting.  With a 12v coil
and ignition system you will see battery/alternator voltage at the coil +ve
terminal.

PaulH.