Voltmeter - what am I missing?

[Soundguy]

Pat.. my guess is there is about 30 different connectorsinline before power gets to that aux lamp wire.. and who knows.. this may be off a tap on an ignition switch.. and they aren't known to be robust.

A simple 5$ addition of a relay will 'solve' his problem of the voltage difference he is seeing, and not expecting.

soundguy

 

[Dyer retired]

Larry,

I looked at the installation instructions, the only paperwork that came with the gauge (a Cyberdyne brand,) and it doesn't have any information about how much current it draws. The display is very bright and I suspect that it would be a rare occasion that I forget to turn it off. I put the toggle switch close to the key and have been shutting the gauge toggle off before I shut the tractor down. My multimeter across the battery when the engine is running is reading 14.46 volts and my gauge is reading 14.3, jumping to 14.4 volts now after the engine has been running for awhile. I'm satisfied that I'm only getting a slight drop in the reading on the gauge, compared to the multimeter.


Pat,

I have been able to test the battery post and clamp connections as you suggested, one probe on the post and one probe on the clamp, and am getting triple zero readings. Same result on both posts, so I take that to mean that those clamps are clean, tight, and performing as they should? I like the alligator clips suggestion and will be getting a set that fit over the probes....these tests will become much easier once I have those. I looked more closely at the 20 amp fuse where I had posted that there were 5 or six wires fed into/out of that. Well, that estimated count from memory was faulty and the I'm finding 1 hot wire feeding into one female side of the blade fuse 20 amp box and 3 wires coming out the other female post, one of those wires is the wire that I had originally tapped for the gauge. Now for the latest.....I pulled the 20 amp blade fuse out, just to look and see what the cluster of wires looked like from the fuse side. I couldn't really tell much from that, so I plugged the fuse back in snug. I tested the battery posts, key and engine off, and got a reading on my dmm of 12.73 volts. I then decided to recheck the voltage drop on the wire that I initially used (before wiring direct) and got the one volt drop. I turned the key on and first checked the two battery posts again and got a 12.32 volt reading. I then touched the positive probe to the unused wire and got the same reading of 12.32 volts. What happened? The only thing that's happened is that I've taken the fuse out and plugged it back in? Dyer, retired

 

[CCWKen]

I'm really getting concerned about you guys. I'll be down right scared when you want to know if you have 20.1 or 20.6 psi in your tires.

 

[mjncad]

Steve,

It's a digital mulitmeter....if that's what you're asking.
If you're asking about the gauge, there is a picture attached to post #6, and it is also digital. Since my first post, I've wired the gauge directly to the battery with a toggle on/off switch and am gettng much more accurate readings...checking against the readings with the multimeter. I was hooked in after the key initially and got the full volt drop with that installation, so I went direct to the battery instead. John

John:

I've only scanned this thread, so if I'm repeating what someone else has written, my apologies.

That being said; the problem could be the gauge's accuracy tolerance. Just because it's digital, doesn't make it accurate. I had a digital LCD give away clock I received when the defunct company I worked for at the time had its 100-year anniversary. This clock would gain 30+ minutes each month and it was digital. It was useless and went in the trash. That was in the 80's before Made in China became the corporate in thing to do.

 

[Steve_Miller]

I might as well jump in here and give my two cents worth as well. When you hooked your meter to the accessory wire the meter is such a light load (probably a very high resistance) that even though you are now completing the path for that circuit, the load (ie. milliamps at best) is light enough that any contact resistance (ignition switch, relay contact resistance) is still significant because of the loading. If you do a simple experiment by adding a marker light (signal light, park light) or something in parrallel to the meter should help load the contacts in the circuit enough to get a more accurate voltage reading. An even simpler test may be to hook the meter to a loaded circuit (marker lamps)and see if you get any different readings. It may prove nothing or it may make all the difference in the world. As patrickg stated, the sum of all voltage drops equals the source. With light loading in your circuit, resistance can be your enemy. Every time you turn your key on you may get a slightly diffrent voltage as the resistance can change each time the contacts make. Sound simple? Better yet, does it make any sense?

Steve

 

[Soundguy]

The only thing that's happened is that I've taken the fuse out and plugged it back in? Dyer, retired

Dirty fuse holder connections?

soundguy

 

[RobJ]

I'm really getting concerned about you guys. I'll be down right scared when you want to know if you have 20.1 or 20.6 psi in your tires.

hehe, me to.

I always amp gauge, simpler, and tells you what the charge rate is.

 

[Dyer retired]

John:

I've only scanned this thread, so if I'm repeating what someone else has written, my apologies.

That being said; the problem could be the gauge's accuracy tolerance. Just because it's digital, doesn't make it accurate. I had a digital LCD give away clock I received when the defunct company I worked for at the time had its 100-year anniversary. This clock would gain 30+ minutes each month and it was digital. It was useless and went in the trash. That was in the 80's before Made in China became the corporate in thing to do.

I tested the gauge before I installed it by running the hot and ground leads directly to the battery posts and got the same reading as my multimeter in key off, key on, engine running at idle, increased rpm's, etc......so I think the gauge is accurate. At least accurate enough for me. I was concerned about the 1 volt drop when hooked to the spare wire for the rear light hookup. I've since wired it directly and am satisfied with the readings I'm getting, but am also using this opportunity to try and learn a little about this stuff. It interests me and I'm picking up a lot from the TBN help I'm getting. Dyer, retired

 

[Dyer retired]

Dirty fuse holder connections?

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

 

[Dyer retired]

I'm really getting concerned about you guys. I'll be down right scared when you want to know if you have 20.1 or 20.6 psi in your tires.

Ha! I'd agree that it seemed picky whining about 1 volt, but wondered what good a voltage gauge was that gave an inaccurate reading? Then, after some very helpful responses, I got nervous that there was a potential problem with the circuit I tried originally to tie into, but really just find it all interesting. I don't think I really needed a Voltage gauge, but it's fun to have, looks good, and will let me know if my new alternator upgrade is working down the road. John

 

[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