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Your truck and the electrical revolution, part two
Diagnosing and fixing common problems with your truck’s electrical system

With the electrical and electronic revolution in trucking, it is increasingly important for operators to know their vehicles’ electrical systems and how to do basic diagnoses. Last month we reviewed basic electrical terms and properties. Using a water-in-a-hose analogy, we examined relationships between force (volts), amount of flow (amps) and resistance to flow (ohms). We described series and parallel circuits, magnetic fields around a wire carrying current, sources of current and grounds. Grounds complete circuits so current can flow.

Now, let’s look at some ordinary problems and basic diagnoses, the right way to use tools and some common (and sometimes costly) mistakes. When I mention some of the tools needed to do a proper diagnosis, don’t run out and spend hundreds of dollars for something you may use only once or twice. You should know about the tools and procedures so you can evaluate service providers.

Tools for basic electrical diagnostics
The first, and the one you might want your service provider to have available to use for you, is a carbon pile load tester. They’re fairly expensive, so fleets, dealers, truckstops and independent repair shops should have them, not individuals. If a shop doesn’t, find one that does for electrical repairs. The other tools are far more affordable. You can get a decent multimeter for under $100. It may not stand up to the abuse of a shop environment, but it will be more than adequate for occasional use. Clamp-on multimeters provide extra flexibility to get reading from wires without having to poke holes in insulation. A clamp-on also can replace a continuity tester in some applications. A hydrometer or, better yet, a refractometer, will help you determine a battery’s state of charge. The final tool you should have for diagnosis is a continuity tester.

How each of the tools operate
The load testertests batteries by creating an artificial load great enough to simulate the demand of starting. It creates a circuit with variable resistance by passing current through carbon discs. A control knob increases or decreases the load. Meters indicate current flow and voltage. Cables are used to connect the tester to the battery.

Multimeters combine the functions of ammeters, voltmeters and ohmmeters in one convenient case. Analog models have mirror surfaces on the dial faces, so you can look straight down at the dial for accuracy. Zero the dial each time you change modes or settings. With analog meters and some less expensive digital ones, you have to change scale settings to match the expected results. If, for example, you expect a wire leading to a 55-watt headlight to carry about 5 amps at 12 volts, you should use the “20 A” setting (up to 20 amps). If you use the 20 mA setting (up to 20 milliamps, or 0.020 amps), your 5-amp current will blow the resistor and ruin the instrument. Better digital multimeters adjust automatically and provide an accurate digital reading so you don’t have to interpret what you see.

Continuity testers do one simple thing. They indicate if a circuit is open or complete.

Hydrometers and refractometers test the specific gravity of fluids. Sulfuric acid, the electrolyte in most batteries, is heavier than water. By measuring the specific gravity (fluid density), you can determine a battery’s state of charge. Batteries create electricity by converting lead to lead sulfate, taking sulfur from and diluting the acid. When the current is reversed during charging, the lead sulfate returns the sulfur to the electrolyte, recreating sulfuric acid. Specific gravity of a battery relates directly to the state of charge, or voltage available between the battery posts:

Specific Gravity
% of charge

These figures are for batteries at a standard test temperature of 80 degrees F. For each 10 degrees above 80 F, add 0.004 to your reading. Below 80 F, subtract 0.004 for each 10 degrees. While there may be enough acid left to produce some no-load voltage, a discharged battery cannot do any useful work.

How the tools work 
A continuity tester, as the name implies, determines if a circuit is continuous or broken between the two points where its leads are connected. It has its own battery and a lamp that glows to signal that the circuit is complete. It says nothing about high resistance due to corrosion or too small a wire, low voltage or insufficient current. It is merely a “go/no go” type device. Never probe into wire to check a circuit. Even the tiniest pinhole will let moisture and chemicals in, and the wire will corrode.

Check specific gravity by drawing battery acid into a hydrometer and reading the state of charge on a float. The denser the acid solution, the higher float will be. Be very careful handling both the glass tool and battery acid. A safer tool to use, and a more accurate one, is an optical refractometer. Light is bent (refracted) differently through different density fluids. A refractometer needs only one drop of battery acid on its glass plate. Close the cover, look through the eyepiece and read state of charge at the line between light and dark. This same tool can be used to accurately measure engine coolant.

Using the tools
Before testing any system (starting, lighting or anything requiring battery power) test the battery to make sure it has a charge. Turn the lights on for one minute to remove any surface charge. When checking fluid, test each cell in each battery. Variation from cell to cell should be 0.05 or less. If there is 0.10 difference, replace the battery. If cells have about the same specific gravity but are below a 75 percent charge, recharge the batteries before proceeding.

If your batteries run down often, you may have high parasitic (key-off) loads. Parasitic loads, named for parasites that live off other creatures, are loads that continually drain your batteries whether the key is on or off. Many are necessary and are usually small enough so they don’t harm the batteries. Examples are current to the clock, satellite communications, some electronic control modules (ECMs) and heaters within ECMs. They ensure operation in cold weather. Electronics don’t do well below zero.

Other sources of current drain could be: a lamp left on, a refrigerator or freezer, an “instant-on” TV, radios with pre-sets or any device left on continually. Make sure the key is off. Test for parasitic loads by disconnecting the cable between the chassis ground and the negative post of the last battery in line. Set your multimeter to 0-20 A and connect the leads between the battery and the cable: red (positive) to the negative (minus) battery post, black (minus) to the ground cable. The multimeter will tell you how much current flows with the key off. If it’s more then one or two amps, you have a significant drain. Some devices, like refrigerators or heaters controlled by thermostats may be “off” when you test, so try to test when they are cycled “on” or you may get false readings. Here’s a hint. When I sit for long periods, I disconnect my electric refrigerator (or turn it off) and put a bag of ice inside. It works just as well as an ice chest. If you find excessive current drain, identify the circuit and remove the problem device.

Sometimes current drain is from corroded or damaged cables. Any blue, green or light gray coating visible on cable ends is a sign of internal corrosion. Even dirt and grime can conduct current between battery posts. Keep batteries and cables clean by brushing them with a solution of three ounces (by volume) of baking soda in one quart of water. Clean with the ground cable disconnected. Flush thoroughly with water and let things dry before reconnecting the cable. If you see signs of corrosion in the battery tray, check for a cracked or leaking battery case.

When you are sure your batteries are in good shape, you can diagnose any other electrical system on your rig. Often, problems arise with the charging system, the starting system, lighting, accessories, and electronic controls. Electronics are best left to specialists, but often the problem is not the computer, but rather power, ground and sensor input. These depend on good wiring, or there will not be enough current to the computers.

Volumes have been written about how to troubleshoot electrical systems. Space constraints and my limited knowledge prohibit turning this into a technical manual. But here are some points to remember. The keys to any successful electrical diagnosis start by examining a circuit or device that won’t work for any obvious physical damage. If none is found, isolate and test each element.

Helpful hints
Whenever you work on wiring, seal all connections from moisture. The days of twisting two wires together and wrapping them with electrical tape are long gone. When repairing wiring on trucks, use the largest wire you can find with properly sized terminals. The best terminals have heat-shrink covers. Once the terminal is crimped to the wire, use a heat gun or, in a pinch, even a cigarette lighter (do not let the flame contact the wiring) to melt the sealant and shrink the cover to form a moisture proof seal. For terminal-to-terminal connections, put a section of heat-shrink tube on one side of the wire before making the connection. Slide the tube over the connection and heat it to seal the entire repair. When making a terminal-to-socket connection, use a liberal amount of dielectric grease to seal moisture out. I keep Truck-Lite NYK in my toolbox. The blades of the connectors will wipe clean as they come together, leaving the remaining grease as a barrier against moisture and road chemicals.

If you find broken lights, frayed or missing wiring or corroded connectors, you won’t need to test a circuit to find faults. If you do test, make sure there is only one item in the circuit you are testing. For example the starter circuit runs from the battery through the solenoid to the starter to ground. If you test and find trouble between the battery and the ground, you won’t know where the fault is. Test each part (battery to solenoid, ignition to solenoid, solenoid to motor, motor to ground) individually. When checking current flow to the starter, use a clamp-on ammeter so you won’t have to disconnect cables. The tester reads the magnetic field surrounding the cable when current is flowing.

Other resources
Before closing, I’d like to mention a few resources available either for you or your service provider. Bruce Purkey developed Intelli-Check, a handheld alternator analyzer that can diagnose a truck’s charging system in just five minutes. It’s available from Purkey’s Fleet Electric, 1-800-219-1269, or at any Delco-Remy authorized dealer. This tool picks up things that are often misdiagnosed or overlooked.

Truck-Lite has a CD-ROM that, besides containing their complete catalog, has a section titled “Why Lights Fail.” It shows common lamp failures so you can take corrective actions to prevent the failures from recurring. After all, if you don’t correct the cause, the item will continue to fail. The CD also has Truck-Lite’s Vehicle life Cycle Cost Estimator so you can evaluate your lighting practices and explore alternatives without spending a penny. For a free copy, call Truck-Lite at 1-800-562-5012 or go to

Sears has a great little book in its “How-To” series. The “Multi-Tester Book” is available from Sears Hardware. It has a section on automotive applications.

Finally, Delco-Remy America has a manual, “Diagnostic Procedures for Heavy-Duty Electrical Systems.” Call 1-800-DRA-2000, ask for Technical Information and request publication DRA/DP1029.