First of all, voltage is electrical pressure, just like water pressure in a pipe. Water pressure makes the water flow in a pipe. Voltage makes electrical current flow in a wire.
Thinking of that water pressure, you have pressure in a garden hose just like you have voltage in a wire. If you partially kink the garden hose, you'll still have pressure in the entire hose, ... Until you open the nozzle and very little water comes out. The kink is restricting the flow of the water. That restriction is not a total blockage. Similarly, a high resistance connection in a wire restricts the current flow. When the circuit is turned off, you'll find full voltage all the way to whatever is switching it off, just like the nozzle on the hose. When you turn the circuit on, the voltage, (pressure) will be lower after the restriction. That restriction limits how much current can get through the circuit and will cause the item to work slowly or not at all.
In this sad drawing, (figure 1), full water pressure is found up to point "A". That's where Ma is standing on the hose! When the nozzle is opened, the pressure will be lower after point "A". You have the luxury of seeing where the restriction is, but if you couldn't see it, as in the electrical problem, you can find it with multiple pressure gauges. Now, keep in mind it's not practical to poke a bunch of holes in the hose to take lots of readings, but use your imagination and suppose we could. You'd find 50 pounds of pressure all the way to the restriction, and lower pressure everywhere after it.
One problem could be the restriction is very small, such as your poodle standing on the hose instead of Ma. Your pressure gauges might look like figure 2. That two pounds of pressure drop would be insignificant if you were watering your plants, but if you're trying to shoot the woodpecker off the roof of your house, that two pounds could mean the difference between him falling off the roof or laughing at you! That very small restriction becomes important in a high-power circuit. That same tiny restriction becomes important in a high-current circuit in your car.
It can be very time-consuming taking all those readings, and you might have to do it in numerous circuits. That can take all day. A faster way is to look for pressure drops. In the second circuit there was 50 psi before the restriction and 48 psi after it. In figure 3, one gauge reading is used to find the pressure drop, or difference. That's only 2 psi. The entire circuit is included in this measurement. All you know is there's a 2 pound pressure drop but you don't know where, ... Yet. To find the location, move the test points toward each other, as in figure 4.
By the time we get to figure 5, the test points are both on the same side of the restriction so the difference between them will be 0 psi. From this we know we just jumped over the restriction. If you want to double-check, in figure six we verified the location of the restriction with the 2 psi pressure drop reading.
Remember in figure 2 where the restriction was so small that only two pounds of pressure was dropped? That only had a noticeable affect when we needed lots of power. That's what happens in high-powered electrical circuits too. The starter and the charging circuits are the two main examples in a car. The undesired resistance in those circuits can be WAY too small to measure, but we CAN measure the results of that resistance in the form of a voltage drop.
Piercing wire insulation is never an acceptable method of taking readings just like piercing the garden hose is not acceptable. Instead, we have to look for accessible test points. In the starter circuit that includes the two battery posts, the two cables bolted to them, the starter terminal in the positive circuit and the engine block ground connection in the negative circuit.
Not to complicate this description further, but when you are measuring pressure in a hose, you are always measuring something in comparison to a reference. In the hose, we have 50 pounds compared to atmospheric pressure. We're measuring the difference between two points; atmospheric pressure and what's inside the hose. In an electrical circuit we are also measuring between two points. When you take a reading AT a point in the circuit, it's typically taken in reference to "ground" which is a common point for all the circuits, and is connected right to the negative battery cable. When you take a voltage drop measurement, you're measuring BETWEEN two points in the same circuit, not with reference to ground. Ideally every voltage drop reading would be 0.00 volts, but since everything, including wires and cables, has some small resistance, there is always going to be some voltage drop. We just want them to be as small as possible. It's when one gets too big that we have problems. Those voltage drops only show up when current is trying to flow in the circuit, so that circuit has to be powered up and trying to operate. In a starter circuit, that means you need to take the measurements while a helper is trying to crank the engine.
The other benefit of using voltage drops is there are too many variables when we measure at multiple places at multiple times. If you take four measurements at four different places in the starter circuit, the battery will be running down during each time the starter is activated. Each subsequent reading will naturally be lower than the previous one. Next, due to the nature of how electrical motors work, the current flow is not perfectly steady. It rapidly rises and falls a little. Digital voltmeters take a reading, analyze it, display it, then take the next reading. That can cause the readings to bounce around and be not too informative. All of those problems are cancelled out when we take voltage drop readings because we're measuring between two points that both see the same current flow at the time. That makes the voltage drops remain very steady and easy to display on the meter.
In the starter circuit, the industry standard is to have no more than 0.2 volts dropped across any one mechanical connection, and no more than a total of 0.4 volts dropped in the entire circuit. One mechanical connection is the battery positive cable that is bolted to the battery post. Electrical Notes 9 is something I put together for my students. What's important here is the drawing in the lower left corner. That shows one voltmeter probe right on the battery post and the other one on the cable clamp. You will find 0.0 volts at first, but when a helper cranks the engine, the high current will cause a small voltage drop across the tiny amount of resistance in that connection. THAT'S the resistance that's way too small to measure, but we can measure the results of it. If you find more than 0.2 volts there during engine cranking, clean and tighten that connection. The same test can be done where the positive cable attaches to the starter but it's usually harder to get to. Measure the voltage drop on the negative battery cable too, and again where it attaches to the engine.
In the diagram on the right, you'll see there's five mechanical connections in the entire positive side of the circuit. Any one of them can have no more than 0.2 volts dropped across them, but for the entire half circuit, test points 3 7, the total can be no more than 0.4 volts. Some of those test points are not accessible on many car brands and models. Points 5 and 6 are not accessible individually. If high resistance is determined to be in one of the contacts, we just replace the entire part. Knowing which contact caused the problem is of no value. Likewise, if no excessive voltage drop is found in the entire positive circuit, there's no need to check those inaccessible points further.
Normally we use the process of elimination. If no excessive voltage drops can be found in the cables and connections, all that's left is a problem inside the starter. It's fastest to just replace it and less expensive than doing a lot more testing, but two quick voltage drop tests on the cables will identify a problem that won't be solved by replacing the starter. That can save a lot of wasted time and the expense of the new starter that isn't needed.
To sum this all up, voltage drop measurements are taken with both meter probes in the same circuit. Logic says you should find 0.0 volts difference because the same voltage should be found in all parts of the circuit, but it's that very tiny undesirable resistance we're looking for. It's much too small to measure with an ohm meter, but we CAN measure the results of current flowing through that resistance. It shows up as a small voltage reading. The lower the voltage reading, the lower the resistance. When the voltage drop reading is too high, the resistance in that part of the circuit is too high.
By the way, some of the causes of resistance that's too high include corroded copper terminals, loose attaching nuts, that black gunk that builds up on battery posts and inside the cable clamps where they contact the battery posts, and an often-overlooked problem is frayed wire strands on the end of one of the cables. Every piece of wire has some resistance. We lower that resistance by using more strands of wire in the cable. That's why starter cables are so fat. When some of the strands break or corrode off, the resistance in that cable goes up a bunch.
Images (Click to enlarge)
Saturday, October 15th, 2011 AT 7:50 AM