automotive electrical grounding 101

[shogun]

The best and most widely used example when it comes to explaining the principles of electricity is plumbing. Ever been in the shower and someone flushes the toilet? It gets real hot real fast doesn't it? The reason for that also applies to grounding. When water flows through a pipe there is a pressure loss along the pipe caused by resistance, the smaller the pipe the larger the resistance to flow. This resistance to flow lowers the pressure at the showerhead from the original pressure at the main. As soon as the toilet is flushed, the flow in the pipe to both the toilet and the showerhead increases and more pressure loss results upsetting your hot and cold water balance because there is less pressure available to the cold water side of shower.

In electrical systems it works the same way. If multiple devices share the same electrical return path (for example the frame), the currents (flow) of all the devices add up on the way to the battery and there is less voltage (pressure) available for each, depending on the total current between the ground point and the battery.

Electrical flow is measured in Amperes (A) or Amps. 1/1000th of an Ampere is a milliamp (mA). In electrical formulas, the symbol for current is (I).

The electrical pressure (equivalent to water pressure in a plumbing system) is measured in Volts (formula symbol: V). Resistance is measured in Ohms (formula symbol: R) The bigger the resistance of a supply wire, the more voltage builds up along that wire (V = R x I). But because there is only a fixed supply voltage available from the battery, the resulting voltage available for the electrical device is less. Bigger wires have lower resistance and therefore lower losses.

Returning to our plumbing example above: If we would run a separate pipe from the water main to each water faucet in the house, there would be no scalding in the shower because the pressure losses from any faucet would only affect that faucet. Of course the other solution would be to run 3" pipes all through the house to minimize pressure losses. This of course would be much more expensive than running only pipes as necessary. It also only minimizes the shared loss effect, but does not eliminate it, like separate pipes would. The grounding systems on the market are the equivalent to that 3" pipe.

In wiring the electrical system in a car the best way is to run separate ground wires from each system to a common ground point. The common ground point should NOT be just a bolt where lugs from each device are stacked in top of each other. The contact patches between the lugs create their own resistance and ground problems. The very best way is to bolt a short piece of copper bar near the battery to the frame. Connect the copper bar to the battery with a heavy duty ground strap. Drill and tap the bar for each ground return and use Dielectric grease on the bolts and nuts. (Thanks Austin).

The heaviest current user in a car is the starter. It can take up to 800 Amps of current. So it should have its own ground strap directly to the battery. So should the alternator. Its wires carry the second most amount of current after the starter. Electronic fuel injection and ignition systems have their own caveats though. Ignition systems create very high current pulses for a very short time. This is especially true for Capacitive Discharge Systems. Those need their own ground wire to the common. EFI systems rely on different sensors in the car. Throttle position sensor, Intake and coolant air temp sensors, manifold pressure sensors and so on. These sensors typically have two or three wires. When they have two wires, one is typically the signal and the other ground. Three wire sensors need a 5V supply, signal and ground. A few sensors have a single wire (power) and use the sensor body mounted in the block as ground. Some two wire sensors have a power and a signal wire, again using sensor body as ground.

DO NOT connect the ground of these sensors to the common ground as described above. The EFI computer, as any electrical device can only see its own ground and references all measurements to that. The EFI computer also switches the injectors on and off. Injectors use relatively high currents, and these currents have to flow back to the battery through the EFI computer's ground wire. This causes a voltage drop on that ground wire. Were the sensors grounded to the common ground as described above, the ECU would see only the sensor voltage minus the voltage drop of its ground. Instead ground the sensors directly at the EFI computer to its ground. Sensors only take a few mA of current anyway, so the additional drop on the EFI ground caused by them is irrelevant.

Another issue arises with sensors that create a very small signal, like thermocouples for EGT and cylinder head temperature measurements. In an engine compartment a lot of current pulses from ignition and injection systems flow around. Any electrical current also creates a magnetic field. The two wires from the sensor (signal and ground) create a loop, which acts as antenna to pick up these magnetic fields. To avoid that, either use shielded wire or simply twist the wires together. Each twist creates a smaller loop, which picks up less of that noise, but also adjacent loops pick up a signal that is inverted from the loop before. This way the induced noise voltages in each loop cancel each other. Audio systems in cars also need to be connected to this 'star' ground. The human ear is the most sensitive organ we have. The difference between the loudest noise (pain threshold) and the quietest noise we can hear is over 1 million to one. So, any electrical noise from inadequate grounding can be amplified by the audio system to hearing level.

Now, regarding the claims of some of the grounding systems regarding ten times better (less) impedance: We talked about resistance of the wires before. Resistance applies to steady-state current (DC). Steel has about ten times the specific resistance of copper. So the material must have ten times the square area of copper to achieve the same resistance. This is not a problem when using the frame as ground.

The material with the lowest resistance known is silver, followed by copper (a few % higher). High frequency currents (and pulses contain high frequencies) make the wire behave differently. High frequency resistance is called impedance and depends on the frequency of the current. Very high frequency currents tend to flow not evenly in the whole cross-section of the wire, but only on the surface. Therefore multi-stranded wire creates more surface area for the high frequency currents to flow, hence lower impedance. But this effect is only important for VERY high frequencies in the upper radio frequency range. If the electrical system in your car produces high currents with frequencies that high, your car would shut down TV and radio reception for miles around. (Ask the military or anyone who works in the Electronic Countermeasures world.) Frequencies that high are just not found in a car. The advantage of multi-stranded wire in a car is flexibility and vibration resistance.

As to claims of better performance and lower fuel consumption: EFI systems inject fuel according to the amount of air drawn into the engine, measured with its sensors. More performance can only be had if more air/fuel enters the engine. No electrical system can increase the air-flow, the same applies to fuel flow. The only affect a better grounding system can have, is if the sensor grounding was so bad before that the EFI computer misread the sensors due to ground offsets. This can be inexpensively remedied by following the grounding guidelines above. Some of the better EFI computers utilize 'differential' inputs. They measure both the signal and the ground of the sensor itself and calculate the difference. This way they become independent of any grounding issues.

As to ignition system grounds, one of the performance parameters in an engine is ignition timing. Timing is derived from sensors on the flywheel or crank. These sensors create reference pulses. A pulse can either be there or not. If not, the car stops. No additional grounding changes that. As to the ignition system itself: as long as there is enough spark energy to light the fire, the engine runs. More ignition energy does NOT increase power, just spark-plug wear. So if the ignition system is adequately grounded no amount of more grounding can do any good.
from: http://fastcoupes.com/reviews/automo...grounding-101/

[whiskychaser]

Its about 40 years since I did my 'O' level physics. Some of it sticks - V=IR, W=I2R etc. Experience has taught me that if you want to install a shower you might be better disregarding paragraph 1
Perhaps its time I revised.