On Mon, 17 Mar 1997, Matt Kulka wrote:
> Being new to the list (having just bought my first MG - a '74 MGB w/
> chrome bumpers) maybe it's fitting I reply to someone who seems new to
> electrical problems.
Be aware that the "B"'s electronics are Lucas brand; made to a price
(low), and prone to intermittent failure.
> The red dash light indicates a low rate of charge (a.k.a. 'current'
Not necessarily!! We ran one alternator in a "B" for years, and it
always lit the red light dimly; BUT the battery never went flat!!!
> It would be worse at night because you have the headlamps on. Second
> only to your horn, these use more current than anything else in the car.
Actually, headlamps aren't that bad... 37.5 watts/12 volts= 3+ amps
Lucas, Prince of Darkness, cheats a bit by using teeny weeny wires to
the headlamps, so the resistance of the wires is in series with the
bulbs, and they end up drawing less due to the voltage drop in the wire
The real power sucker is the heater/demister fan. Once your alternator
diode bridge blows one phase, (and it WILL sooner or later), you'll see
for yourself. Turn on everything else, no sweat! Turn on the fan, and
the wipers slow to a crawl, the turn signals blink every ten seconds,
and the battery is discharging continuously. And that stupid red light
may, or may not light up.
> backfires is interesting - could it be that due to a short, the choke is
> kicking in and out and changing the mixture as you drive?
The hysterisis of the choke mechanism (heated coil) is too great to
allow it to kick in and out. More than likely, there IS an intermittent
short somewhere, and the available voltage isn't enough to fire the
plugs properly when the short drags down the voltage in that circuit.
Your best bet is to buy a cheap lcd vom meter, hook it up in the voltage
function, and take a friend for a drive to check things out while you
drive. Start out cold, and wait for the backfireing.
> setup ... except for the carb, this is how I got the car. :) Alternator
> is a rebuilt Lucas unit, installed last year. Battery is a fresh Series
^^^^^^^^^^^^^^^^^^^
This means nothing!! You could blow a diode in the internal diode
bridge ten seconds after installing the rebuilt alternator. Because the
alternator is a 3 phase unit, you loose at least 1/2 the power when you
drop one phase; so your 25 amp (only for the first 5 -10 minutes, then
internal heating drops the output) alternator will put out about 12-13
amps and then drop to maybe 9-10 amps. Not enough to keep up.
Check things out systematically, preferably with a shop manual to guide
you (NOT one of those Haynes type things), and write down your results.
Let us know what you run in to.
TTUL8r, Kirk Cowen
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