Electrical diagnostics of mining equipment

By Dan Sullivan*

They need to result in confident decisions that minimise wrong turns and unnecessary parts replacement. Production pressures often make it hard to spend the time needed to carry through on repairs after the fault is located, so getting to the answer quickly is even more important. Here are some principles, concepts and tricks you can use to help make the process easier.
Remember that electrical diagnostics and repairs are completely opposite to mechanical diagnostics and repairs. An accurate diagnosis of a mechanical fault can be made within minutes, with the repairs taking hours. During this time no one complains about how long the job is taking because everyone can relate to the process and tangible progress can be seen as it happens.
An electrical diagnosis, however, can take several hours, especially when we don’t fully understand how the system is supposed to work in the first place. But the repair will likely only take minutes to complete, and if you consider your own experience you’ll agree. How many times have you made an electrical repair that was insanely simple - so simple in fact that it was embarrassing to admit once you found it?
I suggest you get into the habit of starting the process by assuming it’s going to take several hours to diagnose an electrical fault, but mere minutes to repair. It’s easier to defend the process if you understand it and realise that a five-hour diagnosis and five-minute repair are normal.
Use the technical data at your disposal, and spend the time needed to learn and understand how the system is supposed to work. Many technicians read schematics incorrectly; you read them from negative-to-positive, not the other way.
There are only four types of circuits on the equipment we service, and they all work the same from machine to machine. These are: analogue circuits switched in either the positive or negative (clutch solenoid for example); digital output circuits (proportional lift solenoid); analogue sensor data (rheostat); digital sensor data (PWM). Don’t assume that there are hundreds of different circuits out there. Why scare yourself? Learn these four and you’re pretty much set.
What’s more, there are only three faults that can occur in a single wire: open circuit (no current); short-to-ground (too much current, usually); and high resistance (corrosion). If you limit your thinking to these faults and can make a definite decision about what you actually have wrong, then you can calibrate your eyeballs to what you’re actually looking for. If two or more circuits touch (crossed circuits) it’s a little different, but the first place to look is where the circuits come closest together, as in a connector.
Best of all, there are only five voltmeter readings that will all immediately point to the precise fault you have. Two of these readings are “ghost voltage” and “zero voltage”. Ghost voltage is where the meter fluctuates wildly when you read voltage, and this is a definite indication of an “open” circuit. If the voltmeter reads 0.000V and locks in, it means you have a “short-to-ground”. These aren’t typical diagnostic readings, but they work well.
If you limit your use of the ohmmeter and use the voltmeter more effectively, the need to tear a machine apart and use jumper wires is almost entirely eliminated. The voltmeter is more accurate, it doesn’t require you to open up circuits and isolate the circuit, and most importantly, it doesn’t require a prediction. The ohmmeter requires you to decide where to start, and set up a test that only examines a small percentage of the circuit.
The voltmeter requires no such prediction, and if you open the circuit, the meter won’t work. Here’s how I teach it, and this might seem a little backward but it works. Think about what it means to read and see system voltage at a component connector. If you put the meter leads directly into the connector (black lead too) then you’re effectively “ohming out” the entire circuit. If you see system voltage (12V or 24V) it means you can’t have an “open” or “short”.
Simply reading voltage means you get to immediately rule out two out of the only three faults possible. High resistance can be seen with a voltmeter equipped with a tool such as TESlite voltmeter leads (Caterpillar P/N 275-9936). These leads allow you to instantly load the entire circuit with every voltage reading, and if you use them properly, you have the ability to confirm whether the wire is the problem, or not. Corrosion is otherwise invisible to a meter, and these faults can result in lengthy and embarrassing repairs.
Other simple techniques you can use to speed up the process are:
. Use a compass on the end solenoids to see if the problem is electrical or mechanical. Solenoids use a magnetic field to move a plunger, and if the field is there, the compass will point to the retention nut ruling out an electrical fault.
. Use the same compass on the ground cable of the battery to test whether an alternator is charging. If the compass needle swings 180º after the engine starts, it means the current in the cables has reversed. This means that the alternator voltage is higher than the battery voltage, and the alternator is charging.
. Remember that there will always only be one load per circuit. When you read the schematic, if you’re reading from the load to the battery and you hit another load you’re wrong. Backtrack until you find where you made the wrong turn and take another path. Try redrawing the circuit as a straight line from the factory print to eliminate all of the unrelated circuits from your thinking.
. And lastly, as you diagnose, work the problem backwards. Don’t start out hoping to immediately know what’s “wrong”. Start out rather by figuring out first what’s working “right”. Remember that the problem will always be something simple and take minutes to repair, and 99% of the circuit will be fine, and will test fine.
Electrical diagnostics doesn’t have to be hard. We go a long way out of our way to overcomplicate the problems we experience. We aren’t always helped by the procedures the manufacturers give us, and when you add in the sheer size of the machinery and the non-stop pressures to always keep digging, the problems we face electrically can seem imposing. But, if you remember that the circuits and the faults are all simple and predictable, the outcome needn’t be so frustrating.

* Dan Sullivan is a professional vehicle electrician and electrical trainer and the founder of Sullivan Training Systems - the only technician-owned training company dedicated solely to electrical training specifically for mechanics in the heavy equipment and heavy truck industries. He is based in the USA and can be contacted at: dan@brighterideas.org