Three-part harmony

| October 01, 2006

The test stand drives the alternator with its own motor. The technician can easily vary the rpm to see how the alternator reacts to speed changes, a good test of regulator performance.

Your alternator has a lot in common with the giant dynamos that power the U.S. electrical grid. In earlier times, cars and trucks had generators – primitive direct current machines that put out so little power at idle they relied entirely on the battery when sitting at a traffic light. But as electronic technology developed the diode – a one-way electrical valve – smart automotive engineers realized they could produce super generators for cars and trucks. These “alternators” would generate alternating current like you use in your house and then convert it to direct current to operate your truck and charge your batteries.

When compared with generators, they’d be more efficient, and would have much more consistent output with changes in engine rpm. This way, the batteries would have almost nothing to do but get the vehicle started. The result would be far fewer battery and electrical system troubles.

We consulted Phil Craft, a vice president and co-owner of Jersey Rebuilding Service in Farmingdale, N.J., and Bruce Purkey, of Purkey’s Fleet Electric, of Rogers, Ark., for the best answers when it comes to preventing, finding and curing electrical system troubles. Craft informed us that alternators themselves are generally very reliable – provided the bolts are tight and the belts aren’t slipping. Problems crop up most of the time when the batteries are not up to snuff, or there are other system problems. His view: You need to see the truck’s electrical system as a whole and to learn to take care of it.

“When people think of maintenance, they think of changing the engine oil and the various filters,” he says. “The electrical system needs maintenance, too, but it’s often neglected.”

Purkey, who solves many of the knottiest electrical system problems for large fleets, agrees that the alternator, batteries, starter and interconnecting wiring, need to be diagnosed as a system, rather than a group of separate components.

Part of a system
“The batteries are there to stabilize the voltage in the electrical system,” Craft says. “But they have to work efficiently.

“Alternators are not designed to be a battery charger. The alternator itself is sized for the vehicle load, not the load plus a large amount of current for battery charging. 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. As the batteries age and experience a loss of their electrical reserve, more and more of the alternator’s output is devoted to charging the batteries. This puts too much pressure on the alternator.”

The effect is like a snowball. Why?

The wiring in the alternator resists the flow of current, and that resistance increases with temperature. And generating all that current in so small a space makes quite a bit of heat, so alternators run hot. Craft says a 160-amp alternator will easily produce 175 amps when cold, but only 150 when hot. If there is too much load because of bad batteries, the excess heat in the alternator itself will actually reduce the alternator’s output and its ability to take care of the problem. It will eventually overheat and fail prematurely.

As both Purkey and Craft mentioned, the system also includes the cables. If there is resistance at connections or within the cables themselves, the batteries won’t get you started, and they won’t recharge, either.

Purkey recommends a three-part method for troubleshooting alternator or other electrical system troubles: “First, test the batteries. Second, test the cables. Third, test the alternator.” It’s no coincidence that this is exactly the process observed at Jersey Rebuilding Service.

  1. Battery testing and maintenance
    If using batteries with removable caps, Craft says you should be sure and keep the cells full by adding distilled water frequently. If using low maintenance batteries, keep an eye on the indicator that shows battery condition. If it loses its green color, get the battery checked out right away.

Comments are closed. strives to maintain an open forum for reader opinions. Click here to read our comment policy.