I love fault finding; it’s sort of like a who-done-it. When working on auto electrical systems, it helps enormously if you have some understanding of what’s going on. It’s not practical to know what goes on inside all the vehicle’s control modules, but it helps to have the basics nailed down. I’m John; I’ve been an auto mechanic for the past 25 years. In this post, I’ll share the basics of car electrics.
While ground and negative are technically not the same thing, they do, however, refer to the same polarity, that being negative (cathode), symbolized by a minus sign (-) on a vehicle battery casing.
In this post, you’ll learn why ground and negative aren’t technically the same thing; even though they are used in terms of fault finding or wiring, it amounts to the same polarity.
- What’s ground?
- What’s negative?
- How does a ground circuit work?
- Common faulty ground circuit symptoms
- Common ground circuit faults
- Ground circuit testing
What is Ground?
The ground is the path voltage takes back to the negative side of the battery. The term ground is used as the vehicle’s metal chassis is effectively part of the ground circuit.
What is Negative?
Negative refers to the negative side of the vehicle’s battery. While it’s technically incorrect to refer to the ground side of the circuit as negative, most folks use both words interchangeably.
How Does a Ground Circuit Work?
Voltage leaves the positive side of the vehicle’s battery, and it has two goals – to reach the negative side of the battery by the shortest path possible.
As you know, we use wiring to channel voltage to the load (load refers to whatever component we want to power, for example, a light or motor, etc.) where we force the voltage to do some work before whatever remaining voltage goes back to the battery negative via the ground circuit.
Circuits are broken into two parts – the power side and the ground side. The power side refers to the circuit from power to the load, and the ground side refers to the circuit just after the load ad and all the back to ground.
Common Faulty Ground Circuit Symptoms
Symptoms of a faulty ground circuit vary widely depending on the load type – light, motor, controller, etc. Broadly, ground issues relate to high resistance. Resistance is exactly what it sounds like, resistance to the flow of voltage.
Here’s a flavor of the type of symptoms a bad ground generates:
|Light||Dim or flickering|
|Motor||Slow or no movement|
|Controller||Fault codes, temperamental performance|
Common Ground Circuit Faults
In general, issues with ground circuits mean there is high resistance in the circuit. High resistance is, as you know, basically a blockage in voltage flow back to the ground. High resistance may be caused by several issues, typically they include:
- Loose ground fasteners
- Corroded ground contacts
- Corroded internal ground wiring
- Broken internal ground wiring
- Loose negative battery terminal
- Dirty/corroded negative battery terminal
The fix for these types of issues is obvious, but finding exactly where the point of high resistance can be challenging unless, of course, you know how to volt drop test a ground circuit. And that’s what you’ll learn next.
Ground Circuit Testing
Before testing an electrical circuit, it’s best to check all the easy stuff first, much of which only requires a visual check. Check your power supply and wiring block connectors are secure and in good shape. Fault finding an electrical component that stops working altogether is always easier than fault finding an intermittent fault.
So, where to start? It’s helpful to think of your circuit as two halves, the power side and the ground side. This type of strategy allows us to test each side of the circuit and quickly reveals which side of the circuit is at fault.
As we suspect, we have a circuit with high resistance; you’d think checking resistance with a DVOM (Digital Volt Ohm Meter) Voltmeter would be a good strategy. A resistance check would work fine if our ground circuit were broken clean, but when it’s an intermittent issue, it’s best to test on a live circuit; doing so stresses the circuit and produces a more accurate reading of the situation.
Volt Drop Testing
The concept of the volt drop test is simple – it checks the voltage difference between two points. The difference in a healthy circuit should not be excessive. A large difference in voltage means there’s excessive resistance.
The power and ground sides of the circuit have different voltage thresholds, and we’ll cover that next.
A reading above .2v means there is a fault in the ground circuit somewhere between your two DVOM test probes.
In the case of our simple test picture, a high resistance reading above .2 would likely be caused by a dirty or loose terminal.
Repeating this test on shorter sections of the circuit helps isolate the exact point of failure.
The same test may be run on the positive side of the circuit, and the same principles apply; however, as said, our threshold is different; on the positive side of the circuit, the voltage difference should not exceed .5v.
As with the ground side test, testing smaller circuit sections helps isolate the fault.
That’s it; that’s the secret to volt drop testing!
Volt drop tests the power side of a circuit.
As per the infographic, a reading above .5v indicates an issue with the cable or the terminal, as that’s the test section between our two test probes.
When volt drop testing, the circuit must be live, meaning under load (light, motor, etc. turned on).
While ground and negative are technically not the same thing, they do, however, refer to the same polarity, that being negative (cathode), symbolized by a minus sign (-) on a vehicle battery.
Ground refers to the path voltage that takes back to the battery negative after the load, and negative refers to the identity of the negative post of the vehicle’s battery.
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