My Fluke 88 does just fine to measure resistance for my purposes
It is not a POS harbor fake multimeter
It’s not special equipment either, for that matter
I agree that voltage drop is a more reliable testing method, and is in fact the preferred method
But I disagree with you 100% that I can’t measure resistance less than 1 ohm with my Fluke
And I disagree with you 100% about that 0.005 ohms of resistance for the battery cable. Ain’t gonna happen in the real world, not on any car you or I are buying. Maybe on some vehicle designed and built by NASA, but not on anything you’ll find at a Chevy, Toyota, or Ford dealership, for example
I like you just fine, and you bring a lot to the table, so to speak
But I profoundly disagree with you on this topic
I’m coming at this from a practical standpoint, somebody who works on this stuff for a living
I wasn’t arguing, just curious. I own many pieces of electrical test equipment, some quite costly, and was wondering what was different about measuring resistance above or below 1 ohm.
Part of my curiousity stems from the fact that there are almost no automotive situations where measuring ohms is important or useful.
It should not be difficult to sort out an electrical problem on a '91 Capri. A simple VOM should be sufficient and resistance readings are meaningless.
Without knowing a few things (voltage at the battery while the engine is running, condition of the belt and tensioner, condition of fusible link, dashboard battery light operative, etc.) I can’t say what the problem is but these cars are simple.
If you think the alternator is not charging turn the key to the RUN position (engine not running) and touch the alternator pulley with the tip of a screwdriver.
If you feel a magnetic attraction at the pulley then in theory the alternator is fine.
asemaster,
it’s been a looooong time since school, but I believe the Galvanometer is what BillRusssell is referring to. It’s an extremely precise meter for measuring very small currents, far more precise than your typical ammeter, but only for very small currents. The current value is then used to calculate resistance.
It has virtually no use outside of a laboratory, and certainly no automotive use. You’re right, voltage and voltage drop is meaningful for troubleshooting, measuring resistance for anything other than open or short circuit is usually useless, for automotive and practically all other electronic applications other than research and development.
asemaster: a 4 terminal ohmmeter is basically a current source, dual connections, and a millivoltmeter. Take, as an example, a piece of copper tubing. Set the current source to 1 amp and connect it to the ends of the pipe. Connect the voltmeter to the ends also, but with separate connections. Measure the voltage and use Ohm’s law, R = E/I.
The separate connections mean the contact resistances do not enter into the calculation. Contact resistance can be larger than the unknown being measured.
Ahh I see. I can see no purpose for this type of meter or testing on automotive charging or starting systems though, because to be useful or accurate the load applied must be equal to the load seen in normal use, right?
I believe that all analog electrical gauges are Galvinometers. The Sun VAT 28’s volt meter and amp meter were somewhat identical D’Arsonval Galvinometers but the amp gauge was connected through a shunt and measured voltage drop. And the gauges on pre 1990 cars were virtually identical volt meters calibrated to the sending unit to indicate oil pressure, temperature,etc.
But as for the OP’s problem, it is intermittant and unpredictable… And shops hate to see a problem like that come in the door. I once charged a man over $100 to diagnose an intermittant short and cut the wire to a vanity mirror. He paid the bill without complaint and I later found out he had a Phd in electrical engineering.
Ase: no, the resistance is fairly independent on current (load), unless the current is high enough that the wire starts to get hot. But, yes, a 4 terminal ohmmeter is not useful in automotive diagnostics.
If you need to calculate a wire size, ohm’s law and a set of wire resistance tables are all you need. For example, if you want to have less than one volt drop at 100 amps in a 2 foot cable, you can calculate that you need R to be 0.01 Ω for 2 ft or 0.5 Ω / 1000 ft, which is AWG #7 Cu or #5 Al. Note that this does not include any connectors or connections on the ends.
It’s true that a 4 wire micro ohm meter has sufficient resolution to see the reduction in cross sectional area that would affect maximum current capability of the wire. I’ve used them to track down solder shorts on circuit boards or disconnected/broken conductors in Litz wire for example, that is how sensitive they are in measuring resistance. But it is not a practical instrument for automotive diagnostics. Handheld DVMs have significant limitations for resistance measurement. Just short the meter’s own leads and tell me what you read It is far more sensitive in voltage measurement and that can be very useful when actual load currents are present in the wire…and it fits in your pocket
And speaking of fitting in your pocket @TwinTurbo, I just ran accross a digital oscilloscope that fits in your pocket for $150. Like all things digital those things have gotten smaller and much cheaper over the years. I paid $2,000+ for a 4 channel Techrtonix scope many years ago. And for anyone patient enough and determined enough to learn to use one those things can identify some outrageously difficult to find gremlins in automobile electronics. The scope that I had seems to have been long discontinued but there are quite a few offerings from many competitors these days. And without one a shop might need to limit their work to classic models.How can anyone diagnose a driveability problem without watching simultaneous traces from TPS, MAF, MAP, ign primary, etc while also monitoring the fuel pressure?
An ohmmeter does have a place in troubleshooting automotive electrical systems, mainly in checking for continuity of wires and connections. But for measuring resistance of a battery cable, I agree, use the voltage setting and measure voltage drops.
Ase: no, the resistance is fairly independent on current (load), unless the current is high enough that the wire starts to get hot.
Yes, that’s the point. I can’t think of a situation automotive-wise where testing continuity of wires is useful or leads to finding the trouble. Often, it’s a false positive and just wastes time. Unless you’re testing resistance with the circuit loaded, there’s no point.
For example, I watched a guy test and confirm the continuity of a wire from under the dash to the tail lights of a car. There was a connector under the rear bumper that was full of green corrosion. It would show very low resistance with an ohm meter but just the current from a brake lamp was enough to make the connection go open circuit.
