I have a 1986/87 Nissan Pathfinder 3.0 liter.  The thing runs very
strong, but it has developed an electrical bug.    First, the wiring
harness was ripped out by a stereo-thief, and so there's a jumble of
wires under the dash, which, in the twelve or thirteen years I've
owned it, I've managed to hook up to three or four aftermarket stereos
and have the thing run great.  Recently, though the thing has
developed some sort of electrical short.  I'm no expert, but I bought
a digital Multimeter (which I've always wanted and am very interested
in mastering) and I figured out how to use the red and black leads to
check the voltage.  I get a 9.5 volts reading on the unit when I check
the battery on its own (negative cable disconnected) but the minute I
hook up the cable, the reading drops to 8.2 or so.  This indicates
some sort of voltage drain within the system, correct?  Once the truck
is jumped, it runs strong (just tuned it up) so the charging system
seems to be fine.

My question has two parts:

1:  What specific advice can you provide to help me solve this
charging problem (step by step please), and 2: provide practical
understanding of the mechanics and electronics of this Nissan 3.0 V6
engine.  I hav toyed with the idea of upgrading this truck with the
same motor, but out of a 300ZX which has the turbo.  Are there weaker
components that might be innapropriate for such a conversion?  What
about a front axle project?  I plan to keep this truck forever and it
will become rougher and meaner as time goes on.

Recently I hooked up a newer stereo, and this may or may not be part
of the problem.  The truck ran fine in this case, but then I had to
leave it sitting for about a year.  Stupidly I didn't disconnect the
battery.  Since I've had it out of mothballs, I put a new battery in
it, new connectors on the cables and cleaned them up real good,
knowing that such issues are often the cause of simple charging
solutions.  This didn't work so I busted out the Multimeter (A Sperry
DM-4400A) and decided to take the plunge into the world of
electronics.  I'm good with computers but new to electronics.

Tip will be given for anyone who can also increase my knowledge of my
Multimeter and using it practically in everyday life.  I have set my
price high because I want a good answer.  Thanks!

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Request for Question Clarification by revbrenda1st-ga on 17 May 2003 07:45 PDT

Hi hamstrung,

My mechanic husband is wondering what exactly is the problem. For
example, does your battery go dead overnight and you need to boost the
truck to get it started, or is the charge light coming on?

Thanks,
revbrenda1st

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Request for Question Clarification by sublime1-ga on 17 May 2003 21:58 PDT

hamstrung...

A well-charged battery 'should' read 12 volts (on the DC,
or direct current, scale of the multimeter, with the negative
cable disconnected from the battery. This may be a simple
matter of 'zeroing' the meter prior to use (making sure the
meter needle is on zero when not connected to anything), or
it may be a reflection of the drainage on your battery. 

A slight voltage drop is possible when you reconnect the 
the ground cable (especially if the 9.5 volt reading is
accurate, and reflects a weak battery - which will show
a drop in voltage much more easily than a strong, fully
charged battery). Such a drop could occur with a weak
battery due to items which are commonly still 'hot'
even when the engine is off, such as the clock and the
alarm system.

I would recommend first testing the meter itself on a
battery you know to be fully-charged and strong - 
perhaps a neighbor's truck. If it doesn't read 12 volts,
you might want to trade your meter in for a new one.

You can also test the charging system by measuring 
across the battery leads with the engine running.
If the meter is accurate, it should be 13-14 volts,
or just a bit higher. In no case should it read less
than 12 volts (assuming an accurate meter), or less
than the voltage of the disconnected battery. 

If this is not the case (*and* you've verified that
the meter will read 12 volts accurately on a good 
battery), then your charging system may not be doing
it's job, and the battery will get gradually weaker
as the engine draws a portion of the voltage for the
spark from the battery as well as the alternator.

If the alternator is not charging at all, a car will 
run for almost an hour, just on the spark produced by
the battery before it goes dead. If the alternator is
weak, but not completely shot, it will take longer to 
drain the battery to the point of uselessness.

That should get you started. Let me know where this
takes you...

sublime1-ga

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Clarification of Question by hamstrung-ga on 21 May 2003 00:48 PDT

Rather than spend a hundred bucks on having a Nissan mechanic fix it,
and my watching or talking to him afterward, I suppose I was hoping to
find a Nissan mechanic online.  I guess Google Answers isn't quite
there yet.  Yes, the vehicle registers 14+ volts under operation.

I am wondering (A) is anyone familiar with this particular model of
vehicle and the specific troublespots?  Specific relays I can check? 
And (B) Is anyone familiar with this particular Multimeter I have
(it's digital, no needle), as the manual says little or nothing and I
really want to get better with it.

Google Answers is really the lazy man's alternative and I admit that. 
I'm halfway there on my own, but if the right person comes along, the
money will be well spent.

---

Request for Question Clarification by sublime1-ga on 21 May 2003 10:45 PDT

hamstrung...

There are somewhere in the vicinity of 500+ Researchers, who have
been screened for their ability to research information on any
topic imaginable. While there are a few who are expert in one 
area or another, I'm not aware of any who are Nissan mechanics.

Even if they were, you say you've had "a jumble of wires under
the dash" for "twelve or thirteen years", and you've hooked up
"three or four aftermarket stereos". This means your wiring is
anything but standardized at this point, so a Nissan mechanic
going by the book would still have problems.

We've established that "the vehicle registers 14+ volts under
operation". This means the multimeter is accurate, and the 
9.5 volt reading, with the battery disconnected from the truck,
is an indicator that the battery is weak, and failing to be
recharged to the nominal 12 volts. This indicates the need for
(ultimately) a new battery - one that will hold a charge.
Naturally, it would be better to find the 'short' or voltage
drain, before doing so.

If your wiring were standardized, this would be relatively easy,
since all electrical items would be 'fused', or, wired through
a fuse, first. Your aftermarket stereo may not be wired correctly
in this manner.

What you want to do next is to disconnect each device which may
be causing a voltage drain, doing so with the meter hooked up to
the battery while the battery is connected to the system. The 
meter should initially read the 8.2 volts you cited earlier.
Upon disconnecting the device which is causing the problem, 
the voltage across the battery should jump up to the 9.5 volt
reading you cited when the battery was disconnected from the
system. It would jump to 12 volts if the battery was new and 
fully charged.

You might start by disconnecting the red, or positive, lead
from your stereo. If the voltage jumps up, you've found your
culprit. Since you installed it, you should be familiar with
the wiring for it.

If the stereo is not the problem device, then the easiest way
to proceed is to remove the fuses from your fuse box, one by
one, until your find a fuse which results in the voltage 
jumping up when you remove it. Then go to your owner's
manual and find out what devices are protected via that fuse.

You can work from there to determine which, of the devices
using that fuse, is the problem.

If removing all of your fuses (and disconnecting your stereo)
doesn't result in the voltage across your connected battery
jumping from 8.2 back up to 9.5 VDC, then it's time to 
look at untangling the "jumble of wires under the dash",
to see if doing so results in the voltage going back up.
If none of the above results in a voltage change, then
I may not be able to assist you any further.

As for the use of the multimeter, I have 6+ years of Navy
electronics in my background. They are basically very 
simple and similar, in that you use a dial to select what
you are measuring, make sure the scale you're using is 
the right range, and apply the leads. They measure AC
(alternating current, like your 120VAC house voltage)
and DC (direct current, like your car battery) voltages;
current, in the form of amps or milliamps (thousandths
of amps); and resistance (to current) in the form of 
ohms or a range thereof (a kilohm is 1000 ohms).

In this particular instance, you would have little need
to measure anything but the voltage of your battery as
you disconnect devices, looking for the one which is 
causing the drain. Once you found it, you could also
use the multimeter as an ammeter or ampmeter, by placing
the meter in series with the power lead for the bad
device. This would allow you to measure the amount of
current drain which the bad device is causing, however
this is unnecessary information.

Let me know where this takes you...

sublime1-ga