Re: Chrysler quality

treeline12345@xxxxxxxxx wrote: 
maxpower wrote:
Check fuses with a load testing device  such as a test lite,  not an
ohm meter. it is possible to ohm out a faulty fuse
Glenn Beasley
Chrysler Tech



Bob Shuman wrote:

"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



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.

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 
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

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.


versus my digital meter which had 10,000,000 ohms
resistance.

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 
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.

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')
.

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