Charge it up! Charging system diagnostics.February 24, 2020
Everywhere you look on modern farm equipment, you will find electronics. From the diesel engine to the planter seed meters, electrons are flowing through wires.
If soil is the foundation for plant growth and nutrition, then the alternator is the bedrock of all things electronic on farm equipment.
When the charging circuit is not functioning correctly, then a myriad of issues can and will occur. Thus, it is essential to understand the synergy between the alternator, battery, and electronics on a piece of equipment.
A basic look
Older equipment was equipped with a basic charging system that included a battery, a generator, and either a cut-out or a voltage regulator.
In those days, once the engine started, the only electrical draw with a gas engine, if you did not have the headlights on, was the ignition system.
If it were a diesel, then there was not even that. The generator could stop charging, and if the battery was in good shape, you could run the tractor for days if you did not shut it off and start it frequently. Not so today!
Whereas a generator produces direct current (D.C.), an alternator produces alternating current (A.C.) that is changed to D.C. The process is called rectification and is accomplished thru a series of rectifiers.
A.C. current produces a sine wave that switches back and forth from positive to negative. The point where it changes is called the zero crossing. The rectifiers end up chopping the negative side of the sine wave off and allow only the positive to be sent to the battery and electrical system. When this happens as intended, the voltage is “clean.”
The electrical system pulls energy from the battery, and the charging circuit’s job is to keep the battery fully charged.
When troubleshooting a charging circuit issue, a system approach needs to be taken.
You can no longer consider just the alternator but also the battery, wiring, voltage regulator, and ground circuits.
If one of these is askew, the alternator may not charge, overcharge, or undercharge.
Don’t always blame the alternator
Even though there are many variations in charging system design, the following overview will apply. But is not meant to be a substitute for the proper shop manual for that piece of equipment.
When facing a no-charge condition, it is easy to jump the gun and replace the alternator without performing any diagnostics.
Today alternators are expensive due to the power they need to produce and that they may include other components internally, such as the voltage regulator.
You do not want to go there first, even though that may be your final destination.
In most instances, the alternator is replaced as a unit. It is not serviceable internally in the farm shop, but that is not always the case. Familiarity with the equipment and the level of service/repair it offers is essential.
Before replacing the alternator
Keep in mind the wiring to the alternator needs to be checked along with the fuses in the fuse box. Many systems are fused or have a fusible link that, if blown, will not allow the alternator to charge.
High resistance to the field circuit of the alternator or to the voltage regulator will cause it to overcharge and boil the electrolyte out of the battery.
In like fashion, a battery with sulfated cells can cause the alternator to overcharge.
Some applications that use a CHARGE indicator light have the bulb wired as part of the field circuit. Keep this in mind since if the lamp burns out, the alternator will not charge. Many Delco charging systems were wired this way, along with others.
Always do a key-on, engine-off test first to check and see if the CHARGE light illuminates. If not, it would be wise to check that first.
A battery that has an open buss bar that ties the cells together will have no voltage. Even if you jump-start the engine, depending on the circuit design, the alternator may not charge due to the broken battery.
If the brushes are worn, the alternator may charge fine at idle and low engine speeds but then lose contact on the slip ring of the rotor at higher rpm.
This will lead to a system that checks fine at idle, but in the field under load will not charge. Always make sure you check alternator output throughout the engine speed operating range.
A weak diode in one of the rectifiers will allow the alternator to charge but at a lower voltage. Most charging systems are designed to put out 14.6 to 14.8 volts. A weak diode may bring that down to 13.8 volts or so.
When a diode in the rectifier becomes weak, the system will have “dirty” output. Though it may keep the battery charged and the engine may run fine, the corrupted voltage can play real havoc with delicate electronics in the tractor, combine, planter or sprayer.
This often goes undetected since the normal response is to diagnose the circuit, such as an electric seed meter, when the problem really is dirty voltage to it.
The voltage regulator is a common area of trouble, especially with an older mechanical design. The newer transistorized voltage regulators tend to be more reliable but cannot be repaired.
Often the contacts in a mechanical regulator would stick, causing either an overcharge or undercharge conditions. All that was required to get it working again was a good, swift tap with a screwdriver handle.
The best way to check a charging circuit is with a dedicated tester that allows a load to be put on the system. Few farm shops have one, but most have a voltmeter.
All you need to do is connect the voltmeter leads in proper polarity to the battery and take a reading.
You can enjoy a makeshift load tester by disabling the engine, so it does not start and crank it over for about 15 seconds while watching the battery voltage on the meter. Then start the engine while still watching the meter. The alternator should be working to replace the energy in the battery, and this will tell you of its efficiency.
This test will show you the battery’s reserve capacity to maintain cranking voltage, the battery’s willingness to accept a charge once the engine is started, and the alternator’s readiness to work.
Alternators do have the ability to fail. Keep in mind that everything else needs to be shipshape before you jump in the pick-up and go to town to spend a few hundred dollars on a misdiagnosis.
If the alternator needs to be replaced, make sure that you invest in a quality remanufactured unit, not a rebuilt one.
You do not want to do the job twice and risk the chance of failure with a poor-quality rebuilt unit.
I always like to say, “I do not have enough money to afford to buy something that cheap!”