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Username Post: Electricity baffels me.        (Topic#344818)
ltlvt 
Senior Member
Posts: 1743

Age: 65
Loc: Wichita Falls Texas USA
Reg: 09-17-00
05-19-17 10:45 AM - Post#2691902    

Okay here is the deal. I was checking a friends truck for and electrical drain on the battery. I disconnected the ground cable from the battery. I tested between the negative post of the battery and the ground cable with a DVM (digital volt meter) and it read 12 volts. But when I connected a test light to the negative post and the ground cable it did not light the light. But when I turned on the head lights the test light would lite up. There is no drain on the battery but could someone explain to me why the meter reads voltage but will not light the test light unless something is turned on? This is a 72 C-10 so it has no ECM.

Runs like a Scalded Dog!!


Edited by ltlvt on 05-19-17 10:47 AM. Reason for edit: No reason given.

 
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JeremyB 
Senior Member
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JeremyB
Reg: 10-31-01
05-19-17 11:09 AM - Post#2691905    
    In response to ltlvt

You should have been checking amps between the negative post and the cable. DVM's can give misleading results because they place no load on the circuit where a test light does. Since the DVM showed a voltage difference there must be a very small drain on the battery.

I did about the same last year helping my uncle with his camper with slide outs that didn't work. My meter showed power at the switch but his test light wouldn't light. There was a loose wire nut holding a bunch of negative wires together at the battery, once tightened down, the slide outs worked



 
Vaughn 
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Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
05-19-17 01:07 PM - Post#2691922    
    In response to JeremyB

Test lights don't normally light up until there is a significant amperage draw across the terminals of the light. It is much more accurate to measure the amperage draw using the multimeter. This is why a test light has only a limited ability to sense a problem, a multimeter is a much better tool to use than a test light.

If you don't have the capacity to measure more than 50 milliamps, use a resistor across the ground cable and the battery terminal, then measure the amount of voltage drop across the resistor. Divide the voltage read by the resistance you use, and you will have the amperage of the current drain out of the battery.

If you have a short, you want to measure the current draw, and then pull fuses one at a time while watching the current draw. If the current drain drops as you pull a specific fuse, there is a drain in that circuit.

The two things that won't register a change in current draw when you pull fuses is the starter and the alternator. You will have to disconnect those from the wiring harness while watching the current draw to identify whether they are the source of the current drain.



 
ltlvt 
Senior Member
Posts: 1743

Age: 65
Loc: Wichita Falls Texas USA
Reg: 09-17-00
05-19-17 03:24 PM - Post#2691930    
    In response to Vaughn

Does this mean the battery is going to be dead when I get ready to start the truck ?
I let it set over night and it fired right up.

Runs like a Scalded Dog!!


 
stumppuller 
"6th Year" Gold Supporting Member
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stumppuller
Loc: Canada
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05-19-17 05:55 PM - Post#2691954    
    In response to ltlvt

  • ltlvt Said:
I tested between the negative post of the battery and the ground cable with a DVM (digital volt meter) and it read 12 volts.



The reason the meter saw 12 volts is because one side is attached to the battery negative, and although the other is connected to the trucks negative lead, it is seeing the positive side of the battery "through" a components wiring, circuitry, etc. Being a 72, I'd guess the radio.

The test light, as stated above, lights when there is sufficient current flowing.

Make sense?

-91 Sierra C2500, now K2500
-81 Chev K20


 
ltlvt 
Senior Member
Posts: 1743

Age: 65
Loc: Wichita Falls Texas USA
Reg: 09-17-00
05-19-17 07:51 PM - Post#2691973    
    In response to stumppuller

Stump Puller could you please dumb it down so i understand it. I am not verry literate when it comes to electrons. Thanks

Runs like a Scalded Dog!!


 
Vaughn 
"15th Year" Gold Supporting Member
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Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
05-19-17 08:52 PM - Post#2691980    
    In response to ltlvt

It will only drain the battery if you see a current draw between the ground cable and the battery negative with everything off.

Just because you could start it the next day doesn't mean that there is no current drain. You need to measure it with an ammeter to insure that there isn't a current drain with everything off.

If there is no current drain between the battery cable and the battery (again, with everything off) that you can measure, then there is nothing wrong with the truck.



 
ltlvt 
Senior Member
Posts: 1743

Age: 65
Loc: Wichita Falls Texas USA
Reg: 09-17-00
05-19-17 10:05 PM - Post#2691989    
    In response to Vaughn

Okay Thanks Vaughn. I have 3 different meters . I will see if i can take an amp reading. They have hacked the wiring. It no longer has the old seperate voltage regulator as did the 72's . As soon as the weather breaks i will take another look at it. I put a new battery on it a few days ago but haven't started it in at least 2 days.


Runs like a Scalded Dog!!


 
bowtie44s 
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05-20-17 06:39 AM - Post#2692015    
    In response to ltlvt

Set your multimeter to amps. You may have to move the red lead to a different spot. Disconnect the battery and put one lead on the negative battery post and the other on the cable. It should be under 50 miliamps or .050 amps. I recently had a 94ma draw and it would drain the battery in a week. I Do NOT turn the headlights on when you do this. It could burn up your multimeter.

Jeff

'88 Chevy K3500, aluminum head roller cam 511inĀ³ stroker 10.5:1 compression, 96 NV 4500, 94-98 grille, 305/70-16 (33x12) BF Goodrich KM2s, 91 cluster swap


 
stumppuller 
"6th Year" Gold Supporting Member
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stumppuller
Loc: Canada
Reg: 11-01-04
05-20-17 01:40 PM - Post#2692053    
    In response to ltlvt

If you look at the picture, with a voltmeter connected inline like you had, the filament in the light bulb doesn't light up, but rather acts as a wire, allowing the voltmeter to "see" the positive side of the battery through it and measure 12volts. Same is true with electronics such as the radio which has a battery "hot" connection.

If an ammeter is placed in line, it completes the circuit and allows the current to flow, which turns the light bulb on.

It is sometimes easier to understand electricity by comparing it to a water hose,

Volts = Pressure
Amps = Flow

I hope I'm not confusing you more. This is a basic way to look at it, but for some people it helps.



Attachment: img190.jpg (445.57 KB) 0 View(s)




-91 Sierra C2500, now K2500
-81 Chev K20


 
thx1138v2 
Forum Newbie
Posts: 95

Reg: 05-01-14
05-23-17 01:59 PM - Post#2692516    
    In response to ltlvt

This might help you understand the DC electricity used in autos.

Think of the electrical system as a plumbing system. The battery supplies electricity like the water tower supplies water.

Electrical voltage is like water pressure. You can have water pressure in your plumbing system without any water actually flowing. You have voltage in your electrical system without any electricity actually flowing.

Electrical current flowing is measured in amperes, or amps for short, and is like water flowing in the plumbing system. So amps are like gallons per minute. When all of your faucets are closed no water is flowing and the flow rate is zero. When all of the devices attached to the electrical system are off there is no electricity flowing and amps measured is zero.

Open a faucet or turn on an electrical device and water/electrical current is flowing and can be measured.

If there is a water leak, water is flowing and can be measured. Likewise, if there is a drain from an electrical device, current is flowing and can be measured in amps.

So volts is analogous to water pressure and amps is analogous to water flow rate.

There are some devices in an auto that use "standby power" so there may be a tiny amount of electrical flow even when everything is turned off. A clock radio is a good example. It takes a miniscule amount of electrical power to maintain the clock setting and radio channel selections. That's why when you disconnect the battery the clock loses its time setting and the radio loses its channel selections. This would be analogous to a very slowly dripping faucet.

Hope that helps.



 
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