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.

I LOOOOOVE THIS!!!!

Good intel. I fought bad grounds on my old 240 for a long time without knowing it. Finally found the culprit and it was a different car after fixing it.
I'm now hyper-sensitive to the issue and replace ground straps fairly regularly, or even add redundant ones just to make sure I'm not going to have any problems.

Hello, just want to say thanks for the information. Looks like something so minut can cause chaos!
Anyway, i was referred here from this post made by me need-help-completely-lost-t631119.html#p6838626
So i took the car to the shop
I told them about your post, and they felt you were wrong. I asked them to check it, they insisted, that wasnt the problem. Changed whole fan assembly, checked trans fluid, and radiator fluid and boom, $640. (A/C blows colder now)
My problem still existed: brought it back the following week, then boom, they said it was the cluster that needed repair and to get another one .

Cluster replaced, all gages are working perfectly, with the exception i have to get someone to fix the mileage. Its showi ng 30k more miles.. Anyway, everything was good (i never drive over 10 miles in one setting) ...... up until July 2, 2022. Took a drive from SW Houston TX to LaGrange TX. By the time i was passing Sealy TX,(1hr away from home), it slowed down on me again. Car wouldnt go past 76mph! No shaking or any other physical issues. It just would not go past that.

Took foot off gas and hit the gas to rev it up, and it would not rev up. It may go from 2200rpm to 2300 then back to 2200. I just thought to myself, my problem is not fixed .
(20 mins later) Weirdest thing happened, im on the right lane doing about 76 in a 75 and boom , i hit rain.. Came down hard for like 10 mins then went away. Then next, the car was fine. I was able to do 80-85 mph again!!!

Sounds: only sound i heard was whining, which may very well mean the tranny needs to be flushed. And i will have it changed in a week or so.. I might google it to see if i can do it in my driveway with 2 jackstands. But that whine almost sounds like those electric cars you hear in the movies.(if u can imagine)

My question to you is, do you really think i still have a ground problem? If so, are there instructions somewhere that te u what to do? Which wires to check, and where the wires are? And what is the best tool to buy to check for home use. I dont mind giving it a try by myself, because this car repair shop is starting to be too high for me! I just know nothing about cars electrical diagram. I know how to install radios and speakers, but never touched an ECM or etc.

You might have more than one thing going on here, but the fact that it's more than one gauge wigging out simultaneously certainly could be a ground issue. Even the cheapest voltmeter from Harbor Freight or eBay will suffice to find a ground issue in the engine box. The wire you're concerned with is the big negative cable coming off the battery. On a gen4 Altima, it leads to a chassis connection underneath the battery tray, then to a lug on the transmission. It's all one cable but there are two lugs on it, one in the center for the chassis. To check it, follow the instructions in my original post. With the car running, measure from the negative terminal to the engine block and from the negative to anyplace on the chassis (I usually use one of the strut tower bolts). If either one reads high, then proceed measuring down the cable to zero in on the problem area. If they both read good, then your problem is somewhere else. That could still be bad ground wires to some individual component like the ECM, but the problem is most likely someplace in the engine box. The cluster on a gen4 gets its engine temperature info from the ECM via the CANbus, and the fuel gauge wire is connected to both the ECM and cluster. So it's unlikely the cluster or anything outside the engine box is the root of the problem.

However, it's entirely possible you have multiple issues going on. The cooling system behavior you originally described sounds like it might be a sticky thermostat or air in the system, and the ECM will definitely respond to wrong temp readings with wrong behavior. So will the TCM, and part of the over-temperature behavior of both is to limit throttle response and RPM. So at this point, if you don't find an obvious ground issue, I'd say it's important to get a code scan on the whole car and not just the couple of systems accessible to a cheap OBD scanner. That would include CVT-A/CVT-B, the tranny's record of overheated fluid which has to be accessed through work support and not a regular scan. So a trip to the dealer for a scan with Consult3+ or a scan with a very high-end shop scanner like a Snap-On Solus is the only way you're likely to get a whole picture of what's ailing your ride.

VStar650CL wrote: ↑

Mon Jul 04, 2022 9:13 am

You might have more than one thing going on here, but the fact that it's more than one gauge wigging out simultaneously certainly could be a ground issue. Even the cheapest voltmeter from Harbor Freight or eBay will suffice to find a ground issue in the engine box. The wire .......

@VStar650CL: Thanks.. I'll go to Oreilly this weekend to purchase a voltmeter.. And from there, i'll do the test. I'll keep you posted in a week as far as my progress. Thank You for responding.. I'm tired of spending unnecessary money.

Wow!

I found An Outstanding amount of helpful data here in these posts about Grounds, & Voltage Drop.

Excellent Posting Vstar!

Happy to help, and especially happy to help those who wish to learn.

wow what a great post!!. I'm enrolled in Mechanic Mindset online courses and they too delved into VDrop...I think you made it even clearer.

"I could tell you dozens more war stories" I'd actually like to hear more of them.

Thank you for this post. Is there a single reference where I can look for all of the ground straps on a 2005 G35 sedan? :-)

You're most welcome. Ground distribution for all N/I models can be found in the PG section of the FSM. Yours starts on PG-29 here:
https://www.nicoclub.com/service-manual ... 5%2Fpg.pdf

you're a saint Vstar!!

cfauvel wrote: ↑

Fri Jan 27, 2023 10:42 am

you're a saint Vstar!!

Nope, just a guy payin' it forward. Carry on....

That's a good read! Thank you for sharing

Excellent post.

some great knowldege