Overlooking body ground checks may be a costly mistake during a difficult electrical diagnosis. Here’s what you should know about inspecting and testing body grounds.
A faulty or missing body ground may cause a wide variety of weird and/or intermittent electrical problems. Experience shows that these odd, erratic symptoms frustrate many technicians because checking body grounds is not a priority for them. Sadly, a tech may burn hours of time on a problem vehicle that should have been diagnosed within 15 to 20 minutes.
Overall, body ground checks are simple and straightforward. To me, the steps are so easy that it pays to perform them very early – instead of much later – in your diagnosis process. Please consider another tip borne of experience: Do not perform body ground checks in front of customers. A car owner may not believe that you were able to solve an elusive electrical problem within minutes. That knowledge is very valuable. It should not be free.
REVIEW OF CIRCUIT OPERATION
A brief review is helpful here. Voltage, which is measured in volts, is electrical pressure. Current, which is measured in amperes, is electrical volume. (I prefer to use a super-simplistic approach called conventional current theory.) In a simple series circuit, current flows from the positive battery terminal to an electrical component called a load. Techs often call this the power supply or “hot” side of the circuit.
A load is an electrical component that does work of some kind. Examples of loads are bulbs, horns, flashers, radios, relays, solenoids, motors, and computers. Current flows from the hot side of the circuit through the load. Then current flows from the load through the negative or “return” side of the circuit to the negative battery terminal. Techs commonly call the return side of the circuit a ground.
It’s easy to envision a hot side where a single wire runs from the positive battery terminal to the load. Likewise, it’s easy to visualize a ground side where a single wire runs from the load back to the negative battery terminal.
However, design engineers save both money and weight by eliminating the ground wire. Instead, they use the vehicle’s body as the return path to the negative battery terminal; all that sheet metal provides an adequate pathway. This is a perfectly valid engineering approach since most vehicles on the road feature unitized construction. (This means the body and frame are welded together during manufacturing.) Marrying the body and frame together provides many benefits. Arguably, one of them is creating a reliable, nearly wireless current return path back to the negative battery terminal.
BODY GROUND FORMATS
There are several ways that a vehicle designer may link the body/frame assembly to the negative battery terminal. For example, a cable or thick wire may connect the negative battery terminal to a frame rail or inner fender panel somewhere under the hood. Or, a ground cable or braided strap may connect a point on the frame to the engine block or transmission case. Then the traditional negative battery cable completes the return path from the engine back to the negative battery terminal.
Sometimes a robust, braided strap parallels each rubber engine or transmission mount. “Bridging” each rubber mount with a ground strap ensures that the return current flows freely from the body/ frame assembly to the engine block – then through the negative battery cable to the battery itself. Last but not least, one or more braided straps may be routed from the firewall to the engine.
Occasionally, you encounter an automaker’s service bulletin that specifies replacing an existing body ground or adding additional grounds to a particular vehicle. But in most cases, the OEM body grounds are adequate – provided their connections are clean and tight, and the conductor isn’t damaged.
I urge readers not to remove OEM body grounds or leave them disconnected after a repair job. Simply put, the vehicle designer invested in body grounds for a reason.
CAUSES OF FAILURES
Carelessness and corrosion cause common body ground failures. Based on my field experience, there’s no contest: Carelessness is the more common reason. For example, suppose that a vehicle arrives at your shop with strange and/or erratic electrical symptoms. Politely but persistently interrogate the car owner about the vehicle’s repair history. All too often, you discover that those electrical symptoms only appeared after someone else worked on the vehicle.
If that is the case, always focus your initial visual inspection on the last place(s) where human hands touched that car. For example, you may find new engine mounts, but the ground straps that used to parallel those mounts are gone.
Patiently check the firewall area and the firewall side of the engine. You may discover that a ground wire or braided strap is still securely mounted to the firewall. But the opposite end of it is dangling – disconnected. At the same time, you may see evidence that someone recently replaced an engine, transmission, cylinder head, head gasket, intake manifold gasket, etc. Very likely, one event caused the other to occur.
Sometimes, limited working space on the firewall side of the engine invites shortcuts such as leaving body-ground cables or braided straps disconnected. I have seen cases where a bellhousing bolt normally secures a ground strap eyelet to the top of the bellhousing. But a tech ignores or overlooks it when he replaces a transmission.
Hopefully, a shop foreman or senior tech routinely inspects every repair job before customers arrive and catches this mistake. Some savvy shop owners keep assortments of new ground straps handy for situations like this one. Instead of struggling with hard-to-reach bellhousing bolts, a tech simply routes a new ground strap from the vehicle’s firewall to the engine.
Always watch for small but revealing clues during an electrical diagnosis, especially when the vehicle exhibits strange and/or erratic electrical symptoms. Furthermore, find out if someone recently replaced a transmission, swapped an engine, or performed collision repairs. Watch for telltale witness marks on the firewall where a body ground eyelet terminal used to be mounted. A witness mark flags a missing ground cable or ground strap.
Snow and road salt may damage vital body ground connections. Road salt also may attack a braided ground strap so severely that the strap literally disintegrates into a greenish powder. As this corroded strap disintegrates, it gradually creates an open ground circuit.
Finally, you may find a body ground strap that appears to have been badly overheated. Or, you see a body ground cable with severely blistered insulation. Usually, these clues tell you that someone cranked the engine with a loose engine-to-negative battery cable connection. Imagine that a careless tech removed the negative battery cable from the engine during a repair. Later, he put the negative cable back on the engine but did not tighten its bolt thoroughly.
Current always flows through the path of least resistance. Experience shows that a loose negative cable-to-engine connection causes the starter current to seek an alternate route back to the negative battery terminal. A convenient alternative path may be through the vehicle’s body ground(s), which are not intended to carry starter current. This excessive volume of current burns and/or blisters the body ground.
Do not be fooled by a cranking speed that sounds normal for the vehicle. Experience shows that a body ground may carry just enough starter current that the cranking speed sounds normal.
Watch out because the starter current’s alternative path may go through components such as a throttle cable, clutch cable, floor shift cable, etc. A seized cable and/or blistered cable jacket are both key clues.
VOLTAGE DROP TEST
Many techs successfully identify a body ground issue via careful visual inspections. But there is an easy voltage drop test for it. Grab a digital voltmeter. Select its DC volts scale and auto-ranging feature. Then securely connect one of its test leads to the vehicle’s firewall. Connect the other voltmeter lead to the negative battery post or the retaining bolt inside a side-terminal battery terminal.
Start the engine and turn on as many electrical accessories as practically possible.
Under these operating conditions, the voltmeter displays the voltage drop from the vehicle body to the negative battery post. An automotive field service engineer taught me this procedure back in 1978. He recommended a working guideline of approximately 0.50 volt or less.
I have practiced this test countless times; the voltage drop across a healthy body ground usually measures 0.10 volt or less. And typically, there’s no guesswork with a failing ground because its voltage drop is sky high – many times greater than normal.
In conclusion, do not let elusive electrical problems – oddball and/ or intermittent symptoms – beat you. Check the body ground(s) early in your troubleshooting procedure. You will be glad you did!
