Voltmeter - what am I missing?

[Dyer retired]

With a lot of help from Shaun (Blackmagicwoman) I bought and installed a 40 amp alternator on my BX 1800, fired up the rig and tested the voltage readings across the battery terminals with my multimeter. I got a reading of 12.6 volts with the tractor off and 14.46 volts with the engine running just above idle and 14.32 volts at idle. I also got a reading of 14.32 volts with the engine just above idle with the lights on. Very exciting, nothing caught fire, no explosions, I finally installed something without a semi-major injury, so I was happy!

I bought a digital voltmeter gauge and tested it across the battery terminals and got the same readings as I had with the multimeter at the various rpm's and with the lights on and off. I then mounted it to the tractor and hooked it up to a keyed power source, so it would only display when the engine was running. For a keyed power source, I used the wire for a rear light that came installed on the tractor, but hadn't been used. I pulled the wire back through to the battery compartment and used only the length I needed for the positive lead and I just hooked the negative terminal to the battery negative auxiliary 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

 

[Skyco]

You are reading the effect of the resistance through the switch and additional wiring, known as voltage drop. The more resistance in a circuit with a load, such as the lights, the more the voltage drops. If you want to know exactly what the voltage is at the battery you'll have to read it at the battery.

 

[Dyer retired]

You are reading the effect of the resistance through the switch and additional wiring, known as voltage drop. The more resistance in a circuit with a load, such as the lights, the more the voltage drops. If you want to know exactly what the voltage is at the battery you'll have to read it at the battery.

Skyco,

I guess it's on to Plan B. Thanks for the reply. John

 

[jinman]

Skyco is exactly right about the voltage drop, especially in a 12 VDC circuit. The small difference you would gain by running wires to the battery might be negated by the wire resistance of the wire to the battery. What I would do is to read your voltage with the drop when you know it is correct at the battery. Use that value as your baseline for the future. If it reads 14.2 or 13.8, you don't care as long as it always reads this amount. If it drops to 12 or less, that would indicate a problem. Don't make it harder on yourself than it needs to be.

 

[Skyco]

The volt meter should have a high input impedance pretty much making wires to the meter from the point of measurement insignificant. The drop, of course, occurs because of the loads on the circuit.
But I agree with jinman If you know what is normal wherever you hook it up then you still have a valid frame of reference...

 

[Dyer retired]

Skyco is exactly right about the voltage drop, especially in a 12 VDC circuit. The small difference you would gain by running wires to the battery might be negated by the wire resistance of the wire to the battery. What I would do is to read your voltage with the drop when you know it is correct at the battery. Use that value as your baseline for the future. If it reads 14.2 or 13.8, you don't care as long as it always reads this amount. If it drops to 12 or less, that would indicate a problem. Don't make it harder on yourself than it needs to be.

jinman, Skyco,

The instructions, and about every post I've read, called for running the gauge through a keyed ignition source, so I used that rear light wire that was factory installed, which fed out from the 20 amp fuse, along with 5 or 6 other accessories. I know very little about electrical circuits, but thought that getting readings that were 1 full volt lower than what was actually occurring at the battery was unacceptable....or at least undesirable. I ended up running the meter directly to the battery with a toggle on/off switch in the hot lead. My readings are now only slightly lower than the multimeter is showing me and I'm much happier with what I'm seeing now. I've run the gauge against the multimeter now with engine off, engine on, lights off, lights on, etc., and am getting consistent readings only slightly lower than the multimeter throughout the ranges. For instance, now the gauge with the lights off and engine running is reading 14.4 volts and the multimeter is showing 14.6 volts. The gauge drops to 14. 2 volts with the lights on and the multimeter shows 14.4 volts, so I'm more comfortable with what I'm seeing. Thanks for the help and advice guys. John

 

[patrick_g]

A full volt of drop at the "ignition" switch seems like a lot to me.

There are two issues. 1. all the advice telling you to note what is "normal" and go from there is basically sound except... 2. I'd be curious to find out why there is a voltage drop of a volt at the source you are tapping.

There is another way to use your new meter. Here is an example: you place one probe on the battery post itself and the other probe on the clamp on that post and note the voltage indicated on the meter when you turn on all the lights and all accessories you have. If the connection is clean and tight (good connection) there will be little or no voltage read (way less than a volt, A millivolt or less.)

Another example: you place one probe on the positive battery post and the other probe on the switch connection that is connected to the wire coming from the battery. Again you turn on all the "stuff" getting power via that switch. If you get more than a very small fraction of a volt then there is a "high resistance" somewhere between the battery and the input to the switch. If no problem detected at this stage put one probe on the input to the switch and then the other probe sequentially on each output of the switch (if there is more than one) with all the current users using current that are fed from each of the switch positions. IF more than a tiny voltage is found there is something wrong, a high resistance like a bad contact or similar.

Anyway the basic idea is that a run of wire or a connection or switch or relay contacts when passing their normal load of current should not have much of a voltage drop across them and if they do then there is something wrong. If a wire is undersized for the current it is trying to pass then you will get an excessively high voltage reading which for a piece of wire should be a tiny fraction of a volt at most.

There has to be a logical explanation for losing a full volt which in my opinion is excessive for your reported situation.

If your trouble shooting volt meter is connected directly across the contacts of the dash mounted meter do they read the same voltage? If not, it is fair to just note the difference, assume the dash meter is slightly off, and live with it as others suggested BUT I would not just ignore a lost volt and live with it. Something is NOT RIGHT.

Pat

 

[kenmac]

could it be that the factory installed some sort of resistor in the wire for the light wire ? I wouldn't think the wire would be long enough to have that much voltage drop. Provided the wire is in good condition

 

[BigE_]

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

 

[Dyer retired]

Pat,

I do plan to chase this around some more and will follow your suggestions. I did test the factory installed wire I used before pulling it back to the fuse location and got the same voltage drop readings using the multimeter as I did after connecting the gauge to it. That same voltage drop was read after the wire was cut to size. Again, I know very little about this stuff and imagined that the drop was somehow related to the length of wire prior to cutting it to what I needed. I ended up using less than 10 inches from where it was attached at the 20 amp fuse. I didn't count the various wires also coming through that 20 amp fuse, but there are at least 5.......all factory installed, nothing I've added. I did not have the gauge on a separate switch, but just hooked to that wire, which powered it up when I turned the ignition on and shut off when I shut the tractor down. I did test the gauge before installing it by touching the leads on the gauge to the battery posts directly and compared that to the reading on the multimeter when testing the same contact points. I got the same readings from both the gauge and multimeter with the engine off, the engine running, and with all the lights fired up, when testing directly from the battery, so I am satisfied that the gauge is accurate. I appreciate the help with this. John