Springtime is a transition time for cooling and electrical systems. Is your truck ready? - Photo: Jim Park

Springtime is a transition time for cooling and electrical systems. Is your truck ready?

Photo: Jim Park


If you have a bad alternator, chances are you’d never recognize the problem in the winter when it’s cold. But the problem will show up pretty fast once the weather warms up and electrical loads increase with air conditioner use. Unless you know what to look for, a dying alternator can be hard to spot in the winter.

Cold batteries don’t charge well because of their chemistry. As truck electrical system expert Bruce Purkey will tell you, “It’s like trying to push energy into a cinder block.”

A typical flooded-cell battery at zero degrees will only take about 2 amps per hour. You might have a 150-amp alternator, but the whole battery pack will take only 8 amps (4 batteries, 2 amps/hour each) Everything may look right, with the volt meter right up there where it’s supposed to be at 14 volts trying to push something into the batteries, but the batteries are losing charge faster than the alternator can make it up, explains Larry Rambeaux, a sales application engineer at Purkeys.  


Dead batteries aren’t always completely dead. They may only need reconditioning. - Photo: Jim Park

Dead batteries aren’t always completely dead. They may only need reconditioning.

Photo: Jim Park


“Doing a jump start for a dead battery isn’t going to solve the problem,” he says. “It will get the truck going again, but unless it’s run for a long time, it’ll be just as dead as before the next time you try to start it.”

Unfortunately, the same thing might happen in summer when drivers start using air conditioning — except now it might actually be an alternator problem. You might miss it entirely because the system appears to be working properly.

“In winter, people get a false sense of security because they see the volt meter is right up where it’s supposed to be,” Rambeaux says. “They can easily miss an alternator with a bad diode that has lost performance, but because the loads are so small, everything looks fine.

“Summertime is when you’re likely to have an alternator change for no reason,” he adds. “If the batteries are low, the alternator will put out everything it can, and the batteries will take everything they can get, but the voltage will be down at 12.5 volts rather than 14 — and it could be really hot to the touch. That leads most people to assume the alternator is bad. That heat is actually just a byproduct of putting out that much energy trying to top up the four batteries.”

For example, if one diode fails on a 135-amp alternator, it will lose about one-third of its output. “A 90-amp alternator is probably fine in the winter, but in summer when all the extra loads come on and the batteries are more receptive to taking a charge, the alternator cannot keep up, and suddenly you have a road call,” says Rambeaux.

Another factor that affects alternator performance and battery life is all the other electrical demand. When manufacturers build a truck, they put in an average battery that will service the cab and chassis requirements with a little bit extra.

“The batteries are usually slightly over-spec’d, but the OEM won’t know, unless you tell them, if it’s going to run a liftgate or an in-cab refrigerator, or if the driver will be using an inverter to power hotel loads,” says Joe Puff, vice president of truck technology and maintenance at NationaLease.

“Those things create significant electrical demand, so you have to compare the draw with the alternator output and the drive times so you’ll have ample time to recharge the batteries. If you have added electrical loads to the original spec and not changed the battery or alternator spec, you could be setting up for a failure.”

In some cases, that may involve adding additional batteries or stepping up to a 300-amp alternator. But, Puff cautions, “Batteries can only charge at a certain rate.”


Alternators sometimes seem fine when they are faulty and show puzzling results when they are working fine. Testing is required to determine the alternator’s condition before replacing it. - Photo: Jim Park

Alternators sometimes seem fine when they are faulty and show puzzling results when they are working fine. Testing is required to determine the alternator’s condition before replacing it.

Photo: Jim Park


If you run trucks through the winter with lift gates and hotel loads and you have weak or inadequate batteries, or an alternator that’s either not up to the task or not at 100% performance, summertime is when the problems will show up.

Springtime Electrical Check

If you’re unfamiliar with proper battery, charging, starting and cable inspection and maintenance, check out the Technology & Maintenance Council’s Recommended Practices 129A and 132B. If your definition of a battery inspection means opening the battery box cover to see if the batteries are still there, you may be surprised by how much you’re overlooking.

Rambeaux suggests using an electronic tester rather than a carbon pile tester, because they can do a much more thorough job and provide greater insights into the condition of the electrical system.

“You have to start with testing the batteries to make sure they are at a minimum state of charge. If the batteries are at 100% SOC, the alternator isn’t doing anything,” he points out. “You have to test with a good state of charge to make sure the alternator is regulating properly, and you have to test the system when it’s under load, so the alternator is working.”

Using a carbon pile tester is complicated and time-consuming, and it relies on the technician to perform the test properly. Rambeaux says the better and significantly more expensive high-quality electronic testers can run all the required tests more or less automatically and consistently, which produces more accurate and documentable results.   

Considering the whole of the electrical system — alternator, starter, voltage regulators, cables and the batteries — there are a lot of moving parts, and each can affect the performance of the other components. Rambeaux says hundreds of alternators, starters and batteries are taken out of service needlessly, simply because they weren’t tested properly. Springtime is an ideal time to turn over a new leaf on electrical maintenance, and all the money you save on non-warrantable replacements will more than pay for the proper tools and the training to get this vital inspection right. 





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