Voltmeter - what am I missing?

[Dyer retired]

The problem is that when you read the voltage on the wire after the key, you are getting a voltage drop due to the current flowing through the key wire. (Aux lights, dash lights, relays, etc.)

If you really want to only be able to read the voltage when the tractor is on, connect your sense wire to one of the relays. Due to its proximity to the battery, you shouldn't get too much of a voltage drop.

I'm assuming this is an analog meter? An analog meter has a much lower impedance vs. a digital meter, since you are using the battery to power the analog meter.

-Steve

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

 

[SPYDERLK]

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

Check out this link. Lots of good info in it relating to finding where voltage is being lost in a current carrying circuit. What patrick g described a few posts back.
http://www.tractorbynet.com/forums/parts-repairs/102900-starter-circuit-problem.html
larry

 

[Dyer retired]

Check out this link. Lots of good info in it relating to finding where voltage is being lost in a current carrying circuit. What patrick g described a few posts back.
http://www.tractorbynet.com/forums/parts-repairs/102900-starter-circuit-problem.html
larry

Larry,

Thanks. I read through the thread you directed me to and find all this stuff interesting. My wife showed up yesterday with a book, "Electronics for Dummies," so I'll be reading through that and seeing if it all eventually clicks for me. I'm going through the test procedures that Pat suggested, but am finding that I need to be a contortionist to do some of these. I still have my mmm deck on and trying to get at some of these contact points requires awkward positioning....none of them comfortable, so it's going to be slow. I'll make notes and report how it's going beginning with the battery post and clamps comparisons....those are at least easy to get at. I appreciate everyone's patience with my severe lack of circuit knowledge. John

 

[Dennisfly]

One thought. If you now have the meter wired on its own switch and forget to turn it off when you shut the tractor down, it will discharge the battery over time.

 

[Treemonkey1000]

That would take a long time to discharge. But it is still a drain on the battery. About the same as most car and truck batteries get keeping the clocks and alarms going while the vehicle is off. Keep a trickle charger going if the tractor is going to be stored for a extended period.
Just to toss in another point the accuracy of the DMM is probably a few percentage points better than any after market voltage indicator that mounts on your vehicle. That would account for differences in readings as well.
One test that may have been mentioned is to take your dmm with good test leads. Probe directly on the battery post. Then probe directly on the battery clamp. You really shouldn't see any voltage difference. If you do then the connections need to be cleaned up. This is especially true if you crank over the starter doing the same test with the DMM. Check both +/- post on the battery.

 

[RWolf]

IMO best way to wire a voltmeter in your tractor is to find which terminal of your ignition switch is at 12v when in the run position. use a crimp wire splice (sold at auto parts store) and your other end to frame ground close to the meter. Much simplier, don't have to worry about switches etc. and the wire from the alternator/battery is much thicker and you won't experience any appreciable voltage drop.

Good luck.

 

[SPYDERLK]

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

Dyer, when you get a chance please look at the specs of that gauge and see what the current is that it draws from the circuit. I know it will be too low to worry about it affecting the battery [even if on all the time] but it will draw MUCH more than a DMM since the DMM carries its own power source and therefore just samples the circuit, drawing about a millionth of an amp. Your gauge is powered from the battery and probably draws thousands of times more. Make sure your connections are good. With it drawing [relatively] so much current you could actually see a voltage drop across a bad connection with your MUCH more sensitive DMM. This drop, summed with all others in the circuit powering the gauge would cause an error in its reading. Except in extremely unusual marginal connection quality situations this error would be too small to see with the gauge resolution of 0.1V. However, where the batt voltage was 14.350, a V loss in your gauge circuit of 0.001V would cause your gauge to read 14.3 instead of 14.4 because of the round off. No serious implication here-just FYI
Larry

 

[Iplayfarmer]

... I'm going through the test procedures that Pat suggested, but am finding that I need to be a contortionist to do some of these. I still have my mmm deck on and trying to get at some of these contact points requires awkward positioning....none of them comfortable, so it's going to be slow.

I completely empathize with what you're going through. It's not easy to reach some of these connections.

I don't know what kind of meter you're using, but I really improved the usefulness of mine by cutting off the "needle" probe on the black wire (negative) and attaching an aligator clip in its place. It eliminated the need for an extra hand. I have an extra set of leads from an old meter that I can still use if I need the needle type.

 

[Soundguy]

I agree with skyco.. the aux lamp wire you are using is undoubtedly run off either the igntion switch or a relay, that is being fed from , probably, a fuse panel.. and that fuse panel. which runs all the electrical loads onthe tractor except the starter.. is 'probably' powered from a single 10ga wire...

Thus with the low wire gauge, and all the contacts inline.. you are indeed getting a voltage drop. If you want to get by that.. buy a cheap 12v foglamp relay from an auto store.. should be less than 5 bucks... have your switched hot wire power the relay coil, and ground the other side of the relay coil. For the contacts ont he relay coil.. hook one to battery, and the other to your voltmeter.. thus when the keyed power wire comes on, it energizes the relay.. which then feeds 'bat' power to the voltmeter... the relay itself draws very litle so it should not overtax your fuse panel.

soundguy

post. I then turned the key on and got a reading of 12.0 volts with the engine off and 14.1 volts with the engine running above idle. At one point, and at a little higher rpm's, it displayed 14.2 volts. When I turn the headlights on, the reading dropped to 13.2 volts on the newly mounted meter. I ran my multimeter across the battery terminals again while this reading was displayed and got a reading of 14.32 volts on the multimeter.

I know from other threads that I should be reading about 14.5 volts and feel confident that I'm getting that from the testing directly across the battery terminals with the new gauge and with the multimeter. Can someone help me, or explain to me how to correct this, or am I missing something? It seems that the new gauge should be reading accurately and did so before I ran it through the key?

Plan B would be to run the leads to the battery directly, but through a switch that I can kill the power to it when the engine is off. Any help is appreciated.

Dyer, retired

 

[patrick_g]

I completely empathize with what you're going through. It's not easy to reach some of these connections.

I don't know what kind of meter you're using, but I really improved the usefulness of mine by cutting off the "needle" probe on the black wire (negative) and attaching an aligator clip in its place. It eliminated the need for an extra hand. I have an extra set of leads from an old meter that I can still use if I need the needle type.

There are probe kits that have special alligator clips that slip over the needle tips provided in the kit. This gives you lots of options on how to use your probes. The ones I have seen and or used also have slip over tips that are like spade lugs, and bananas too.

To Larry (the OP)

The current draw of your panel meter is NOT MUCH but over a long enough period of time it will pull the battery down some (aided by self discharge.) The battery maint, i.e. float chargers are ideal for offsetting this and will keep your battery charged without overcharging and rapid electrolyte loss. I have used several different brands and ALL WORKED FINE. HF has them on sale every so often under $10.

They aren't intended to charge a flat battery but do keep a decently charged battery topped off, even with alarms and such constantly trying to drain the battery. Even with two large (100 AH) diesel starting batteries in my Dodge-Cummins truck, my extensive alarm system would pull the battery down if the truck wasn't used frequently which it wasn't. My $7 HF wall wart type maintainer has been keeping it topped off for years now. Always ready.

And Larry, in a series circuit the sum of all the voltage drops is exactly equal to the applied voltage. So as you move along a circuit if you find ANY voltage drops and you add them all up, including the voltage across the panel meter's power connections the total should be the same as the battery voltage.

I again assert there is something wrong in there somewhere. The current consumption of the panel meter is so low that it should not significantly load wires that were intended to run a relatively heavy current using light.

When you measure the battery voltage directly at the battery posts (not the cable clamps) and again at the power connections of the panel meter, the difference in volts will have to appear somewhere along that series circuit. It could even be a poor ground connection for the panel meter.

Also, always remember , "We are all ignorant, just about different things." Ignorance is curable but stupidity is permanent. Luckily you were ignorant and not stupid and are well on your way to not being so ignorant about electrical stuff.

Beat of luck to you,

Pat