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.
- 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.
Clean the top surface of every battery frequently. That layer of dirt is a path to ground and will add to battery charging load. Also, clean the terminals frequently. “Corrosion is a no-no,” he says. Make sure the connections stay tight, too. Corroded or loose connections will keep the voltage from getting to your starter or accessories, and will also keep the batteries from being recharged after they’ve cranked the engine.
Load test the batteries annually, before winter. They must first be disconnected to isolate them from one another. A carbon pile – a big electrical resistance – is used to put 1/2 the rated load on each. Checking the output voltage for 20 seconds of continuous loading tells a lot about whether or not the battery is starting to lose it. If the voltage stays at or above 9.5 volts for more than 20 seconds, the battery will get you started. The voltage on a battery that is close to failing will fall off by the end of the 20 seconds.
But batteries are sneaky, Craft says. Even if they pass this test, their reserve capacity is likely to be diminished after a lot of time and miles – this is a process that starts the day you put them into service – they are a maintenance item. The ideal, he says, is to just replace them on a regular schedule – before they start to drag the alternator down. For most trucks, this would be every four or five years.
He says, “The owner-operators who are very maintenance savvy and who I see replacing their batteries regularly have very little electrical system trouble. Those large fleets who try to skimp are the ones who end up having breakdowns.”
And both Purkey and Craft are clear on one thing: It makes no sense after long service to replace just one failed battery in a group of three of four. If you do that, the new, powerful battery will end up doing too much of the work and fail prematurely. Replace them all at the same time so they will work as a team.
Purkey adds that they should all be of exactly the same brand and rating – even batteries of similar rating but different designs may fail at the necessary teamwork. If the dealer can’t supply all new units that are identical, go elsewhere or ask him to get the necessary units in stock before replacing them.
- The cables
The cables that carry the power from the batteries to the starter and from the alternator to the batteries are critical, since voltage drop can both reduce starter cranking power and the ability of the alternator to charge the batteries back up. In fact, Craft says his shop often tests the batteries and cables for capacity together.
As Purkey says, “If the alternator puts out 14 volts, but you’re only getting 12.5 at the batteries, the system won’t charge them very well.” Craft and Purkey report that the way they test the cables is to have the engine running so the alternator is charging the batteries, all accessories on to maximize the load, and then measure the voltage drop from the alternator connection to the batteries. It should not exceed 1/2 volt.
Craft advises that you physically inspect the cabling, as well. Replace cables if the connections look worn. Clean everything up, as road salt often gets in the act at connections. This includes ground straps, especially where they ground to the frame. All connections must be clean and tight and not show damage from corrosion. Remember that voltage drop across ground straps does just as much to restrict flow as on the positive side of the system. Measure voltage drop here, too, if in doubt about the condition of a ground strap and its connections.
Once all the wiring is in good condition, you can check out the alternator itself.
- Alternator maintenance
The most damaging thing that happens to an alternator is a slipping belt. Craft explains that slippage means heat, and the heat goes right into the alternator shaft or “rotor.”
Purkey, who worked for an alternator manufacturer in the past, explains what happens when the heat travels down that metal shaft. “A slipping drive belt can create so much heat, it will boil the grease out of the shaft bearing, causing it to lock.”
Purkey sees the new automatic tensioners as a great solution to this problem. Although belts have been greatly improved and the makers say they won’t stretch, the reality is that they do need to be kept tight enough and may need an occasional adjustment if there is no tensioner.
With an automatic tensioner, there are markings showing whether or not it’s at the right angle, which indicates the torque and belt tension. The mark on the tensioner must be between the two on the block or mounting. Critical here is replacing a bad belt with a new one of the right length, or the marks won’t be lined up right. If you’re in doubt about a tensioner’s performance, remove the belt and check how it responds when turned with a socket drive in the direction of removing belt tension and then gradually released. If it rotates smoothly and returns consistently to the same position, it’s OK. If there is any binding evident, it should be replaced.
While traditional mountings take more maintenance, using the right bolts and torquing them properly will cure most problems. It’s also essential to get the belt tension right. Too tight is almost as much of a problem as too loose, because it puts excessive “radial side load” on the bearings, which can also be damaging, says Purkey.
There are two alternator mounting systems. One uses a pad mount, preferred because the unit is bolted at four points directly to the engine block, reducing vibration. The bolts pass down through the mounts at front and rear on either side.
The other system uses the J180 style mount. Purkey points out that this type of mount is sensitive to vibration if the bolts get loose. Using good bolts is crucial – he recommends Grade 8 – if you’re to get the right torque and have it hold. Don’t use a lock washer, because they are not hard enough to hold the torque. And, he says, if there is evidence of an egg-shaped hole, you’d be smart to replace the unit because it won’t stay tight.
Belt tension is a large issue, and it needs to be just right. Craft says an experienced technician can put high thumb pressure on the middle of a span and if the belt will only depress a maximum of 1 inch, it’s right.
Purkey thinks checking with your thumb is no more precise than banging on a tire to check its pressure. He strongly recommends that you leave nothing to chance and get a belt tension gauge because they only cost about $25. One made by Gates Rubber clicks like a cricket when the belt reaches proper tension. Replace belts that are glazed (the covering looks smooth instead of rough where it rolls along the pulley), because they will slip even when at the right tension.
Both experts agree that proper bolt torque is critical. The good news is that, if you use the right bolts and torque them properly, the unit is likely to remain properly mounted. You need a long wrench handle, because the torques are in the range of 80-100 pounds feet for the J180 mount. Pad mounting torques are lower – 35-40 pounds-feet. Check your maintenance manual for the right torque figures. Too loose and the bolts will loosen up, risking vibration and loss of belt tension. Too tight and you’ll break the ear off the alternator body.
Retightening the bolts in the proper order on a J180 mounting system after working on it is also critical, Craft says. The two hinge bolts should be only slightly snug while you adjust belt tension. Once the alternator is in position to create the right tension, torque the single bolt that allows adjustment. Then torque the front hinge mount on the other side and, finally, the rear hinge mount with the split bushing. If there is a single through bolt on the hinge mounts, it’s torqued after the adjustment bolt.
Craft’s only other suggestion in terms of routine maintenance is, surprisingly, to prevent fluid leaks in the engine compartment.
“Modern, aerodynamic trucks have very little air moving in the engine compartment,” he says. “Air that comes in through the radiator is directed downward. The alternator fan provides just about the only air motion near the engine, so any leaking fluid tends to get drawn right into the alternator.”
Curing such leaks will keep the alternator clean inside. But a buildup of dust that adheres to the windings because of a coating of oil will soon cause them to overheat and fail.
Craft’s shop has a test stand that runs an alternator at various rpms and applies a load to see how it performs in terms of voltage and amp output. It even has a gauge that will indicate extraneous waves in the output current.
Purkey says you can purchase alternator testers that will indicate these waves, which he terms “high ripple.” He says any failure in the diodes, or stator windings (the stationary wiring where the output current flows through the unit), will cause ripple and this can easily be detected by a quality instrument.
Craft reports that high ripple will cause the unit to overheat. Also, a bad diode, often indicated by ripple, allows the batteries to discharge through the alternator circuit when the engine is off.
You can also run a simple alternator test with a voltmeter connected between the alternator output connection and a good ground, and an inductive ammeter around the output line. First, start the engine and idle it with all 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 .2 volts. If voltage rises with rpm, the regulator has failed and is not controlling it properly.
Then 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. The amperage should be + or – 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. If the stator windings are bad, the unit is normally replaced.
If you replace, Purkey reports that there are two types of alternators – those with brushes and brushless units. 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 have a lot of lights on the truck and are concerned about horsepower draw and fuel consumption. But brushless units will give a longer service life, according to Purkey.
If you’ve added lights or other accessories and need more alternator capacity to power them, it makes sense to install a bigger unit than the 160-amp ones Craft says have become standard.
While he reports oversize cables are sometimes used during original build, Purkey says it’s critical to make sure the cabling is right for a larger unit and to upgrade if necessary. Otherwise, it’s like “putting in a larger water pump but using the same size pipe to carry the water.” Craft agrees and reminds you that longer cables need to be of a larger gauge than shorter ones.
Clearly, keeping your belts, alternator mounts, cables and connections, and batteries in good condition will go a long way toward guaranteeing long alternator life and fewer electrical problems.
For More Information:
Jersey Rebuilding Service, Inc.
Purkey’s Fleet Electrics, Inc.
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