Basic electrical theory, which many of us learned early on in our mechanical education, is important for proper electrical diagnostics of school buses. This, in conjunction with the use of a dependable digital volt ohm meter (DVOM), will help us handle electrical problems easily.

Failure to follow simple routines can result in more work than is necessary and can create additional and unnecessary problems. One problem is the replacement of good parts.

If you do not know how to perform basic electrical tests, learn how — they are quite simple and only take a short time to learn. An inadequate understanding of basic automotive electrical principles and theory can cost transportation departments thousands of dollars in already strapped budgets.

Start with the basics
The additional electronics and new system circuitry, including multiplexing, in school buses can be daunting when trying to diagnose a problem, but they do not have to be. Remember the basics, and start with the basics first. Two important rules to remember are:

1. Components (circuit loads) must have clean power and ground to operate as designed.
2. Most electrical failures are the result of high resistance in wiring and/or connections, not a faulty component. When a component does fail, many times it is the result of high resistance in wiring and connections. I have witnessed, all too often, repeated replacement of a component that was not functioning as designed, not because it was a defective component, but because high resistance in the control circuit caused it to malfunction. Failure to repair the source of the problem causes higher-than-necessary repair bills, excessive down time and a lot of frustration.

Power side faults
Testing for voltage drop within a circuit will point you in the right direction and narrow down the location of above-normal resistance.

Normal voltage drop between two in-series connectors should only be a few millivolts. This takes into account the normal inherent resistance in the wire and the joint resistance in the connectors or switch contacts.

One method of measuring voltage drop is shown in Figure 1. The first test measures available battery voltage. This is the reference voltage you should use for the entire test. Measure at the battery post, not the terminal. Voltage readings will drop slightly as the probe is moved down the circuit.

As seen in Figure 1, test point #4 shows a large voltage drop, which indicates higher-than-normal resistance between the last test point (#3) and #4. Simply backtrack now to locate the resistance point in the circuit.

In an actual circuit on a school bus, you may have to separate the wires in the harness and do a visual inspection along the length of wire inspecting for penetrations or corrosion. One very important note: The circuit being tested has to be powered up and a load applied in order to measure voltage drop.

Each load device requires a specific amount of current (amps) for it to operate. If resistance is higher than the normal few millivolts anywhere in the circuit, the required current needed to power the load component is held back (restricted) from completing the circuit back to ground, and the load fails to operate as designed.

Powering up the circuit — applying a load to it — then measuring voltage at connection points from the power supply toward the load, and through the load to ground, will indicate where the problem lies.

More diagnostic tools
There are two tools in addition to a DVOM that I utilize frequently when diagnosing circuit issues.

One great tool is called the LOAD PRO test lead. It connects directly to your DVOM in place of the standard test leads. Your DVOM will still read voltage, but the tool has a built-in load feature that allows you to apply a load in the circuit by depressing a button on the red lead. This is very helpful in finding high resistance in a power wire prior to the component load or one that has the load device removed from the circuit. Testing at the power and ground connector that plugs into the load (load removed) tests both power and ground sides of the circuit.

The other tool I use frequently is the Power Probe III circuit tester. This tool not only reads voltage in an LCD display, it tells you if the circuit is grounded, powered or not powered. It also enables you to ground or power the circuit by the flip of a switch and is fuse protected. This tool is great for powering up open circuits and checking normal circuit operation — such as heater motors, lighting, window buzzers, etc. — by applying power to the circuit at any connection point or applying a ground where a ground circuit is questionable.

One example of this was when we had no dome lights in one section of a bus. After locating the correct output plug and jack at the back of PCB#2, power was applied to the power pin in the plug by depressing the button on the tool with the plug removed from the jack. This caused the lights to turn on and indicated that the circuit out of PCB#2 was working properly, eliminating questions about that side of the circuit. Since the Power Probe tool also reads circuit voltage on the LCD display, we then turned our focus to the input side of the circuit to discover that we had an open circuit  inside the circuit board itself. Input was being sent to another jack by the light switch, but no output was coming out of the output terminal at the output jack.

Ground side resistance
When measuring voltage (circuit turned on), normal voltage should be used up by any load that is operating as designed. Voltage on the input side of the load should read battery voltage minus normal resistance of a few millivolts. Voltage on the ground side of the load should read zero.

On a good circuit, all voltage is used up to power the load. On a circuit that has resistance in the ground side (bad ground), voltage on the ground side (circuit turned on) will show some voltage, as illustrated in Figure 2. This is voltage drop.

As shown in Figure 2, 3.5 volts are reading on the ground side of the load. This means that the difference between battery voltage and 3.5 volts is being used up to power the load, and 3.5 volts are lost through resistance. In this case, the motor in the illustration would run slow or not at all.

If you find an instance like this where voltage reads on the output side (ground side) of the load, the problem lies in the ground wire or connector.

Recently, a bus was diagnosed for an intermittent transmission problem. The transmission selector keypad indicated “XX” in the LCD display. It would not shift into gear, and an occasional “check engine” lamp would appear.

Technician A suspected a faulty transmission control unit (TCU). Technician B suspected high resistance in the power or ground circuit to the TCU.

Technician B performed voltage drop tests on the TCU circuits and found two high resistance areas in the trans./batt. wire leading to the TCU. After repairing these resistances, the system worked normally. Purchasing and replacing a TCU would have wasted considerable money.

Electrical preventive maintenance
Electrical system preventive maintenance (PM) should be a part of your overall PM program. Doing electrical PM will help prevent component failures and increase uptime.

Always perform a thorough inspection of the power and ground circuits before any other circuit diagnosis is performed. Performing this inspection may solve the problem at hand. I recommend doing this inspection in the following order:

1. Visually inspect the batteries for cleanliness. Are the cables tight? Any moisture on the battery between the battery posts is a potential voltage draw. Are the terminals and cable ends clean, free of corrosion, tight and coated with a battery sealer? Terminals should be removed at least once a year, terminals and posts cleaned, batteries tested, then reassembled and coated. Regardless of battery type, always coat battery terminals and connections with a battery terminal sealer.

2. What is the condition of the clean power positive and negative wiring and fuses? Have you ever wondered what the smaller gauge wires are attaching to the battery terminals or running off of the back of the battery cut-off switch? These smaller wires are direct feeds to the engine control module (ECM) and the TCU. They must have their own direct contact with the battery terminals. These smaller wire connections must be clean, tight and have good clean fusing. These are critical and are often overlooked. They are often the sole source of no-start conditions. Intermittent no-start issues may be caused by high resistance in these wires. If the ECM and TCU do not receive clean power and ground, they will not power up or can lose power intermittently, causing the motor to die or the transmission to malfunction.

3. All battery cables leading away from the batteries, including grounds, must be protected against chafing and secured properly.

4. Frame and body ground cables must have clean, tight connections on the frame and body. This means the terminal lugs must be bolted to bare metal and not a painted surface. There must also be a sufficient-size cable running from the frame to the engine block. Everywhere a cable connects must be inspected, including the battery cut-off switch, starter terminals and alternator. Some models of buses use a dual power relay box, a shunt or a cable junction post inside the frame rail, which are unprotected against corrosion and are often overlooked. These items should be disassembled and cleaned at least annually. Only when you are 100% sure that all cable and wire connections are clean, tight and sealed should you go to step 5. If replacing an alternator with a larger-output alternator, be sure that the attaching cables are sufficiently sized to carry the additional current.

5. When the inspection and repair has been completed, measure and record battery voltage at the batteries. If battery voltage is lower than 12.5 volts, a problem may exist with the batteries and/or charging system. The battery and alternator should be tested at least annually. The battery voltage reading becomes the base voltage for all other diagnostic testing. The bus power supply needs to be in good condition prior to troubleshooting any other electrical malfunction. Never replace a battery or alternator until the battery is fully charged and the system tested with full battery power. Cables and connectors must be clean, tight and the proper size.

Don’t overreact
Quite often technicians will overreact to an electrical problem due to not fully understanding the circuit function. Before diving into a problem blind, take a few minutes to think about the system you are working on.

If necessary, draw it out on a piece of paper, including circuit numbers. For accuracy, obtain a printout of the schematic of the circuit. Most circuit schematics for school buses produced during the last 15 years are available from the OEM websites. Learning to read schematics and understanding the meaning of the various symbols therein only takes a short time.

The basic tools needed for electrical diagnostics include a DVOM, a portable load device such as a headlight, a couple of test leads with alligator clips, some back probes for connector testing and an understanding of basic electrical circuit design. Other tools are available that may be helpful and increase your ability to diagnose.

Keep in mind that there are only three types of circuit failures to deal with: an open circuit, a short or high resistance.

When you master the proper approach to electrical diagnostics, you will be the technician everyone comes to for electrical issues — and you will be well equipped to help your operation save money on maintenance.

Brad Barker has more than 35 years of experience in school bus maintenance as a shop manager and technician. He has written numerous articles for SBF.