Reliable automotive connections depend on a good grounding system and recognizing common ground-related problems.
By John Yurtin
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The term "Ground Zero" has become a somber reminder of the devastation of the World Trade Center in New York. But when it comes to automotive wiring harnesses and automotive connections, it forms the basis for dependable electrical system performance. Probably the most misunderstood part of a wiring schematic is the part you don't see. There is plenty of information on how power is supplied, how juice reaches various components, and how various devices interface with each other. What is typically missing is the information about the "grounds" that form the other half of electrical circuits. What is ground to one device may not necessarily be ground to another. The loss of ground may cause a circuit to generate false or noisy information or, worse yet, stop working altogether.
Ground (or earth) as an electrical term, really has its origins in the early days of radio. Back then, a wire stretched between the trees on a farm, say, made up one-half of the aerial; the other was literally a rod or pipe driven into the ground. But this did not always work as intended. When reception was poor, for instance, the farmer would often have to pour a bucket of water on the ground rod to improve the quality of the connection to earth.
In the dictionary, a ground is a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth. In the context of automotive electronics and electrical systems, ground simply means a common reference point. When we say an automobile has a 12-V electrical system, the part we leave out is that it is 12 V positive with relation to the ground, or common, connection.
 FIGURE 1. Ring terminal performance relies on good metal-to-metal contact and proper torque. |
The ground system gives a common reference to the electrical system in automobiles and acts as the return path for the flow of electricity. Because the car itself is not connected to the earth, the true ground is the negative post of the battery (see Fig. 1). By design, however, all circuits are not wired directly back to the battery, but often use the metal of the car body or chassis as ground, so the wiring harness needs only to consist of wires for supply of B+ voltage as well as carrying information between the sensors and the computers (see Fig. 2).
 FIGURE 2. Various vehicle electrical circuits have different grounding points that all end up at the battery ground post. |
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Down to earth
Most automotive grounding systems are not ideal. One reason is that steel is not nearly as good a conductor as copper. Electricity must pass through the typical spot welds holding the steel body panels of a car together from one grounding point to the next. The problem is, both the steel and the spot welds have electrical resistance. Therefore, different parts of the body can be at slightly different electrical potentials relative to each other. Another factor is the ground cable arrangement. The negative side of the battery is typically tied directly to the engine block via a cable to provide the return connection for the high-amperage starter motor. But this cable often has some resistance. This means that the car's body is not necessarily at exactly 0 V relative to battery negative. For heavy power loads such as solenoids, fan motors, and the like, the consequence of the ground connection not really being at exactly 0 V doesn't matter most of the time. Instead of seeing the total system voltage, the device will see the difference between the true ground value and system voltage. When the loss is a volt or so, the load is designed to still operate properly. The trouble comes when the cables and connections from the battery become excessively resistive. The voltage drops across these poor connections and can rob the starter motor of the current needed to crank properly.
The story is different when it comes to information lines—say, a signal from a sensor to a powertrain control module (PCM). Digital information, for example, is in the form of a pulse train of 1s and 0s represented by 5 V and 0 V, respectively. There are two things to notice about the choice of voltages used: One is that the difference between the two is much closer than the difference between system voltage and ground, so an offset of 1 V becomes much more significant. The second thing is that the plus value is typically 5 V. Theoretically, it could be 1 V that equals a 1 and 0 V that equals a 0. The chosen 5 V value provides noise immunity. A design feature built into logic ICs called a comparator looks at the information waveform and compares the received voltage to an internal reference voltage. The comparator is set so that any signal received that is less than 0.8 V is counted as a 0 and anything above 3.5 V is counted as a 1. This allows a noisy signal to still transmit its information. Analog sensors are much more susceptible to ground offsets and electrical noise in the system. The output of these sensors is often low and any loss due to poor connections can result in misinformation being sent to the computer.
Several aftermarket grounding systems claim to improve performance by increasing horsepower and torque, improving gas mileage, allowing quicker starts, and reducing audio noise. The installers of high-performance audio systems in vehicles go to great expense to run all grounds back to the battery.
Reliable grounding presents several inherent basic problems. First is the issue of the mechanical connection itself. For example, the typical tin-plated brass-ring terminal is crimped onto one end of a wire to be grounded and fastened to the automotive body by a bolt or a self-tapping sheet-metal screw (see Fig. 3). Many things could go wrong: The crimp to the wire could come loose, corrode, or become mechanically damaged in a collision. The sheet-metal screw could back out as the body flexes during use and thermal cycling. The terminal and the place on the body to which it attaches could corrode due to salt, water, or battery acids.
 FIGURE 3. Ring terminals come in a variety of shapes and sizes. Although very basic in design, the plating, crimp, and features significantly affect performance. |
Secondly, many devices achieve their electrical connection to ground by way of a bolt or screw attached directly to the frame, body, or drive train. Much like the ring terminal, the integrity of these grounds depends on the quality of the mechanical connection. Efforts to improve ring terminal performance include the addition of features cut into the metal to resist vibration and maintain contact force. Finally, many cases of poor grounds have been related to improper torque of bolts and screws during assembly.
While identifying ground-related problems is not simple, some scenarios automatically raise a red flag. Multiple, seemingly unrelated failures are often ground-related issues. Mysteries such as the dome light flashing in unison with the turn signals are often fuse- or ground-connection-related. Sensor outputs that are either out of range high or a constant value can be an indication of a ground problem. Poor performance of power loads such as the starting system, solenoids, and fan motors may also be the result of a poor ground connection.
Finding bad grounds is not always easy. Fortunately, recent service literature includes ground location information.
The good ground
 FIGURE 4. Once a faulty circuit is understood, simple methods can be used to find bad grounds. |
The easiest way to test for a bad ground is to attach a wire to a known good ground and to the groundside of the circuit in question (see Fig. 4). In our example, that would mean connecting another wire from the negative side of the battery (point F) to the groundside of the light bulb (point E). When the switch is turned on, does the bulb light? If so, then one of the ground connections was bad or the ground wire was broken.
The ground can be checked with the voltmeter. Connect the voltmeter's red (positive) lead to point E. If the meter does not read 0 V, it means that the ground is bad. There appears to be something else (a resistor) in the circuit. A resistor is an electronic component that resists current flow. While a resistor is a component in and of itself, any electrical device such as a motor or a light can electrically be considered a resistor or a load.
To test the ground with an ohmmeter, break the circuit by turning the switch off. The safest and smartest thing is to disconnect the battery. Connect the ground (black) lead of the ohmmeter to a known good ground. Then connect the positive (red) lead to the groundside of the light (point E). If the meter doesn't read 0 W, then the ground is bad. A bad ground is often the cause of electrical problems in a car's electrical system. The only problem is finding out where the ground is. Usually there is a short black wire coming from the load (the light bulb in our circuit), bolted to either the chassis or some metal part (like a fender well or the firewall) that is mechanically connected (bolted) to the chassis. To fix a ground connection on a car, first remove the bolt attaching it to the metal. Clean the connector on the end of the wire and the metal where it attaches with a piece of sandpaper until they are shiny. Reattach the connector to the metal and check the circuit again.
In many cases, the ground circuit runs through a connection system and the ground connection is handled smuch like the positive circuits are. The performance requirements for the ground, power, and signal circuits may depend on the performance of the connection. Splices and grounds are engineered as part of connection systems to simplify the wiring harness design, and can take the place of the ring-terminal type grounds.
The key to identifying and fixing ground problems is to remember that the ground system provides the return path for power and information distribution and forms the reference point for voltage. When your diagnosis says that neither of the previous two things is happening, it's time to go hunting for that bad ground connection.
ACKNOWLEDGMENT
This article, an edited excerpt of "Eye on Electronics," appears courtesy Mike Dale, MOTOR magazine, 2001.
JOHN YURTIN is a staff product specialist in the Connection Systems at Delphi Automotive Systems, P.O. Box 431 Warren, OH 44486. Tel: (330) 373-3947; email: john.yurtin@delphi.com.
