SPYDERLK
Super Star Member
- Joined
- Feb 28, 2006
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- Tractor
- JD2010, Kubota3450,2550, Mahindra 7520 w FEL w Skid Steer QC w/Tilt Tatch, & BH, BX1500
Yes, the hi input impedance [resistance] of a V meter ensures that it has little effect on the circuit it is measuring if the circuit is much lower in resistance than the meter. [This is always the situation in a tractor wiring harness unless the circuit is "broken" by a switch - or some other point that has developed "near infinite" resistance.] A voltmeter is used to sample across selected parts of a current carrying circuit to determine the V drop in that part. In so doing, the meter is put in parallel with the circuit element[part] of interest. This gives the electrons another path - some go thru the meter, effectively diverted around the part you are measuring. The meter uses this current to make its measurement. Since the meter ismuch higher in resistance than tractor circuit elements, the miniscule change that it truly makes is as if no change has occurred. Nevertheless, the change has increased the current flowing in the circuit. This is because the part, while being measured is replaced by a new circuit element comprised of itself and the meter in parallel.Right you are Rob.
What does this tell us? It tells us the resistance of the battery terminals that permit starting the engine or running the headlights is much much less than 0.1 Ohms. A decent meter has a resistance of several hundreds of thousands of Ohms and more likely millions of Ohms. Just for a simple case lets say the meter has a resistance of only a million Ohms so with a perfect battery connection the total resistance in the circuit (ignoring wire) is 1,000,000 ohms. With a super terrible (you can't start the tractor type battery connection) the total resistance in the meter circuit is 1,000,000.1 so just how much effect do you suppose that will have on the voltage reading? Too small to notice!
That is a reason you want to draw a substantial current through the suspect connections in order to measure a voltage drop.
In the case reported by the OP his corroded fuse to fuse block connections were quite high in resistance, high enough to effect the meter's operation and or reading. The resistance of the fuse connection was NOT just an Ohm or three but several (quite a lot actually), a very bad connection.
Those of you following this thread for credit... note: this material may be on the exam. For you auditors, enjoy.
Pat
Calling them Resistance 1 and Resistance 2, the new R value presented is R1R2/R1+R2. Note that this is always less than either constituent resistance. I think Pat had some alternate avenue of explanation in mind when he presented an addition of resistances - - perhaps V splits in a series circuit being proportional to resistances.
To move on: A Digital V meter is extremely sensitive. Even a cheap one will measure down to 0.1mV and as a result it can respond to very small currents flowing thru very small resistances. Since E=IR, for the meter mentioned the product IR has to exceed 0.00005 in order for you to detect it [a round up] on the meter. You are putting the meter across a circuit element having unknown conduction quality [resistance]. Your batt is applying a voltage to the whole circuit of which the element is part. This causes a current to flow thru the whole circuit. You need to have some ballpark clue as to the amount of current. How much current do you need flowing to sure to be able to get a meter reading on any tractor circuit element when using a meter having 0.0001 resolution? Literally, a tail light will do it to test a battery cable. No point in going quite that lo, but the point is you can. With an amp flowing a battery cable wire will give readings on the last digit. Factoring in [adding] the individual measurements taken from the cable to lugs and lugs to post, you may flick the second to last digit. Not good if you do--a volt lost per thousand amps is a bit more than you want to see. At this point tho, you have figures of merit on each junction. With this you can figure out how much heat will come from the junction during cranking:
As a realistic max ballpark in a tractor lets assume max cranking amps is 500A. We have a measured cable wire drop of 0.0001V @ 1A. At 500A The drop will be 500X or 0.05V. Power W= IE = .05x500= 25Watts is dissipated as heat in the cable wire. If this is a short cable this could be a problem [undersized], but if 2 or more ft long it is not because the heat is well spread out and wont amount to more than a few degree change. But what about the crimps? We dont want even a last digit "1" at the cable to adjacent crimp lug because all the heat from the IE is going to be concentrated there, causing greater V loss as it heats which in turn causes more heat. If the meter flicks between 0 and 1 in the ten thousandth place the crimp is suspect. If it reads a solid 1 it will give trouble. In this case a higher test current is a good solution -- turn on the headlights to make the test setup as sensitive as if you had suddenly substituted a meter with 0.00001 sensitivity. Handheld meters like that are readily available but cost agood bit more.
In the case of the OP, the gauge he is sampling the circuit which is also powered from the circut. This means it is drawing thousands of times more current than would a standalone handheld powered by its own battery. Enuf perhaps to influence the V it receives thru a bad fuse connection to be 0.1V or so lower even if the fuse connection was just 10 ohms or so.
larry