If the cable between the alternator was 0.3 ohm, calculate the voltage drop across the cable end to end when it is charging the battery at 30 amps . This is something my truck does all the time, it has a alternator charge rate meter right on the dash, and displays 30 amps charge rate when I first start the truck after it has been sitting for several days. So 30 amps isn’t an unusual current between the alternator and the battery.
voltage drop = current * ohms, that’s just ohm’s law, so from the alternator to the battery, with a 0.3 ohm resistance, let’s see …
voltage drop = 30 amps * 0.3 ohm = 9 volts.
A 9 volt loss between the alternator and the battery isn’t going to fly, right? There’s no way to charge up the battery with 9 volts being lost before it even gets to the battery.
The type of wire used in high current paths like the alternator to the battery and especially the battery to the starter motor – where the current is much higher than 30 amps – has a resistance on the order (ok, I’m making an educated guess here) of 0.001 ohms. The main source of resistance in the path between the battery and alternator/starter motor isn’t in the wire, but in the various connections along the way.
I think what you mean @db4690 is that your meter can’t accurately measure very low resistances. No disrespect to your meter or its manufacturer, even very expensive meters can’t do that. None of my meters can measure accurately at the 0.001 ohm range.
To measure really low resistances with the kind of meters to which most people have, it has to be done by measuring the voltage drop when current is flowing, in voltage mode, not measuring the resistance directly using the meter in resistance mode.
If anyone here likes to learn more about this, suggest to google “how to measure very low resistances”.
“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.”
Ever hear of temperature sensors . . . ?
Ever hear of NTC . . . ?
Seems to me measuring resistance DOES have its place in automotive applications
Measuring resistance does have some limited use in automotive applications, but in most cases I use an ohmmeter just to establish continuity. You would use it for measuring coil resistance for example. Anything less than 0.5 ohms requires a microohmmeter. Pricey little piece of equipment.
Sure, but temp sensors (or any sensor for that matter) doesn’t report in ohms, it reports in volts. So measuring the output while operation means measuring volts. I don’t care what the resistance is, I just need to know whether the voltage output is correct.
You would use it for measuring coil resistance for example.
Maybe, but why would I? If a coil (or anything) has proper voltage and proper grounding signal it will either work properly or not work properly. That’s all I need to know.
OK, I take that back. I can think of one example. On the GM Multec injectors of 25 years ago it was common to have the injector wiring shorted internally. We used to check injector resistance one by one to find the bad ones. But it’s been years since I’ve seen one of those.
Temp sensors? Sure. They’re variable resistors that report a voltage. You can test them in a pot of water for resistance if you like, or you can measure their voltage in circuit.
Measuring voltage is more reliable than trying to measure resistance. Measuring resistance is using a meter’s 9 volt battery to try to replicate the circuit, and it rarely relates.
And no, I don’t know what NTC is. I do know my way around electricity pretty well though.
Yeah, I use my DVM for measuring resistance all the time in diagnosing faults in my vehicles. My only point was it is not the right tool for assessing micro-ohm wire resistance. Seen plenty of problem connections that looked reasonably fine using the ohmmeter but failed under load…I’d rather do a quick measurement of the resistance of a temp switch/sensor with it unpowered than to try to get in there and read voltage under use for example. Heck, just verified a marginal HSI in my FIL furnace by measuring its resistance cold…
Some devices, like thermistors for instance, can have what is called a NTC. That stands for negative temperature coefficient and that simply means as temperature increases the resistance of the device decreases. With a lot of things, their resistance increases with an increase in temperature. Coolant temperature sensors can be made to work either way. Using a negative temperature curve or a positive curve.
Measuring the voltage from something like a coolant temp sensor (ECT) has a lot of merit in the sense that it is the voltage that the engine computer measures. By measuring the voltage you are measuring everything that could affect the voltage, stuff in the circuit besides just the ECT. But the voltage method is not as easy to do. You can measure the ECT resistance by just disconnecting the connector and measuring the resistance of the ECT all by itself. No need to even have the ignition on, let alone the engine running, or dealing with back-probing while the connector remains on.
