Understanding Electrical Diagnostics
So your forklift won’t move, where do I start? Whether electric powered or engine powered the problem could be your electrical system. In this article I will explain the basics of electrical diagnostics.
Because electricity is not visible, it is not easy to diagnose. Compounding the problem is that electrical schematics and diagrams can be confusing. I will try to equip you with the tools needed for proper diagnosis. A technician has to rely on those tools to show where electricity is, or where it isn’t, and then draw accurate conclusions from that knowledge.
If you can learn to “think” like electricity and skillfully use your multi-meter. With an understanding of some basic electrical principles you can then apply them to testing electrical systems.
Have you ever witnessed a technician spending time looking at wiring diagrams or drawing sketches on a piece of paper. He was actually diagnosing the problem at hand. A big part of electrical diagnosis is done at the bench before ever lifting a wrench. A multi-meter (VOM) is the second most useful tool in your arsenal after your brain.
Computer controlled systems offer another complexity to the way electricity works. It is easy for technicians to replace a control board, often because they don’t understand what it does or how it works. This often results in unnecessary expense and can also cause the new part to be damaged because the original problem was not corrected.
By understanding what a control board needs in order to function properly and then ensuring that it has everything it needs to do its job, a technician can successfully determine whether the board needs to be replaced.
OK, so let’s get started. A simple understanding of electricity and basic terms is in order.
What is VOLTAGE?
Voltage can be thought of as electrical pressure. Voltage is the force that pushes electrons along an electrical conductor. The measurement for voltage is called the volt. There are two types of voltage: alternating current and direct current. In forklift circuits we are use both direct current and alternating current.
Direct current voltage (DC voltage) is voltage that is applied in one direction all of the time. Most forklifts use direct current to get their jobs done. Industrial forklift batteries or automotive type batteries supply direct current to the electrical circuits.
Alternating current voltage (AC voltage) is voltage that is applied in both directions in an alternating fashion. This is similar to the electricity found in the electrical outlet of your house wiring.
Some modern forklifts use AC powered motors as they require less maintenance and can be more efficient than their DC counterparts.
What is CURRENT?
Where voltage is the driving force in an electrical circuit, current or amperage is the measurement of how much electricity (electrons) is flowing in a circuit.
We measure current in amperes (amps). Current is the quantity of electrons passing a single point at any given time. Current can be measured in both AC and DC circuits.
What is RESISTANCE?
Resistance is the opposition of flow in an electrical system. This is sometimes built into the circuit for various functions. All loads create resistance that is normal in a forklift circuit. Resistance can also be caused by lose connections, faulty components and faulty wiring. Understanding resistance can be a key component in understanding how to diagnose electrical circuits.
Resistance is measured in OHMS Ω.
How can we see these different parts of electricity? Using a VOM we can measure the existence of voltage, amperage, and resistance.
To measure voltage; place the black lead of your meter in the com jack of your meter and place the red lead in the V jack. Now adjust the meter to AC or DC depending on the circuit you are testing. Set the meter on auto range. If your meter does not have this functionality, you should set the meter to the highest voltage that you expect to be reading.
There are two types of voltage measurements that are useful in forklift diagnosis: available voltage and voltage drop.
Available voltage is voltage that is present at any given point in the circuit. You can measure available voltage by placing the red meter lead at some point in the circuit (switch contact, fuse, connector, etc.) and the black meter lead on a common terminal. This test will tell you how much voltage, or electrical pressure, is present to do some work at a given point in the circuit.
The available voltage test is good for determining if you have voltage present, but it does not do much in the way of diagnosing what is wrong with the circuit.
Available voltage is a good test to use when you want to see whether voltage is present at a point in the circuit or not. You can think of the available voltage test as a "go, no go" type of test.
Do not use the available voltage test to determine whether a circuit is functioning properly or not. There may be voltage present at a point in the circuit, but that does not mean that the circuit has what it needs to work.
The second voltage test, and by far the most useful, is called a voltage drop test.
We say that voltage is “dropped” when some form of load uses all, or a portion of, the voltage available.
Loads come in the form of motors, solenoids, contactors, lights, etc. Corrosion, open circuits and damaged contacts can also be considered loads on the system as they can cause voltage drop also. When a load of this type “uses up” or drops voltage, then it may not leave enough available voltage to the rest of the circuit for the circuit to work properly. A voltage drop test will help you to isolate where voltage is being dropped, or used up.
Unlike resistance measurements where the circuit is not turned on, a voltage drop test is performed with the circuit working or turned on.
To perform a voltage drop test you must place the meter leads at two points within the circuit. Remember, during the voltage available test we place the leads at one point in the circuit and the other to common. In the voltage drop test, both meter leads will be placed in the circuit.
With the meter leads placed in the circuit, you are now measuring how much electrical pressure, or voltage, is present between the meter leads. If you measure any voltage at all on the meter display, then there must be some sort of load between the meter leads.
Your next job is to determine whether the load that is represented on the meter display is a load that is supposed to be there, or whether it is a load that may be harmful to the circuit. Remembering a few simple rules will allow you to determine if the voltage drop you are measuring is "normal" or not.
A meter reading of less than 0.1 volts represents a wire, or switch that is operating normally.
A meter reading of full source voltage represents all of the available voltage being dropped. This could be due to an open wire, or it may be a "normal" load that happens to be the only load in the circuit.
The third measurement that can be useful in electrical diagnosis is the measurement of current, or of how many electrons are flowing. Current is measured in amperes, or amps for short. Like the voltage drop test, an advantage of measuring current is that the circuit will be performing its function, making this a "real world" test.
The technician must first determine what current or amperage draw is normal for the component or circuit he or she is working on. Current specifications for even common circuits can be hard to come by. If specifications are not known they can sometimes be found printed on the component as on a coil or motor. You can also compare to a known good component.
The second drawback to testing current during diagnosis is in that the meter must be installed in series with the circuit you are testing. Since current measurements are actually measuring how many electrons are flowing in the circuit, all of the electrons in the circuit must pass through the meter. This means that the meter must become part of the circuit. An ammeter must be installed in series with the circuit, at a location where the maximum current within the circuit will flow through the meter. This involves opening the circuit up and connecting the meter leads in series with the circuit.
There are a number of good locations for opening up a circuit up to install the meter, with the most popular, and often the easiest, being at a circuit fuse. This is done by removing the fuse and installing the meter leads on either side of the fuse connectors.
The third drawback to using current as a diagnostic tool is that you may not know how much current is flowing before you install your meter. The current flowing is often more than what your meter can handle. Although a quality meter is fused, often these fuses will blow when trying to measure current because the circuit being tested uses higher current than what your meter is designed for.
To get around these last two drawbacks, technicians will use an "amp clamp" or an inductive ammeter. This device uses the electromagnetic field that forms when electricity flows through a conductor in order to determine how much current is flowing in a circuit.
Amp clamps have two significant advantages: firstly the circuit does not have to be opened up to install the ammeter, and secondly the amount of current flowing in the circuit cannot damage the meter. The disadvantage is that you must purchase an amp clamp that measures the range of current you expect to find. It is not possible to purchase a single clamp that will measure all current ranges.
With these basics, a good Multi-meter, and a wiring diagram or schematic the technician can navigate circuits to determine where voltages or currents exist in there correct specifications. Before replacing components a good mechanic will assure that all necessary wiring and circuitry is in place that could cause component to fail.