On 7 Oct 1998, DANMAS@aol.com wrote:
> It is normal for the smaller, underpowered, alternator on a TR6 to experience
> a small voltage drop with all loads on.
Small?
If I remember correctly, we are talking of more than 1 volt below proper
charging voltage, which is not what I would call small. For winter use,
such a mediocre voltage means constant reliance on home charging for
trouble-free starting at all temperatures and long battery life (a good
battery should last 10 years, insufficient charging will half that).
The problem is inherent with the design of the built-in regulator. I would
imagine the early, seperate regulator units to be better, but I've never
tried them. The Lucas DC generator with relay-based regulator does
definitely maintain the charging voltage over load much better then do the
Lucas alternator! With the exception of when the engine is running at
idle, of course.
I should hasten to add that is not only Lucas alternators that have this
problem, most brands of the 70s and 80s are just as bad (I'm not really
familiar with 90s).
Not being able to leave things alone, I've replaced the poor built in
regulator with a seperate, adjustable PI regulator measuring actual system
charging voltage (PI in this case meaning Proportional/Integrating).
With such a regulator, the Lucas 18 ACR alternator is able to keep
correct charging voltage even with all loads applied (PI pump, heated rear
window, headlights and extra driving lamps, fan and wipers).
Now enter the following tale of car electricals engineering: It turns out
that the rectifier diodes have been dimensioned with the limitations of
the built-in regulator in mind! With the increased output commanded by the
new regulator, the diodes get so hot that over time the solder starts to
melt! After having replaced the diodes once, before I understood what the
problem really was, I resorted to installing proper Schottky rectifier
diodes instead. They have the additional benefit of increasing useful
alternator output by an additional 50W due to significantly lower losses.
Of course, fitting a big GM unit is probably more inexpensive and simple,
but I'd just wanted to say that you it is in fact possible to have
satisfactory charging with a Lucas unit (much modified, admittedly).
A much simpler (and quite customary) approach to increasing poor
alternator output is to mount a silicon diode in series with the
regulator, increasing the charging voltage by 0.7 volts. Just make sure it
doesn't bring the charging voltage sans load too high, though.
Ensuring first that the brushes are OK goes without saying - they do not
last particularily long on the Lucas unit, but are cheap and easy to
replace.
Egil
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