Chrysler quality

I'm uncomfortable with Walmarts because of their employee practices and how they treat their employees. The parts are very uneven, that I'll grant you. Sometimes good and sometimes not so good. An auto parts store if a real auto parts store, that is connected to a machine shop somehow, tend to have much better parts.

I'm not familiar with the phrase, "ohm out." Is that to burn out a fuse since some older meters have quite a lot of current in their resistor testing circuits, like inexpensive analog meters.

I think I see your point. A test light would require current or load to light the bulb. I wonder what the load would be for a small 12 volt test bulb versus the load from an old, inexpensive analog meter with its ohmmeter scale.

"Ohm out" (Glen's phrase, not mine, but was perfectly correct) means to measure the resistance in ohms using an ohm meter. No ohm meters ever produce high current. The older analog units used a higher voltage battery compared to newer DMMs of today, but they still had extremely high resistances so the currents were negligible.

Bob

Bob Shuman wrote:

I am not arguing about whether the phrase is correct. But it definitely seems that Glenn is says NOT to use an ohmmeter but a test light instead. Now whether this means to ohm out or not. I don't know. But intuitively it sounds as though the meter might affect the readings. And I could see this easily with cheap analog meters. I have used such meters. In fact, I used one to anti-polarize electrodes it used so much current in its readings. As I vaguely recall, expensive impedance meters could get around certain problems but that's not the case for simple continuity testing.

The older AND inexpensive analog units did not have extremely high resistances and did have rather high current requirements. Offhand, I can recall using a cheap analog meter that had only 50,000 ohms resistance versus my digital meter which had 10,000,000 ohms resistance. This is a tremendous difference, of 200 times more resistance - assuming my recall is correct. My oscilloscope also uses

10 megaohms of resistance. But that was a very high end analog Tektronix 'scope for its time.

Both my analog and the digital each used the same voltage battery or batteries. I don't think I have ever seen a meter using high voltage batteries. What brands are you thinking of that do this? I have seen 5 or 6 digit Flukes which use AC voltage but they are $1000+ meters. I do have a special HP millimeter that does not do resistances but uses AC voltage and is quite big and originally rather expensive.

As Bob said, the phrase means to measure continuity with a handheld multimeter. When Glenn said "it is possible to ohm out a faulty fuse", he was saying that a bad fused could read good on an ohm meter because it (the meter) is pulling negligible current (i.e., the fuse could have a weak or intermittent connection that will show "good", that is low ohms, whereas if you run any sizable current at all in a weak or intermittent fuse, it will generally clear from the higher current thru the weak connection). You have to look at numbers - in some contexts,

100 ohms is 'low resistance'. In other contexts, 0.5 ohms is 'high resistance'. Language alone doesn't do it.

I don't know. But

That's huge relative to the less than 1 ohm of a good or partially blown fuse. Like I said, in a discussion like this, words like 'high' and 'low' are meaningless unless you define them. 50,000 ohms is at least 5 orders of magnitude higher than an automotive fuse. Do some calculations for a resistor divider with a few volts across 50k ohms on top of 0.5 ohms. The voltage dropped across the 0.5 ohms (fuse) is Vapplied x 0.5/(50,000+0.5) . Even if you vary the 50,000 ohms by a factor of 10, the voltage across the fuse will be very small. Also the current is small. If V is 5 volts, the current would be 5/50,000 - a fraction of a milliamp.

So now the current in the same scenario is 5/10,000,000, or 1/2 a microamp. Several orders of magnitude difference - yes - but to a good or weak fuse, both are as close to negligible as you can get.

This is a tremendous difference, of 200 times more

Special purpose resistance meters may have to pump some current in - in fact - that's what you have to do to get a meaningful reading on something as low in resistance as a fuse. In essence, that's what you are doing with the test light - you are putting enough current thru it so that you have some observable effect. But your typical hand-held meter is going to read close to 0 ohms regardles of the scale you use. ANd what little it reads above zero is going to be meter error (test lead resistance) - not anything meaningful. Also, with resistnaces ars low as fuses, you would need a four-lead probe for reasons that I won't go into here (hint: The lead tip contact resitances are significant relative to what you are trying to measure, so you have to somehow cancel those restistances out).

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

It only takes one strand of wire to for an ohm meter to show continuity, place that one strand of wire in a circuit and it could fail causing the technician to run around in circles. I find it easier to use a load carrying device like the old test lites or fabricating a dome lite bulb with alligator clips to use as a testing device. I never use an ohm meter on fuses

Glenn Beasley Chrysler Tech

Bill Putney already wrote a detailed reply and Glen explained his preference for the use of a higher current light to test fuses so I will not address those here. What I do want to respond to is your one comment on your need to "anti-polarize electrodes" due to the meter "using so much current in its readings".

This is an obtuse statement and not clear to me at all what you were attempting to convey here. My guess is that at the time you might have been attempting to read an "in circuit resistance" using a meter and that when you changed the polarity of the test leads you got different readings. If so, then this was because the circuit had "active" semiconductor devices (e.g. diodes or transistors) and the voltage across the junction of the device (about .7 volts for silicon) was sufficient to generate a current through it thereby causing the meter to read differently when the junction was forward or reverse biased.

Keep in mind that most ohm meters are simply a DC voltage source and a very high precision resistance. As such, the current flow is what is being measured and this is then converted to resistance using ohm's law by dividing the known voltage by the current flow that was measured.

Regarding my statement that some older analog meters used higher voltages, I had an older Simpson analog meter that used a 9V "transistor radio style" battery. This compares to most DMMs today that use 2-4 standard 1.5V AA or AAA style batteries. As Bill accurately pointed out, the term higher is relative when comparing the 9V to the "lower" 3-6 volts used today.

Bob

Not if you do it right! If you mean it is possible for a bad fuse to look good if you just shove an ohmmeter across it without removing it from the car, I agree. But if you remove the bad fuse from the car and check it with an ohmmeter, you WILL NOT get a false-good reading.

You sure could get a good reading with an ohm meter as you test a bad fuse in your hand.

Glenn Beasley Chrysler Tech

No way in hell, so long as the ohmmeter is on a low impedance setting (say 0-100 or 0-200 ohms) that pushes more current than using "autorange" which tries to measure in the mega-ohm range. Auto-ranging meters are of the devil. :-) Well, they do have uses, but you can't just leave them on autorange all the time. That's what gets people into trouble when testing high-current devices like fuses.

Let me quickly answer this because my use was definitely very different from the usual. I was building or maintaining silver silver-chloride electrodes used in recording microvolts (millionths of a volt). Because of the sensitivities involved at that time, the DC offsets were a tremendous problem. They could be designed out of an analog pickup circuit, but that presented other problems.

The current in the resistance circuit of the cheap Radio Shlack meter was enough to further polarize the electrodes. The electrodes were inclined all on their own to develop a cell effect, that is, up to several hundred millivolts all on their own. So what I would do, crude but sometimes effective, was measure the DC offset and resistance and then reverse the leads using the Ohm/Resistance scale to zap the DC offset. But this was short term. I found another technique to bleed off the voltage remaining on the electrodes. I can't remember now the connection between voltage and resistance in regards to these electrodes. High resistance was one problem. DC offsets another problem.

Better to bleed out the DC offset. Let me see, it was a standard practice, depending on whose standard, to measure all the electrodes while applied and get the resistances down below 1000 ohms if doing exacting work, even 500 ohms in the past. An impedance meter was best for this because it did not introduce any current that appears to upset the silver silver-chloride electrodes. Sometimes I used gold electrodes which had fewer problems, no DC offset, but only silver silver-chloride would record the best down to DC, in case DC recordings were required for some reason.

This does not have much to do with cars since these problems exist at levels that are not really relevant to this discussion but I was curious.