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 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 the main 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 (including me) use both words interchangeably, as the ground side of the circuit leads back to the negative battery post.
How Does a Ground Circuit Work?
Voltage leaves the positive side of the vehicle’s battery, and it has two goals:
- Reach the negative side of the battery
- Reach negative by the shortest path possible
We use copper 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 the power source to the load.
The ground side refers to the circuit just after the load to ground.
Common Faulty Ground Circuit Symptoms
Symptoms of a faulty ground circuit vary widely depending on the load type – light, motor, controller (computer), 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. Several issues may cause high resistance, 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 pin-pointing 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. Go ahead and 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? When fault finding, 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 reveal 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 .2v could only be caused by a dirty or loose battery terminal, as there are no other components between the two test probes.
Worth noting a volt drop test only works when the circuit is live, meaning the circuit being tested is in operation; in our case above, we cranked over the engine, so voltage flows through the cables.
Repeating this test on short sections of the circuit helps us isolate the exact point of failure. I did say it was simple!
The same test may be run on the positive side of the circuit, and the same principles apply; however, as said, our failure threshold value is different; on the positive side of the circuit, the voltage difference should not exceed .3v.
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 .3v would indicate an issue with the terminal, as that’s the only component between our two test probes.
As with the ground side volt drop test, checking small sections allows us to pinpoint the fault.
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 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.
You may also find the following posts helpful:
- About the Author
- Latest Posts
John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.