Avoiding an electrical meltdown

This Volvo D16 engine has a state-of-the-art pad-type mount for the alternator. It also has an automatic tensioner.

The electrical system’s heart is the alternator, and understanding its limitations is the most critical part of system maintenance.

“The alternator is not designed primarily as a battery charger,” says Phil Craft, vice president and partner at Jersey Rebuilding. “A 160-amp alternator could be expected to use as much as 145 amps output for the load, leaving as little as 15 amps for charging. The maximum an alternator would ever be expected to devote to charging would be 25 percent of its output.”

That’s why it’s imperative to maintain a balance between alternator capacity and battery need. Key to doing this, Craft says, is knowing how perishable batteries are. Batteries require maintenance because their reserve capacity diminishes with time. As miles accumulate, batteries become more difficult to charge, thus wasting energy like a worn engine with low compression. The result is alternator stress.

Alternators naturally run hot because wires resist current flow and are crammed inside a small space. As the wires get hotter, they resist current flow more and produce even more heat, creating a vicious circle. Craft says a 160-amp alternator will easily produce 175 amps when cold, but only 150 amps when hot.

Therefore, if there is too much electrical load because of bad batteries, excess heat in the alternator cuts output and its ability to take care of the problem. The alternator will overheat and is doomed to premature failure.

Here are the steps for proper maintenance.

Test your batteries.
Craft suggests load testing the batteries annually, before winter. To do this, disconnect the connectors and use a carbon pile or battery tester to put half the rated load on each. Check the output voltage for 20 seconds of continuous loading to determine whether the battery is starting to lose capacity. If voltage stays at or above 9.5 volts for more than 20 seconds, the battery will get you started. A battery that is dead or close to failing will have its voltage fall off within 20 seconds.

Whenever exploring electrical problems or performing preventive maintenance on the electrical system, test batteries first, says Bruce Purkey, president of Purkey’s Truck Electric and an adviser to many large fleets. Several good-quality battery testers are available at retail, he says.

Even if your batteries pass the voltage test, they may be deteriorating in subtle ways, Craft says. That’s why it’s ultimately smarter, he says, to replace batteries on a regular schedule, before they begin to drag the alternator down. For trucks without abnormal battery stress, this would be every four or five years.

“The owner-operators who are very maintenance-savvy and who I see replacing their batteries regularly have very little electrical system trouble,” Craft says. “Large fleets that try to skimp are the ones that end up having breakdowns.”

Craft and Purkey strongly advise against replacing just one failed battery in a group of three or four. Instead, replace all at the same time to avoid putting a disproportionately heavy workload on the newer, more potent battery.

When grouping batteries, be sure to use the same brand and rating. Batteries of similar rating, but different designs, may fail when working in tandem. Use high-cycle, dual-purpose batteries, rather than either deep-cycle units or high cold-cranking-amps designs, Craft says. High-cycle units store energy more efficiently and recharge quickly, thereby keeping the alternator from working too hard.

Regular battery maintenance also includes removing caps on traditional designs, refilling the cells with distilled water, and checking even “maintenance-free” batteries to see whether the color in the indicator eye is still green, which indicates the right specific gravity. The eye typically turns black when there is a problem.

Frequently clean the top surface of each battery with a mixture of baking soda and water to remove dirt and grime, which can add to battery charging load. Regularly wire-brush the terminals because “corrosion is a no-no,” Craft says. Corroded or loose connections will keep the voltage from getting to your starter or accessories and hamper battery recharging after the engine has been cranked.

It’s also important to check the hold-down brackets, says Scott Faris, Detroit Diesel’s technical services manager. Loose batteries are susceptible to vibration damage. Make sure the connections stay tight; some lead connectors stretch continuously.

Inspect the wiring.
Will Dunn, service engineer at Daimler Trucks North America, recommends inspections of battery and starter wires and connections every 100,000 miles for trucks that are regularly exposed to road chemicals and 200,000 miles for trucks that exclusively run in warm climates. Inspect and clean all exposed ground and power connections.

Protect the connections with a brush-on dielectric (insulating) enamel and the battery terminals with dielectric grease, Dunn says. Inspect the smaller alternator wiring and connectors at the same time because they are just as critical.

Dunn also recommends inspecting the wiring harnesses for insulation worn either because tie-downs have failed or because the wiring may not be adequately fastened. If insulation is damaged, new wire should be shunted in with crimped and soldered connections covered with heat shrink material.

Other problems that can cause a truck to shut down or run poorly are low electronic control module voltage, a poor ground or resistance in sensor wiring. In those cases, a similar inspection and repair process is advised.

“Everything harmful you can do to wiring happens on a truck,” Purkey says, listing vibration, extreme temperatures and exposure to road salts. “If you have 14 volts at the alternator output and only 12.5 volts at the battery positive terminal, those batteries are never going to get charged back up after a start.”

Also check, clean and repack tractor-trailer connectors with dielectric grease every 15,000 miles.

When pressure-washing, avoid the alternator and all the electrical connectors because they are not sealed against water under pressure, Dunn says.

Purkey highly recommends testing and maintenance that follows a logical sequence. The second step in checking how the system is performing is to do that check of battery and starter wiring.

Test the cables.
Craft and Purkey recommend testing cables with the engine running to ensure the alternator is charging the batteries. All lights and accessories, including the air conditioner, should be on to maximize the electrical load.

Then measure the voltage drop from the alternator connection to the batteries by wiring a voltmeter between the alternator output terminal and the lead battery positive terminal. The meter should indicate no more than half a volt difference.

Visually inspect the cabling, Dunn says. Replace cables if the connections look worn. Always include ground straps, especially where they ground to the frame. All connections must be clean, tight and free of damage from corrosion. Remember that a voltage drop across ground straps does just as much to restrict flow as on the positive side of the system. If you’re in doubt about the condition of a ground strap and its connections, measure its voltage drop.

Inspect the drive system.
The most damaging thing that happens to an alternator is a slipping belt. The slippage friction goes right into the alternator shaft, or “rotor,” Craft says. This “can create so much heat, it will boil the grease out of the shaft bearing, causing it to lock,” Purkey says.

The new automatic belt tensioners are a great solution to this problem, Purkey says. Although belts have been greatly improved and manufacturers insist they won’t stretch, the reality is they might need an occasional adjustment if there is no tensioner.

With an automatic tensioner, markings show whether it’s at the right angle, which indicates the torque and belt tension. The mark on the tensioner must be located between the two on the block or mounting. Replacing a bad belt with a new one of the right length is critical, or else the marks won’t line up correctly. As the belt ages and inevitably stretches, the markings will show a slackening in tension and the belt will need to be replaced.

Pad type mounts, where the alternator is bolted to the block at four places, minimize vibration. If your alternator is mounted at top and bottom, away from the block, pay careful attention to bolt quality and torque, usually 80 lb.-ft. to 100 lb.-ft.
You can use a $25 click-type gauge to get proper belt tension on traditional systems. Testing tension strength with your thumb is no more accurate than thumping a tire to check its pressure, Purkey says.

Check your owner’s manual for any specific alternator maintenance requirements. The Bosch T1 alternator used on Detroit Diesels, for example, should get a new voltage regulator assembly, including brushes, every 100,000 miles and new bearings, seals and a new spacer ring every 200,000 miles, Faris says. Similar recommended maintenance on any alternator is bound to improve reliability significantly.

Craft’s shop has a test stand that measures voltage and amp output as the alternator is run up and down the rpm range. It also has a gauge that will indicate extraneous waves in the output current. Too much wave activity, or “high ripple,” will cause the alternator to overheat, experts say.


Alternator test tools: voltmeter and ammeter
NO-LOAD TEST. Start the engine and idle it with all electrical loads off. Run it for one minute to stabilize the alternator, then check the output voltage all the way from idle to governed speed. Voltage should stabilize at 14 volts and should not vary more than 0.2 volts. If voltage rises with rpm, the regulator has failed and is not controlling it properly.

FULL-LOAD TEST. Apply maximum load and run the engine at a fast idle. The voltage will drop to about 12.5 volts. Use an inductive ammeter, one that just clips around the output line that runs from the back of the alternator to the battery positive connectors. The amperage should be plus or minus 10 percent of the alternator’s rating, such as 160 amps.

An existing unit can sometimes be saved if it just needs brushes, bushings or a new armature, which is the rotating shaft. But if the stator windings are bad, the unit should be replaced.

If you replace it, you can choose between two types of alternators: those with brushes and those without, says Bruce Purkey of Purkey’s Truck Electric. The brushes carry current into the rotor so it can create a powerful and controllable magnetic field to make current flow through the stator windings.

Units with brushes are more efficient and should be used if you’re concerned about horsepower draw and fuel consumption. These units also last longer, Purkey says.
It’s vital to install a larger alternator, and the wiring to go with it, if you add a number of lights to your truck, Purkey says.

If you use your batteries to power your sleeper while shut down overnight, you need an inverter or other power supply with a low-voltage cutoff.


When best practices don’t protect you
Almost all electrical-related breakdowns can be prevented if you replace batteries periodically, check the alternator drive belt and mounting or tensioning system for torque and alignment, perform routine maintenance on the alternator, and inspect and repair wiring and connectors. But, just in case, add these to your in-truck toolbox:

  • A new drive belt, along with the necessary tools needed to replace it.
  • A box of electrical connectors and a crimping tool, along with some electrical tape, battery terminal cleaning tools, and a voltmeter. You may be able to locate frayed insulation or an open circuit with these and make repairs.

You also should consider installing an APU with a high-capacity alternator capable of charging the vehicle batteries while you drive. If your alternator fails, the APU’s power may be able to get you home, or at least to a repair facility.

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