From Aaron Turpen on Quora.com: The average modern car has about a mile of wiring, totaling about 40–50 pounds of copper and 30–50 pounds worth of computing components and controls.
Every one of those computers relies on a solid, dependable ground wire. No exceptions. Most of the ones running your engine and drivetrain can experience problems from as little as 0.005 ohms. I'll explain the math later, but that's all it takes, five thousandths of an ohm in your main ground cable. It can literally make your ECM sense things that aren't really there, or miss things that are there. It can be a gremlin you chase for months that only comes out at night. And the funny thing is, verifying the links between your block and chassis and battery is about the simplest operation a tech can perform. Literally five seconds with a voltmeter. Yet it may be the most overlooked favor you can possibly do your car.
There's only one "right" way to check the integrity of grounds, and it isn't an ohmmeter. Five thousandths of an ohm is mighty small, so it takes a mighty fine meter to measure it. Even then, it doesn't tell you the quality of ground your system requires. That depends on how much current is being drawn, so the only good way to test it is dynamically, with the engine running and juice flowing in the circuit you want to test. That means a voltmeter and not ohms. The reason is simple, enshrined in Ohm's Law. When a resistance exists (bad) and current is flowing, voltage will rise across the resistance. So if there's resistance in your main cable from block to battery, you'll read a difference between the block and battery post that corresponds to how bad the resistance is. The bigger the resistance, the higher the difference. In the trade we call this "voltage drop" or "Vdrop". Even a giant chunk of metal like an engine block will have a microscopic amount of resistance, so you'll always see a small voltage between the post and block no matter how good your cabling is. But "small" is the active term. Fifty millivolts (0.05V) is considered the safe limit for most vehicles. Above that, items that are particularly sensitive to ground quality, like spark coils, will begin to fail. Poor grounding is where the infamous "warts" on spark coils usually come from, and also the myth among technicians that coils "fail in groups". They don't. Every case of "coil-eating vehicle" I've ever seen had an underlying ground issue. Often it's fixed by accident when the car no-starts at a later date, and the victim never realizes the conditions were related. The car just mysteriously loses its coil-appetite.
Back to 50 millivolts. What happens inside the ECM when a ground is "soft" is that it will see one value from a sensor when less current is flowing and a different value when extra current is flowing, such as when a coil charges. This is because the rise in ground voltage directly subtracts from the system voltage. It happens on a millisecond basis across the electrical system, so it's easy to appreciate why it can make an ECM or TCM "see things". Only a good oscilloscope is even fast enough to catch it, but if your ECM could talk, it would tell you its glasses were foggy. And it takes a miniscule amount of resistance to cause it. Applying Ohm's Law to a car with an engine drawing 10 amps, 0.05V / 10A = 0.005 ohms (yep, that figure again). That's why a measly 5 milliohms can give your ECM a giant, invisible headache.
Once you learn to use it, Vdrop can even tell you exactly where a bad ground is, not merely that you have one. Say your engine shows an unhealthy 100 mV drop. Put one meter lead on the battery post, one on the battery lug. Reads 1 mV. Nope, that's not it. Go from the lug to the chassis lug. 3 mV, nope. Go from the chassis lug to the engine ground bolt. 4 mV, nope. Go from the ground bolt to the block. Ahah, 80 mV. That sucker is corroded underneath the bolt, remove and clean it. It really is that easy.
Here's why you should do it: Once upon a time there was a beautiful old '07 Maxima, like a sparkling gem inside and out. Metallic maroon, gorgeous color on that MY. The woman never passed a car wash. Trouble is, she did pass a bad motor mount. Gradually, the rocking motor broke the ground cable strand by strand where it attached to the tranny. Pretty soon it was hanging by a thread and the engine was getting most of its ground through the 18-gauge auxiliary ground strap. Not much on cars, the woman kept driving it until three warty coils turned it into a no-start. By then, the bad coils had also melted both cats. That beautiful old ride went to the scrapyard, from a $30 ground cable.
Maybe you'll never see anything as extreme as that, but here's another true story. This one is a Murano, and it seems to be eating crankshaft sensors. The original plus two replacements, and the customer is hot. "Get to the bottom of this," the manager tells us. Because the customer is screaming, we take the time to check everything -- and what do you know, 63 mV between the block and the skinny ground wire to the sensor. The sensor draws about 12 milliamps, so Ohm says that wire has about 5 ohms resistance. Not much, but Hall Effect sensors are ground-sensitive just like coils are. The car was out of warranty, so we spliced a better ground direct to the trans housing. The customer never had to scream again.
I could tell you dozens more war stories, but you get the point. You don't have to put up with invisible ground gremlins. It only takes the smallest effort to make them visible.