I have used my Fluke 189 meter to test for bad grounds on light, medium and heavy-duty trucks. I have seen more electrical problems – i.e., ECU (Electronic Control Unit) communication & data link issues – stem from bad grounds than any other failure, close to 75 %.
What's a "bad ground"?
Bad ground is the general term that I use to briefly describe the problem to the customer. By bad ground, I mean a ground wire or circuit that has more resistance than normal due to corrosion (oxidation) in the wiring harness or pin connections caused by a break in the wire's insulation or faulty connector seals. This causes a big issue on today's trucks, especially when everything is computer controlled or monitored with an ECU.
A typical Volvo truck has an ECU for every major component; engine, transmission, ABS (anti-lock braking system), vehicle ECU, instrument cluster, LCM (lighting control module), body builder control module and SRS (supplemental restraint systems). Sometimes the problems are there after 250,000 miles, sometimes they come straight from the factory with literally 30-300 miles on the truck when the problems arise. I have seen a truck with 18 miles on it; our shop had to replace the front chassis harness due to corrosion that infected the whole harness stemming from a faulty junction connector seal.
If you think about it, the wires are like little pipes. If there's a break in the wire or a faulty seal, water gets in and travels down the wire, "infecting" everything in its path. If there's a problem up higher in the harness, gravity will work to its benefit.
Problems I see
As for the problems that I see, some could be caused by the customer, typically from improperly installed aftermarket equipment. Or, they can stem from a harness that is not secured and will eventually rub through anything it lays against. The larger diesel engines vibrate more than a gasoline engine, so if things aren't secure, it doesn't take long for damage to occur. I have also seen harnesses that are overly secured. At the factory, zip ties may be installed too tight, causing a break in the wire, which is harder to find because it's inside the insulation of the wire.
The easier problems to spot are the corrosion issues. If you see a hint of light green, chances are, there lays your problem. I also see a light film on pin connections from time to time. This must be scraped off to improve the connection.
The grounding problems I see the most are high resistance values between the truck cab and the frame and between the frame and the engine. Anything over .3 ohms is going to cause problems.
When it comes to communication issues between ECUs, that's when problems get more complex. There are two different data links throughout the truck: the J1939 Control Link and the J1587/1708 Info Link. I tell customers to think of the data links like a phone wire connected to all the ECUs in the truck. The wire sends information such as diagnostic trouble codes (DTCs), vehicle speed, ABS wheel speed, engine pressures, temperatures and other variables. All the information is relayed to the instrument cluster. The J1939 data link is supposed to have a sustained reading of 60 ohms. If it differs at all, there's a problem in the circuit or terminating resistor.
As for troubleshooting, my main source or tool for information is the wiring schematics that are provided by the engineers when the truck is built. This shows wire/circuit numbers, connector locations, pin locations, and wire colors. With the cluster in the latest model of truck, you can read the DTCs and interpret the specifics to help troubleshoot the problem in depth.
For example, an electrical fault or mechanical fault, voltage high, voltage low, circuit shorted high, circuit shorted low, data erratic, or communication error are all descriptions linked to certain fault codes, which in turn makes it easier for diagnostics to be performed. Those codes also help me determine which function on my Fluke 189 DMM to choose to look for that problem.
I solve most of the problems by process of elimination. I start by checking fuses. If the fuses check out, then I check what that wire is controlling; light, sensor, and motor. Next, I check the circuit, to make sure the wires are not damaged and have good continuity. There are miles of wires and plenty of potential problems areas. If this area checks out, I continue to the beginning of the circuit; ECU, switch.
Sometimes, I can't trace the problems, or I can't duplicate them, which is very frustrating. Variables that affect troubleshooting "invisible" problems include temperature, pressure & vibration. Some faults can be found by connecting a DVOM and wiggling the harness, looking for a jump or dip in voltage or a complete loss all together indicating an open circuit.