In a message dated 98-11-27 13:05:30 EST, bills@pagesz.net writes:
> Good morning Listers,
>
> I left the lights on in my TR6 a few weeks ago and got a battery jump
> from a Honda. I don't think my alternator charges the battery anymore
> and the battery goes dead overnight. Evidently there's a short or
> something causing a constant drain. The car will start after I charge
> the battery and it runs real well. But as I said, the battery is dead
> again the next morning.
>
> With the engine running, I read about 12.3 volts across the battery
> terminals. I beleive that should be considerably higher. The Smith's
> ampmeter is always to the left of center and never moves. I tried
> reading across the alternator leads on the back of it. I see little or
> no output from there, about .16 volts. I'm not sure where I should
> measure from, however.
Bill,
You should measure more than 12.3 volts - that's roughly the correct voltage
for the battery, indicating that the alternator isn't charging. The voltage
should be over 14V at anything over about 1500 RPM.
> If I have a constant drain from a short, does anyone have suggestions on
> how to isolate that?
Probably the best way to do this is to use an ammeter in series with the
battery and the positive battery cable. Observing the proper polarity
(positive lead of the meter on the battery, negative on the cable), connect
the ammeter in between the two points, and look for a current reading. There
should be none, but you may have a very small milli-amp reading. If you have
anything more than that, start pulling fuses to see if any one of them stops
the current. If so, look for your leak in the wires fed from that fuse. Of
course, if you have a clock, or a radio with memory, you will get a current
reading from them. No problem, just subtract the current draw from them to get
the leakage current, or just disconnect them for the purpose of this test.
If removing the fuses doesn't stop the leak, then use your wiring diagram to
trace all the wires that go directly from the battery (either connected to the
battery, the starter solenoid, or a distribution block). These wires are
usually Brown or Brown with a tracer. Lift these one by one till the leakage
stops, and trace the wire that stopped the leakage till you find the short.
> And, is there a way to isolate the alternator to see if it is working or
> would work without the drain?
A coupe of things you need to check for. First, is the warning light coming on
when you turn on the key, but before you start the engine? If not, there is
your problem, as this lamp is required to make the alternator work. If it
doesn't come on, pull the plug to the alternator, with the engine of, and
short the brown/yellow wire to ground with a temporary jumper. Depending on
the year of your car, you will have either one or three brown/yellow wires -
ground anyone of them, as they are all connected together. Turn the key on,
and look for the light to come on. If it doesn't, check the bulb to be sure it
is good. If the bulb is good, trace the wires to/from the bulb to be sure
there is not a break somewhere (there will be one if you've gotten this far).
One side of the bulb has a white wire to the run position of the ignition
switch, and the other side has the brown/yellow wire you just grounded.
If the light does come on, check for volage on the bown or brown with a white
stripe wires at the alternator. you should have 12 volts here. If not, trace
the wires looking for the break or bad connection. You should have 12 volts
with the ignition key on or off.
If all the above tests pass, you have proven your wiring is correct (but not
necessarily good), so now we need to test the alternator. Start the engine,
and check for voltage on the large brown wire, with the engine at about 1500
rpm or so. If you have around 14 volts, the alternator is 'probably' working
OK. If you have 14 volts or so here, but still have only 12.3 volts at the
battery, you have some very bad connections between the alternator and the
battery, which need to be cleaned and/or repaired.
If you don't have 14 volts, then your alternator is bad, or the connections at
the alternator plug are bad. Clean and/or repair them if needed, and repeat
the last test. If you still don't have 14 volts, then the alternator will need
to be fixed or replaced.
If you do have 14 volts, and you have made sure the charging wires and
connections are good, turn on the lights, heater motor, and any other
accessories you can, and try again, with the engine running over 1500 rpm. If
the alternator is OK, it should still hold the 14 volts, or maybe a little
less, but certainly more than 12.3. If the voltage drops to battery voltage,
the alternator is not keeping up, and needs to be fixed.
I think that about covers it, I can't think of anything else. From your
symptoms, I would suspect a bad alternator. Possibly a weak battery as well.
You may want to go to the VTR web site, and check out the article on
alternators. It will help explain some of the stuff I've talked about here.
The URL is:
http://www.vtr.org/maintain/alternator-overview.html
> Thanks in advance for the help I'm sure I'll receive.
You're welcome, I hope it helps.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/index.html
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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