All automotive electrical circuits have desirable resistance which are the loads or current-limiting devices that are designed in. Undesirable resistance in the form of corroded connections, burned switch contacts, and partially broken wires also limits current to the point the circuit doesn't work properly. Locating that undesirable resistance is not difficult in most circuits but there are new challenges in high current circuits. Starter circuits are where you'll run into problems where you will need special troubleshooting procedures but they can be used in other circuits as well. Starters for small V-8 engines can draw 300 amps or more until they begin to develop back EMF. Using Ohm's Law, that means the entire circuit has only 0.04 ohms of resistance. That is way too small to measure with our equipment but we can measure the results of that resistance. Current through a circuit causes a voltage drop across any resistance. Even though the undesirable resistance may be too small to measure directly, the current flow in a starter circuit is so high, the resulting voltage drop can be measured.
Voltage vs. a Voltage Drop
First we must clarify the difference between voltage and a voltage drop. Measuring voltage at a point in a circuit typically involves connecting the voltmeter's negative lead to a reference point which we call "ground". The positive meter lead is used to probe the place in the circuit at which we want to measure. When searching for the cause of a dead circuit, one procedure involves working your way down the line from the beginning of the circuit to the end, taking multiple voltage readings. The location of the problem is isolated between the two points where voltage is present and where it is not. The voltmeter was used to measure the voltage at numerous points in the circuit.
A voltage drop is simply the difference in voltage between two adjacent points in the circuit. Ground is no longer used as the reference. Both meter leads are connected to two points in the circuit that are next to each other and should have the same voltage. Examples of these are the two mating pins in an electrical connector, the two contacts of a switch or relay, or most commonly, the connection between a battery post and the cable clamp.
Why Voltage Drops Are Valuable
To use standard voltage measurements, you would need to make two of them, one on each side of the suspected undesirable resistance. When doing this in a starter circuit, that resistance will only cause a change in voltage when current is flowing through it. That means cranking the engine and running the battery down. The lower battery voltage alone can be responsible for the second voltage reading being lower, giving the impression you found the problem.
Two voltmeters could be used to measure the voltages at the same time, but the obvious thought has to do with the calibration and accuracy of each meter. A less-known characteristic of digital voltmeters called the "sample rate" presents a problem. The meter measures the voltage, analyzes it, then displays it in the readout while it takes the next reading. This process repeats multiple times per second and two meters being used at the same time will not take their readings at exactly the same time.
The voltage at any point in the starter circuit fluctuates slightly during engine cranking. As the armature rotates, the brushes change where they are contacting the commutator segments. Sometimes a brush will contact two segments at the same time, sometimes only one segment. This causes very small changes in current flow. Also, every time a piston comes up on top dead center on the compression stroke, the starter has to work harder so it slows down. That means less back EMF is generated to oppose current flow from the battery. The voltage at any point in the circuit will pulsate in time with the pulsing changes in current flow. The readings on the voltmeter will pulsate too depending on when it takes the measurements. Two meters used at the same time will naturally display different readings so the information they provide is useless when looking for undesirable resistance.
All of these problems are avoided by measuring voltage drops. Only one meter is used, current flow is the same at both points in the circuit when the reading is taken by the meter, and even though the voltages are pulsating, the difference between the two voltages will be constant and steady. That means the voltage drop will be easy to measure, accurate, and it will have meaning.
The large battery cables going to the starter and the engine block have very little resistance as long as the strands of copper wire aren't broken. Most problems are caused by a very tiny amount of resistance between two parts where they form a mechanical connection. These include the connection between the battery post and the cable clamp, the battery cable and the terminal bolt on the solenoid, two mechanical connections where the contact disc in the solenoid touches the two contacts, and where the starter wire bolts to the second solenoid terminal. All of these points are accessible in the GM system with the solenoid on top of the starter and in the Ford system where the solenoid is on the inner fender. The two solenoid contact disc connections and the final connection to the motor wire are not accessible on the older Chrysler starters, but they rarely cause slow cranking. It is more common for a burned-away contact to cause a no-crank condition.
Procedure for Measuring Voltage Drops
The secret to obtaining correct measurements is in the placement of the voltmeter probes. The easiest way to find an excessive voltage drop is by testing one mechanical connection at a time, so most of the time the two meter probes will be just a fraction of an inch apart. It can be very time-consuming and difficult to connect the probes so they stay in place by themselves. Instead, it's quicker to hold them in place, then have a helper crank the engine while you take the reading. The voltage drop will show up during cranking when current is flowing in the circuit.
Disable the ignition coil to prevent the engine from starting. On fuel injected vehicles you could remove the fuel pump relay. On Chrysler products, removing the automatic shutdown (ASD) relay is quick and effective. You can also jump the connections at the starter relay. That eliminates the need to turn the ignition switch on so the engine will not start regardless if it has a carburetor or is fuel injected. Carbureted cars should have the throttle propped open to prevent raw fuel from flooding the engine.
Figure 1 shows the proper placement of the two meter probes to measure the voltage drop between a battery post and the cable clamp. To obtain a positive meter reading, the negative meter probe must be on the side of the connection closest to the negative battery post. In the positive half of the circuit, the positive meter lead must be on the side of the connection closest to the positive battery post. The nice thing about a digital voltmeter is the probes can be connected backward without damaging it. The reading in the display will include a minus sign indicating a negative voltage, (reversed polarity), is being measured.
Figure 1. Measuring the voltage drop across one mechanical connection, the battery post and cable.
Figure 2 shows the points to take voltage drop readings in the starter circuit. Start with the negative meter lead on the negative battery post and the positive probe on the negative cable clamp. The meter should be on the lowest scale that will provide the most accuracy yet display a usable reading. For most meters that will be the 2 volt scale. Read the highest voltage while your helper cranks the engine. Usually that will occur at the beginning when the starter hasn't picked up speed yet and developed maximum back EMF.
Figure 2. Voltage drops in a starter circuit.
The next mechanical connection is where the negative battery cable is bolted to the engine block. Place one meter lead on the terminal end that is crimped onto the cable, and the other lead right on a clean, paint-free spot on the engine, then take the new reading during engine cranking. The positive battery post / cable clamp and the other end of the cable at the solenoid are tested the same way. You can't get to the individual connections inside the solenoid, (points 5 and 6 in Figure 2), so you'll have to measure the voltage drop across the pair of contacts at the same time. Place the meter leads on the two large diameter studs, (not the cable and copper strap), and take the reading. The last reading with the GM-style solenoid is between the stud and the copper strap or braided wire going into the starter. For Fords, that connection is at the solenoid on the inner fender. The final connection to test on those is between the cable and stud right on the starter.
Interpreting the Results
Unless the manufacturer specifies something different, the industry-accepted standard is a maximum of 0.2 volts per any one mechanical connection and a maximum total of 0.4 volts in the entire positive circuit and in the entire negative circuit. Any connection with a voltage drop greater than 0.2 volts must be disassembled, cleaned, and tightened.
To speed things up, you can test the entire positive side of the circuit at once, then the entire negative side to identify which part of the circuit has too great a voltage drop. Place one voltmeter probe on the battery post and the other one on the last point in that circuit. If you find less than 0.2 volts during cranking, that part of the circuit is okay. If you find between 0.3 and 0.4 volts, it would be smart to measure the individual voltage drops to see if one is more than 0.2 volts. If you find more than 0.4 volts, you may need to look at multiple mechanical connections in that circuit if you can't identify just one that has too much resistance. With a slow-cranking starter and good battery, you typically will not have much trouble finding the cause with voltage drop measurements. You'll usually find considerably more than 0.2 volts dropped across the bad connection.