Troubleshooting Car Electrical Issues: Understanding Grounding and Wattage

  • Thread starter mearvk
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In summary: Start at the battery negative terminal and work your way around to the clamp, to the frame, and then to the engine.
  • #1
mearvk
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1. I have a car that has some electrical/electronic weirdness; the basic issue is apparently parasitic draw of about 2.5amps @ 12v. When I connect the (problematic) component the chassis ground (connected to battery ground) what I observe is that the battery chassis jumps from 0v to something like 1.1v and I see the 2.5 amp draw. I connected the related chassis ground connection directly to battery ground and saw the amp draw go to 0.5 amps. This tells me there is some sort of grounding weirdness. My basic question is what kinds of things could/would cause chassis ground and battery ground voltage disparity? I imagine that a bad diode would let current go back into a circuit creating a potential loop where the voltage never got back to battery ground. Ideas?

2. Unrelated to the first question. V = I/R is Ohm's law but if you take a 12w fluorescent high efficiency bulb and run 120v into it the math says there's a 1200 ohm resistor sitting in the bulb given by 120v * (120v/1200ohms = 0.10 amps) = 120v*0.10 amps. Likewise you might have a 100w bulb that would have a smaller resistance. My question is does wattage take into account heat generated at the resistor(s) in the circuit or just I*V? If wattage doesn't define energy but power then what formula defines total energy consumed as opposed to power?

Forum looks great. Sorry about undescriptive title.
 
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  • #2
A lot of what you wrote does not make a lot of sense, i.e.
When I connect the ... component the chassis ground ... what I observe is...
Can you see how someone would have trouble seeing what you are describing? What did you do with the component and the chassis-ground? Presumably you connected it - but how? Where else is it connected? For that matter - what is the component?

I connected the related chassis ground connection directly to battery ground and saw the amp draw go to 0.5 amps.
Does this mean that an ammeter read 0.5A or that an amplifier drew 0.5A? Is the "related chassis ground" not the same thing as the "chassis ground" above? Must be because you said, before, that it was connected to the battery ground and now you are connecting it!

i.e. please post a clear description.
It just sounds like what you said - there is an electrical connection someplace you have not accounted for. I'm guessing you have not been careful to isolate the variables - perhaps you are unclear about what the different "ground"s represent?

Part 2.
V=IR and P=VI ... P=ΔE/ΔT

R is a constant for ohmic conductors only - the resistance of a bulb filament depends on it's temperature.

Light bulbs are usually rated with a power level in Watts as well as a voltage or a current level.
These are ratings not actual values. If you turned down the voltage, the bulb dissipates less power (it gets dimmer).

Relationship between power and energy: 1W = 1J/s
1kWhr = 1000J/s drawn for 3600s or 3600kJ

This help?
 
  • #3
I'm assuming you have a steel car not a fiberglass corvette or other exotic.

Old cars get corrosion underneath the ground lugs that connect chassis to a wire.
Especially when the last guy who had it off leaves out the star washer whose purpose is to dig into the metal for good contact.

Another problem is when somebody is tapping into wirres that he only THINKS he knows what they are...
A "ground wire" that's intended to carry say 1/10 amp from a gas gage will develop significant drop when somebody taps into it for a 50 amp boomer-amplifier. It'll probably melt and destroy nearby wires too. Don't even think about using the ECU Sensor Common, even though by ohmmeter it reads close to chassis.

Battery has only a positive and a negative terminal. Negative is usually connected to chassis and mis-named "Ground", 'cause the rubber tires separate chassis from "Ground". Pre 70's VW's are positive "ground".
Using your voltmeter determine where along your wire that voltage is appearing - start from center of battery negative terminal to the battery clamp, then to frame then to engine.

Many older Fords have a peculiarity in their negative battery wire -
one end goes to battery, other to engine block, AND -
there's a third terminal in the middle of the wire for connection to car frame. It looks like it's just a physical restraining clamp but it's an important electrical connection.
When somebody replaces that OEM cable with an aftermarket cable lacking the middle terminal,
the engine is now connected to car frame only through transmission bearings and the tiny voltage regulator remote sense wire which soon melts. That causes multiple troubles.

So take your voltmeter and figure out where your voltage drop appears. You can go from a screwhead to the metal immediately adjacent to check for corrosion underneath it - a pocket knife or ice pick is handy for scraping through grease and rust, clip your meter lead to it and use it for a test prod..

Good luck and have fun !

old jim
 
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1. What is the difference between voltage and current?

Voltage is the measure of electrical potential difference between two points in a circuit, while current is the measure of the flow of electricity through a circuit. Voltage is responsible for pushing the electrons through the circuit, while current is the rate at which the electrons flow.

2. How does resistance affect an electrical circuit?

Resistance is the measure of how difficult it is for current to flow through a circuit. The higher the resistance, the lower the current will be. This is because resistance causes a drop in voltage, which in turn decreases the flow of electrons through the circuit.

3. What is Ohm's law?

Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance between them. This means that if the voltage increases, the current will also increase, and if the resistance increases, the current will decrease.

4. How can I calculate power in an electrical circuit?

Power is the rate at which energy is transferred in a circuit, and it is measured in watts. To calculate power, you can use the formula P=VI, where P is power in watts, V is voltage in volts, and I is current in amperes. Alternatively, you can also use the formula P=I²R, where R is resistance in ohms.

5. What is the difference between AC and DC electricity?

AC (alternating current) electricity flows in both directions in a circuit, constantly changing its direction and magnitude. This is the type of electricity used in most homes and buildings. DC (direct current) electricity flows in only one direction and is commonly used in batteries and electronic devices.

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