This causes the current in the ground loop to be constantly changing. So now a voltage level (ground) that all of the individual circuits inside a module depend upon is now actually a source of induced or injected noise. This is why it is so important for all grounds to be clean and tight.
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Then there are the more exotic sources of noise. Induction sources like EMI (electro-magnetic interference) and RFI (Radio Frequency Interference). I recall helping someone on the ATSG tech line where the customer would bring the vehicle back two or three times a week with a code for an input speed sensor fault. This went on for weeks. The sensor, computer and even the o for that sensor were replaced. The technician finally did a better interview with the customer and found out that the problem always occurred at the same place on his way to work.
The technician arranged to have use of the vehicle over a weekend. He hooked up every scope, meter and scanner he had to anything that might be causing the problem and drove the vehicle on the same course that the customer took to work. Low and behold the problem occurred at the same spot every time. The technician pulled over and recorded his various readings a few times, then he looked up to view the scenery and discovered that the road went right down the middle of a huge electrical generating windmill farm. He called the following Monday and I gave him a part number for some hard core MILSPEC microphone cable with 100 percent shielding to replace the speed sensor cable. Also made sure that he only grounded the shield in one place and made the leads to the sensor and computer as short as possible. Problem solved.
There are plenty of less exotic and unusual EMI issues. Often a spark plug wire or a wiring harness will not get properly mounted after some work and will get too close to each other. This can cause a small voltage spike in the signal wire every time that plug fires. Sometimes two signal wires will come into contact with each other injecting each other’s signals into the others.
Then there is RFI. This almost never happens in today’s modern electronics. This is when a radio signal gets induced into a signal wire. For this to happen the signal in the wire has to be extremely low (or unusually weak), and the radio signal must be very strong. The thing is that very is a relative term. Would it be possible for your laptops WiFi transmitter to induce a signal into say a speed sensor signal. Well, it is possible, just highly unlikely.
Another RFI scenario involves harmonics and resonance. The wavelength of a radio wave is the distance that the wave travels in one cycle. Mathematically it is the speed of light divided by the frequency (?=?/f). If the effective length of a wire just so happens to be the exact wavelength of the radio wave it makes it resonant and some crazy things can happen. This is the reason why use of radios is not permitted in blasting areas and singers with perfect pitch can shatter a crystal glass.
The defense against EMI and RFI is all about positioning and shielding, and shielding is all about grounding.
One last topic on fixing noise issues is filtering. Filtering is electronically removing the noise while leaving the original signal intact. A filter is a circuit that is added to the signal wire (Figures 5, 6, 7 and 8). There are an unlimited number of filters in the world. Some are specifically designed to solve a particular problem, and some are more general in nature.
The bottom line is if you are having generally wild and crazy things happening, use a scope or AC meter to check for noise. The usual causes for noise are failed or overloaded parts, or bad connections, especially grounds. I know I am not the first person that has harped on grounds as the most popular electrical problem in automotive electronics today. Here is yet another reason.