Hi again,

I am sorry I did not get the point across well. What you want to do is string your meter down one side of an operating circuit. A circuit that has a voltage applied from the battery or turned on from the battery.

I wanted to keep it simple and that always seems to get me in trouble as I edit things along my train of thought. I will try to express things with examples to visualize.

In the first post, I told you about two methods with different purposes and that might have confused you.

One was for displaying current drain using a light bulb on the groundside. You can use an “ammeter” to do the same thing as Trev29 stated in his post. You attach the leads between the open end of the cable and the battery negative post. This way if anything leaves the battery it has to go through the bulb or the ammeter. This is an “in-series” connection of the meter. Like Christmas tree lamps, if one burns out or “opens” all of it quits.

I was leading you to open the fuse panel and remove a fuse, one at a time, until the test light bulb went out or the ammeter goes to zero. That way you narrow down the drain to circuit load like a dome light, cigar lighter, radio or frayed wire to ground. That was where I should have wrote, The END of this method.

In the next step, I was trying to say that using an “ohmmeter” to check large cables used for the starter and alternators are useless. This is because the ohmmeter uses a small low voltage output current and will see only the smallest good strands in that big wire and will show very low resistance. This will give you false information.

Visualize, if you like, that if you connected twenty-five bulbs side by side in a bundle with one lead on one side of all of them and the other lead on the connecting the ends of them all.
All the bulbs are then in “parallel” for the passing current from one side to the other of a battery that has plenty of capacity to power them and even more. They will conduct the current from the ohmmeter the same. The meter reads that value out, near to zero resistance or almost considered a short circuit. A #20 gage or a #6 gage the meter cannot see the difference but we know there is a big difference in what they can do with amounts of current.

Next questions are! How many bulbs would have to bad or open until the restriction show as a resistance higher from a zero that slows down the current from going on to some other load, down stream, on that circuit? What kind of reading would it have to be? How many bulbs or strands in that wire will have to have corrosion to do that? There are answers but I will not go there. High current draws like a starter, fan motor or headlights get our attention first and forget the calculations.

The battery is a system shock (load) absorber or reserve tank. The alternator pushes electrons back out of the lead plates and into the acid solution for another replay upon discharge. Any extra goes to other things in the car.

The voltage regulator is the intermediary that holds limits on the system so things do not get to cooking or burning out. It does this by sensing the “voltage drop” of the system; it drives more current into the windings of the alternator, increasing the magnetic fields and therefore the output in current. The big wires connecting them are like a two Way Street for communications and work requested from changing loads.

I think you understand the importance of voltage regulation from your post. You use you voltmeter across the battery post to read its “state of rest” of its potential charge. If it is at 12.6 volts, you consider it full. Now drop that down to 11 volts and you have a dead battery or open cell. 1.6 volts spells out a no go anywhere situation.

Let us go back up to 12 volts, now it is a very weak battery and some may say a dead one. That depends on what you are trying to get out of it and how it travels out some wires.

I am getting to the point of, what a few tenths of a volt really means. I consider a battery at 12.3volts, as half-full of capacity. .6 volts is a magic number for the battery status.

Voltage drop is important stuff! If you do not have enough pressure, you do not get large amounts of water out of a pipe. If you do not have a larges enough pipes, it does not matter what the pressure is anymore.

Think of the tenths of a volt on 12-volt car systems as a large resistance reading on an ohmmeter. Put it this way. When you apply the voltmeter leads across the battery post, you are in affect putting a very high resistance wire (almost an open circuit) on the battery. It shows you a large voltage drop of say 12.6 volts on a good battery or 13.5 to 14.2 on charging system.


What I want you to think about is how do I check the electrical system as a whole like the battery?

One answer is I go from one side of the system to the other with my voltmeter.
We do it all the time and we expect to see 12.6 volts when the circuit is on or we want it to be! As long as we are real close to the battery or we have a real, good circuit to that point there, should be little change.

I said, “Should” because with a sensitive enough meter you can always find some, called distance with resistance, is just the nature of things.
Long wires are like long highways, it cost more to make them. Put to many people on one and they get hot about it.
Put in a bigger road all is good until you have a traffic lane closure, which is like CORROSION and then the people get hot again or do not GO when they want too! When there is enough pressure (voltage) to push people (current) then they try harder. Things get to hot then the weakest or thinnest part of the road gives up. Something like a bridge of the road gives up. We hope that that was a fuse! I will come back to checking “fuses” later with the use of a voltmeter.

A voltage drop means DOT is at work or a device is restricting flow and or using energy. By looking for a voltage drop, we are trying to find the item or problem. We are looking for the unexpected lane closure.

Now this is the other answer of how to use the voltmeter. Across a circuit, we read battery or source. You got that down!
Now applying the meter leads “parallel” with only one side of circuits wiring that is “complete” the meter reads zero or real close due to its internal circuits.

Visual example is to take a wire, short it across a battery and tried to read that circuit, WHILE IT IS FRYING, the meter would read near zero because you are no longer across the battery posts. The voltage reads zero on the meter. There is no longer a “potential to do something” (volts) as almost all the current is going through the wire.

The closer the meter shows to zero volts, the better the wire is performing its work.
The more voltage goes up above zero volts, the worse the wire is performing.

What you want to do is string your meter down one side of an operating circuit, along the wire(s) under test, sort of replacing it with the meter on top!
You need a circuit that has a voltage applied to it or is on, of course. Use only the lowest range of analog meters. You can set the meter to a low range or trust the auto range feature of digital voltmeters, which is nicer.

If it is a good or turn on working circuit, the voltage drop will be equal to the resistance of a load or a wire that acts a resistance due to poor connections or having corrosion. If it is, a motor or bulb in between, there will be an expected drop or lack of voltage is expected and is good.

When checking fuses, with out pulling them, you can use the voltmeter (across the top) and the fuse that is good will show zero volts when you put the leads across them. An open, bad or corroded fuse connection will show some a voltage reading.

I like testing the negative side first or more often, because all the battery current leaves out from that side. No switches to think into the trouble shooting, just looking for mother natures dirty deeds.

When it comes to straight plain old wire and expected good connection or grounds we do not want any reading of voltage, if possible. A reading of 0.020mV and up to 0.130 mV is good, anything that is greater, start cleaning stuff!

On the positive side of the circuits wiring, the limit is 5% of the applied voltage or less over or up and down the lengths of the wire or side of the circuit that connects to a load.

Now, where did I get that 5% number! Remember the part about the battery loss from 12.6 volts to 12.0 volts. The loss of 0.6 volts tells us the status of the battery charge. Five percent of 12v is .6v. This limit equates some allowances for contacts and normal resistances but any higher means, keep a looking!

Personally, when it is just wiring involved I still like my zero volts approach but I do allow some forgiveness for lead lengths say to the back of a car. Voltage drop readings I have found to be reliable for guidance during a hunt.

I hope I did not confuse or bore too many posters or readers. If I put an answer in here, for you folks, I hope it was not to obscure.
Again, I read stuff and got most of my information from an article written by John Armstrong who along with others writes for the Auto Restorer, a guide for car and truck enthusiasts.
I, like Ray on Car Talk, twisted it with help from my head and hands on experience to type this.

Phil

I should have said to apply them “parallel” to the cables. Guess I tried to over simplify my vision. The polarity does not matter. You are looking for a ratio of voltage drop, which equates to resistance. It takes very small part of a volt to mean a lot of resistance.

On a large cable an ohmmeter only needs to measure a few good strands and it will show a good small wire! Battery cables go bad under the insulation and under the crimps. Then you can throw in the different metals that they mount to and here we go or not!

You want to do this on both sides of the battery circuit to rule out bad cables, engine grounds, positive side wiring and their connections.

I read about this method, some time ago, in an Auto Restorer Magazine article. They deal with worse corrosion than what is on our cars. They have older technology and in most cases lower voltage systems.

Phil

Perhaps Machine Man is suggesting you check voltage drop in the charging circuit from alternator to battery. You can do this by placing the + Positive lead of the meter on the output terminal of the alternator, and the negative on the battery's positive post. On the POST, not on the cable clamp.

With the engine idling, or maybe revved to 1500RPM, there should be very little voltage showing...say less than 0.1 or 0.2 volts.

Much more than that and you should narrow down by placing the positive meter lead on the next connection point going to the battery - the starter terminal, then on to the cable clamp.

Also check alternator ground by placing the meter positive on the alternator case and the negative on the battery negative POST. Then same procedure down the line if there is more than 0.1V or 0.2V drop.
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Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.