Voltmeter - what am I missing?

[Soundguy]

Soundguy,

Would that account for the voltage drop I was seeing? The drop doesn't seem to be there now though and that's the only thing I've done beyond testing. John

Absolutely.. dirty contacts = higher resistance.. resistors drop voltage.

ever had a car with a dirty battery cable not crank, then you wire brushed the post and ran some sand paper inthe cable and bolted it up and fired right up.. same deal... voltage drop across a bad connection.. sliding the blade fuse in and out scraped oxide off the fuse and holder contacts..

soundguy

 

[Dyer retired]

Absolutely.. dirty contacts = higher resistance.. resistors drop voltage.

ever had a car with a dirty battery cable not crank, then you wire brushed the post and ran some sand paper inthe cable and bolted it up and fired right up.. same deal... voltage drop across a bad connection.. sliding the blade fuse in and out scraped oxide off the fuse and holder contacts..

soundguy

Thanks for the response and, yes, that does make sense...the analogy with the car battery hit home, since it described every car I ever had in High School, ha! The Tractor is 5 years old and that is the first time the fuse has been removed. I've removed the rest of the fuses now and cleaned the blades. Thanks to everyone for the responses. John

 

[RobJ]

= and will let me know if my new alternator upgrade is working down the road. John

Actually the idiot light will tell you that. If it goes to ground the light will stay on.

 

[RobJ]

Absolutely.. dirty contacts = higher resistance.. resistors drop voltage.

ever had a car with a dirty battery cable not crank, then you wire brushed the post and ran some sand paper inthe cable and bolted it up and fired right up.. same deal

A little apples to oranges...500 amps is a little different than 100 mils..
With a dirty cable, you can get the full 12 volts, but not enough good contact to carry the starting load. Draw has a lot to do with it to.

 

[patrick_g]

A little apples to oranges...500 amps is a little different than 100 mils..
With a dirty cable, you can get the full 12 volts, but not enough good contact to carry the starting load. Draw has a lot to do with it to.

Right you are Rob.

Also, for the OP's continuing education:

A decent voltmeter has a very high impedance, way higher than the resistance of a corroded battery terminal in most cases. In a series DC circuit the voltage is distributed across all the resistances in the circuit in direct proportion to the percent of the total resistance each individual resistance comprises.

We talk about the resistance of corroded battery terminals in very general terms but how many Ohms is reasonable? Lets say your starter pulls 100 amps. If (and in this case a bad assumption) the combined resistance of the battery terminals was 1.0 Ohm then with 100 amps the voltage lost at the terminals would be 100 volts (see Ohms law.) But wait where did the extra volts come from? You don't get any extra volts and with a 12 volt battery to get 100 amps with a dead short in place of the starter you need a resistance of 0.12 ohms.

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

 

[Soundguy]

I'll kick in some more info that goes right along with pat's.. a real world application.

A common 2-post series ammeter.... know how it works? Most of the common cheap ones are a 'shunt-voltmeter'.. IE.. there is a shunt in the ammeter.. it's a very low resistance material.. but does have a set resistance that is exact enough that the meter is calabrated to it.. when current passes thru the shunt, there is a votlage drop..like with all resistors.. this voltage is what the gauge is displaying, and is calibrated to display it in amperage.. the more v-drop.. the more amperage is flowing..the more the needle moves etc. ( that's a bit over-simplified.. but is enough to give the non electronically minde dthe gist of how it operates.. so all you EE's out there.. put the sticks down.. )

soundguy

 

[patrick_g]

Hey Soundguy, I used to use a file to "tune" home made shunts made out of "sheet metal" (cut from soup cans.) A meter sold to read 0-1 amp can be "adjusted" to read 0-10 or 0-100 (briefly as the 100 amp shunt may get pretty hot pretty fast.)

Pat

 

[Soundguy]

I hear ya. I sometimes wonder about the 0-80 amp scale 2" meters i see for tractors.. and those anemic mounting blts.. and just know that if it was displaying 80 for any length of time, that the plastic case would come apart..

soundguy

 

[Treemonkey1000]

I hear ya. I sometimes wonder about the 0-80 amp scale 2" meters i see for tractors.. and those anemic mounting blts.. and just know that if it was displaying 80 for any length of time, that the plastic case would come apart..

soundguy

I work in a calibration lab where we test and certify current shunts along with every other electronic device in the world from DC to 100 Ghz. I have seen enough shunts come back from the users that pumped too much current in them.. Brass is pretty stable in resistance until it get's too hot. I don't think that the Tractor guages are worried about super duper accuracy just an indication. I do agree about cheesy mounting methods though.
Nice to see the wealth of knowledge out here on this forum to help out.

 

[Soundguy]

I agree.. most tractor applications won't see super high loads.. but I have seen some where an amperage gauge is used to gauge total flow from the alternator, vs a 0-center gauge showing net charge / discharge.. and in some cases.. with lamps and fans and solenoid controls back tot he rear implement.. I have seen sustained loads int he 60-80 amp range.. and i just wonder how long the abs housing on that little 2" meter would hold up.. or the little paper thin plastic grommets around the mounting studs will hold up to that heat before menting, and then making one sid eo fthe meter 'common' to the dash! ( Seen that too! no fun when there is an arc welder going behind the dash! )..

soundguy