When it comes to electricity there are two potential energy
sources you have to keep in mind. One, being the positive side and
the other, the negative side. Just like a magnet, these two opposite
forces react the same way. That is, two positives or two negatives
will not meet, while a positive and a negative source of energy will
attract to each other. This attraction will continue until all the
potential energy is equalized.
Looking at a lightning bolt the potential energy is of a positive
nature and of course, what does it want to do? It wants to
find its counterpart ... negative. Which is good old terra firma, or
in other words ground. In a car the positive energy wants to do
the same thing, find ground. And since a car is more or less its
own little portable planet the car itself is the ground. Or in
other words the chassis is the ground. In the modern car negative
is the chassis ground, however if we go back a few decades you’ll
find some vehicles with positive chassis grounds. For all intents and purposes the concept is the same which is to provide a path to allow the energy to reach that equality state.
Defining A Chassis Ground
A chassis ground basically means that the chassis itself is the conduit for that potential energy. That also means that anything be it metal or some other material that will conduct electrical potential and is attached to the chassis is a potential ground source. So, the importance of the chassis ground can’t be overlooked when diagnosing either a voltage drop or lack of current flow in a system that needs a path to a ground source. In most cases the wiring diagram will show the locations of these grounds or where a wire eyelet is attached to the chassis. Most manufacturers will use the universally excepted ground symbol or assign a letter and number combination to the ground lead and then list these ground locations in the locator section. A bad chassis ground is without a doubt one of the most common problems in today’s vehicles. Since most of these ground leads are exposed to the elements and are in a spot where vibration, movement, and fluid contamination can weaken if not break the connection all together. Obviously, a weak ground can result in a voltage drop, but the lack of a ground has even more issues. Once the ground path is no longer available for certain electrical circuits the first thing the positive energy wants to do is find another way to get to ground. This could mean a back feed through another circuit or perhaps not work at all. While a weak connection may allow the system to somewhat function but not to its designed potential. Such as headlights that comes on but are not very bright, or a blower motor that seems to run at half speed. Tracing these ground faults can be a challenge, but... you can make the job easier if you follow a few key steps.
Ground Circuit Diagnostics Simplified
As with any repair it all starts with verifying the complaint. Be it an intermittent situation or a full on failure. The next step that I tend to go for is the wiring diagram. I’d like to know how and where the circuit I’m testing gets its powers and grounds. Once I’ve got that sorted out I’ll do a quick check of the connections. Of course, paying close attention to the ground connections. Any sign of corrosion or frayed wiring will need to be taken care of before proceeding any further. It’s not uncommon to have a braided ground strap or wire that has only a few strands left holding it together cause a voltage drop in the circuit. The next thing to do is to actually test the ground signal with the tools of your choice. Since I prefer to keep things simple I generally will start with a test light. Placing the test light on a good positive source of battery voltage I’ll probe the connection of the ground leads. (Never stab the wire. Always go to the connector.) Do this before you disconnect anything. The idea is to see if there is or isn’t a completed circuit from positive to the negative source. If your test light or probe is on positive and you make a good clean contact with the negative wire the test light will light up. If it doesn’t, well then, you haven’t completed the path.
Another good way is to have your test light attached to a good chassis ground and probe the ground lead at the affected circuit. If the path is broken the test light will light up because the positive electricity has made its way through the circuit and is still looking for that ground signal. That “ground” wire is no longer grounded. This is a handy test when you’re under a dash and trying to find a good source of a positive signal to test it as I described early and it requires the efforts of a contortionist to find one.... which ain’t me.
Of course there are a lot of various tools from meters to scopes and other various probes that can make the job easier for you. It really depends on what you feel more comfortable with. I’ve used all of them from time to time. It really comes down to which method is going to get me the results the quickest in a given the situation, or the location, or the ease of accessibility.
Sensor and Signal Grounds
There are other different forms of a ground we should mention. That’s the sensor or signal ground. Yes, they are grounds but they are not directly attached to the chassis as in the main ground leads would be. They are a ground signal generated by a circuit in a computer system. The computer controls the signal by various inputs it receives and sends out a ground signal at the appropriate time and strength depending on the circuit it is designed for. A lot of times this ground signal is momentary or can be turned on and off in milliseconds by the computer. Such as duty cycles for solenoids or fuel injectors.
Some can be just a few volts while other will be battery voltage. These sensor grounds can be read with a basic test light in some instances but by far the best method is with a scope or scanner. At this point you need a bit more information than you’ll find on a wiring diagram, you’ll need the diagnostic procedures to find the correct values in order to know what you’re seeing on the scope or scanner is correct. If the information isn’t available in the diagnostic procedures there a several websites and companies that provide libraries of known good scope patterns, such as IATN, Autonerds, Identifix, and many more. These sites can be invaluable to you and I certainly encourage using them.
The main thing to keep in mind is not to overlook the ground signal as the source of the problem. Sure, positive connections can get hot, melt and cause a voltage drop just as well as the negative side. But, performing your diagnostics on both the negative and the positive side of the potential energy source for that given circuit is going help keep you grounded with some excellent diagnostic skills... and that ain’t a bad thing to be.