Here is some good info.
Part One
Electrical Troubleshooting
Diagnosing Voltage Drop
One of the most rampant electrical maladies showing up in automotive service bays today is the phenomenon known as voltage drop. Left unchecked, voltage drop causes countless unsolved electrical mysteries, especially when it infects the ground side of a circuit. It can also trick you into replacing parts that are not bad.
The more connections and wiring a vehicle has, the more vulnerable the electrical system is to voltage drop.
To contain electrical voltage drop, practice safe electrical service. This means measuring voltage drop before reaching any conclusions. "Voltage dropping" a circuit tell you when the circuit is too restricted to operate a component (motor, relay, light bulb, etc.) or operate it correctly. If the circuit is restricted, repair it and retest. If there is no restriction and the component still does not run or run correctly, then replace the component.
In this example, if the water pipe completely collapses, water stops flowing, pressure drops to zero and the water wheel stops turning. Electrically, the same thing happens when a wire falls off or a connection breaks. Current stops flowing, voltage drops to zero. A starter motor would quit or a headlight would go out.
Symptoms of voltage drop
Often confusing and contradictory, electrical voltage drop symptoms vary according to the circuit's job and the severity of the voltage drop.
inoperative electrical parts
sluggish, lazy electrical devices
erratic, intermittent devices
devices that work sluggishly or erratically during periods of high electrical loads
excessive radio interference or noises in the radio
damaged throttle or transmission cables or linkage
repeated throttle or transmission cable failures
damaged drivetrain parts
engine or transmission performance complaints
no-starts or hard starts
high sensor or computer voltages
erratic engine or transmission computer performance
false trouble codes in the memory of any on-board computer
premature or repeated A/C compressor clutch failure.
This symptom list brings up several points.
Visual inspections miss most cases of electrical voltage drop. You usually can't see the corrosion inside a connection or the damaged wire that is causing the problem.
Ground-side voltage drop, a commonly overlooked cause of electrical trouble, can cause most of these symptoms. Any circuit or component is only as good as its ground.
The more sophisticated electrical systems become, the more important their grounds are. The number of electrical components has increased rapidly and most do not have separate ground wires. Instead, these devices are grounded to the engine or body. Rust, grease, vibration and/or careless repairs often restrict the circuit from the engine/body back to the battery.
Many components such as engine sensors share a common ground. Therefore, a bad ground complicates diagnosis because it affects several components at once.
Some shop manuals and diagnostic charts or fault trees recommend checking grounds last. In reality, it is much quicker to check ground circuits before you climb that fault tree.
It's quicker and smarter to routinely check a circuit's voltage drop than it is to memorize long lists of symptoms. If experience has taught us nothing else, it's that chasing symptoms is no substitute for routine and thorough voltage drop testing.
Experience has taught us other reasons to check voltage drop first. Voltage drop, usually on the ground side, causes inaccurate or bizarre voltmeter readings and oscilloscope patterns. Moreover, when you connect a voltmeter or scope to a system with bad grounds, the test equipment itself can create a good substitute ground. This can be frustrating: as long as your equipment is connected, the circuit works and you can't find anything wrong!
Basic procedures
Whenever an electrical problem gives you fits, take a deep breath and think of the basic electrical building block, the series circuit. Drawings 1 through 7 show basic series circuits. No matter how complicated a system is you can always simplify it into mini-series circuits. Then, inspect each circuit for voltage drop.
Also, relate electricity to water flowing through a water circuit. Water pressure inside the reservoir pushes gallons of water through the pipe. The water turns the water wheel and then flows back into the reservoir. In an electrical circuit, electrical pressure (voltage or volts) pushes electrical volume (current or amps) through the circuit, operating a load. The load may be a computer, a motor, a lamp, a relay, or other device. In the water circuit, the water uses up most of its energy turning the water wheel. Water continues flowing toward the reservoir, but it flows at a lower pressure.
Likewise, electrical pressure (voltage) is used up operating the load. Therefore, voltage falls to about zero on the ground side, but current keeps flowing toward the battery. Because the voltage in a healthy ground circuit should be about zero, some technicians call it ground zero.
A kinked return pipe restricts water flow back to the reservoir, slowing down the water wheel and causing a pressure reading on the return side of the wheel. Likewise, ground side voltage drop hurts load performance and causes a voltage reading at the ground side of the load.
Resistance—Restriction
When you think of excessive resistance, imagine a dent or kink that is restricting water flow through a pipe. Common sense should tell you that a kink anywhere in the water circuit (supply side or return side) restricts water flow, causing the water wheel to slow down or stop turning.
Excessive resistance has the same effect on an electrical circuit. Bad connections and broken or under size wires act like a pipe with a kink, restricting current flow. Like the water circuit, restricting current flow anywhere — hot side or ground side — hurts the performance of the load. The effect on the load is hard to predict because it varies with the severity of the restriction. For example, the motor in a restricted circuit may stop working or just run slower than normal.
A restricted circuit can cause an A/C compressor clutch to slip and prematurely burn out. A computer on a restricted circuit may shut off or else work erratically. When corrosion, loose connections or other types of resistance restrict a circuit, volts and amps both drop. If volts drop, amps drop too. That is why when you find a voltage drop in a connection or cable, you know the connection or cable is restricted.
Look at the water circuits in our drawings and remember two critical points. First, a free-flowing ground side is as important as a free-flowing hot side. Second, a ground side restriction is the only thing that causes voltage readings greater than 0–0.1V in any ground circuit.
A completely collapsed return pipe stops water flow, stalling the water wheel and causing a system pressure reading at the return side of the wheel. Likewise, a broken ground wire totally blocks current flow, shuts off the load and causes the ground side of the load to read system voltage.
