Don't dread electrical-system troubleshooting
For many of today's operators and mechanics, electrical systems can be one of the most intimidating areas of your equipment on which to work. After all, you can't see electricity flowing through a wire, most people don't have a great deal of training in today's electrical systems and electrical components have a reputation for being finicky. However, without a properly functioning electrical system, your machine will not work the way it was intended, if at all. Fortunately, even though the electrical-system section in your operators' manuals has gotten bigger in the past few years, the basics have not really changed much. And, with a few simple tools, you should be able to perform most of your own troubleshooting.
For the purposes of this article, we will break down a machine's electrical troubleshooting into five main systems. They are the ignition system, charging system, cranking system, safety-interlock system (including PTO and fuel shutoff) and the accessory system. The only tools you will need--aside from basic hand tools--would be a good quality VOA meter (also called a multimeter) or a voltage probe light and a spark tester (see Figures 1a, 1b and 1c, page 34).
Ignition system On a gasoline engine, if you have no spark, your engine won't run. Obviously, then, your ignition system is critical for engine operation (see Figure 2, page 34). However, just because your engine will not start or run does not mean your ignition system is not functioning properly. Your ignition system could be working correctly, but you could have a problem with your fuel system, compression system or engine timing. Those are topics for other articles, and we will assume that these systems are functioning properly. You also may have a problem with your cranking system or safety-interlock system, which affects your ignition system. We'll address that topic later in this article.
Your first step is to determine if you are getting a spark. Make sure any throttle controls, ignition switches, safety interlocks, etc., are in the run position; no engine will have a spark if the ignition current is being sent to ground by a throttle interlock switch. Do not test for spark the old-fashioned way of holding the high-tension lead a little ways from the spark-plug terminal and cranking the engine to see if a spark is present. This method not only is dangerous, but on many of today's newer, solid-state ignition systems, you can damage the solid-state components using this type of test procedure. The only correct way to test for spark is to use an approved spark tester. These inexpensive testers--available from most tool suppliers and automotive stores--help ensure that you don't create tremendously high resistance that could damage your solid-state components.
If your spark tester shows a consistent, bright, blue spark, then you know you are getting enough voltage at the spark-plug terminal. What you do not know is if the spark plug is delivering a good spark to the combustion chamber. Do not test for spark at the spark-plug electrodes the old-fashioned way of grounding the plug to the head of the engine. This is dangerous and could also damage certain solid-state components. If you still suspect your fuel mix is not being ignited properly, then play it safe and ch ange your spark plugs with the proper new plug. They are relatively inexpensive but critical to the proper functioning of the ignition system.
If your tester does not show a bright-blue or consistent spark, check your electrical connections, armature air gap and flywheel key, and clean off any rust from your flywheel magnets. If it is an older breaker-point ignition system, change your points and condenser. If it is a battery-ignition system, make sure your battery is fully charged. If none of these areas corrects your problem, it is time to inspect your ignition coil.
Testers are on the market, and you should learn test procedures for ignition coils. However, given the high cost of this test equipment and the infrequent use most equipment operators would give these testers, it may be more economical for the equipment owner to keep a small supply of new ignition coils on hand as replacements for suspect coils. If all of your other ignition-system troubleshooting does not find a problem, and replacing the coil with a new coil corrects your problem, then you can be assured the coil was the original problem.
If your ignition system functions properly, and your fuel, compression and timing are all good as well, your engine should run. This does not mean, however, that all other electrical systems are necessarily working properly. You still may have a problem with charging, cranking, safety-interlock or accessory electrical systems.
Charging systems The charging system consists of a stator or alternator, a regulator/rectifier, wiring and a battery. It also may include a keyswitch or an ammeter (see Figure 3, below left). The alternator or stator produces AC voltage, which is sent to the regulator for conversion to DC voltage. The rectifier senses battery state-of-charge and sends DC voltage to the battery when it becomes discharged. Some systems tie the keyswitch into the charging system so the battery cannot accidentally discharge when you shut off the engine, and some systems also may use an ammeter to tell you if the charging system is functioning during engine operation. These two components are usually not a problem with charging systems unless one of them has a bad connection.
The first step in troubleshooting a charging system is to determine if the stator is putting out enough AC voltage. To do this, simply locate the stator leads (usually two wires together in the same harness coming out from beneath the flywheel or out of the alternator), run the engine at full throttle, disconnect the stator leads from the wiring harness to the battery (which temporarily will not harm the components) and, with your VOA meter set at AC voltage, touch the tester leads to the two stator leads. You should read a relatively high AC voltage--somewhere in the 27 to 45 volts AC (VAC) range. If you have a technical manual available for your machine, check the specification in it or call your dealer to see what it should be. If this test is good, then your stator should be okay.
Next, locate where these two wires go into your regulator/rectifier. You may need a wiring diagram to do this, or you may be able to trace the wires on your machine. Check for the proper AC voltage going into your regulator/rectifier. If you do not have about the same voltage there as you did at the stator leads, then you have some bad wiring between the two.
Assuming you have the correct AC voltage going into your regulator/rectifier, you must then find the wire that sends DC voltage to your battery. It is usually red, and you can normally trace it on your machine or use a wiring diagram to locate it. Set your VOA meter to the DC volts scale, clip the black lead of your tester to a good ground on your engine (or your battery's negative terminal), run the engine at full throttle and touch your tester's red lead to the charge-wire terminal on your regulator/rectifier. You should read about 14 to 17 volts DC (VDC). If you read less than 12 volts, then your regulator does not allow enough voltage through to charge your battery. If you read high voltages, you may be overcharging your battery. Check your technical manual or call your dealer for the exact specification.
Next, repeat this test, but this time hold your red tester lead at the battery's positive terminal. If you get a very different reading from the one above, you may have some faulty wiring or a bad connection between your regulator/rectifier and the battery (possibly at the keyswitch or ammeter, if present).
If any of the above tests indicate a faulty component, then replace that component. You can conduct other tests on the stator and regulator/rectifier (such as various resistance tests), but to perform these, you would need to know all of the test points and specifications for those components. Unless you have the technical manual or are a servicing dealer, it is unlikely that you would need or want to do these tests.
Don't forget to look for mechanical problems, such as a loose alternator belt, which could affect the functioning of your charging system. The last component in the charging system is the battery, which we'll discuss in the next section. If the battery is good, and all of the above tests are positive, then your charging system is functioning properly.
Cranking system Some people refer to the cranking system as the starting system, but an engine may not start due to other problems, such as improper carburetor adjustment or engine timing. Therefore, all we are concerned with in this troubleshooting section is whether the engine will turn over or crank. The cranking system consists of a battery to supply power, a starter motor and solenoid, a keyswitch and a wiring harness (see Figure 4, above). The safety interlock system also will affect whether the engine will crank, as will mechanical problems such as a seized engine, dirty bendex or sheared teeth on the flywheel or starter gear.
First, you need to determine if your battery is fully charged. Use your VOA meter, set on the DC-volts scale, to check the battery's voltage. A fully charged battery should show around 12.8 volts. If your battery is fully charged, you may proceed with the rest of the cranking system's electrical tests, described below. If not, you may want to hook your battery up to a charger for a short time to bring it up to at least 12 volts. Also, check the battery's electrolyte level to make sure it is above the plates but not all of the way up to the top of the filler neck. Use only distilled water to refill the battery cells. As long as you have the battery on a charger, clean the terminals and the top of the battery of any corrosion or dirt.
If your battery was discharged or if you continually have problems with a dead battery, you will need to perform a load test on it. Testers are available that can perform this load test for you, but given the cost of this test equipment, you probably won't own it. However, one test you can perform will tell you the same thing. Simply disconnect the high-tension lead from the spark plug(s) so the engine won't start. Next, attach the plug to an approved spark tester so you won't damage any of the solid-state ignition components. Then make sure all the safety-interlock conditions are met (that is, the parking brake is engaged, the PTO is off, the transmission is in neutral, etc.). Set your VOA meter scale to DC volts and clip it to the battery terminals. Turn your keyswitch to the start position and crank your engine for 15 seconds. Watch the reading on your VOA meter. The voltage should not drop below 9 volts in 15 seconds (assuming the battery was fully charged to begin with). If it does, your battery probably is defective, and you will need to replace it.
If your battery is good, voltage should be available at the positive terminal of your starter motor. Use your VOA meter or test light to check for battery voltage at the heavy red cable that connects to your starter motor. If battery voltage is not present, check your connections and wiring. Battery voltage also should be available at the keyswitch's positiveterminal. Because this often is difficult to reach, you can skip the keyswitch a nd check for battery voltage at the starter solenoid's switch terminal. To do this, you will need to locate the wire that sends voltage from the keyswitch to the starter solenoid. It is usually a lighter-gauge red or a brightly colored wire, or you can trace it from your keyswitch. Disconnect the wire from the starter solenoid, make sure all of the safety-interlock conditions are met, connect your VOA meter with the black test lead to ground and the red test lead to the wire, and turn the keyswitch to the start position. You should now see battery voltage at the wire. Battery voltage should not be available when the keyswitch is in the run or off position. If any of these conditions are not found, then either your keyswitch or the wiring between the battery and starter solenoid are defective.
Assuming your engine has no mechanical problems as discussed previously, if the battery, wiring and keyswitch are all okay, but your engine still won't crank, then your problem is most likely in the starter motor or solenoid itself. It is unlikely that you have the tools and know-how to repair these components yourself. Therefore, if you suspect the starter/solenoid, call your servicing dealer or put on a replacement. Before putting on a replacement, however, make sure the safety-interlock system is checked out, as described below.
Safety-interlock system Safety-interlock systems vary widely from one machine to another, depending on the manufacturer and the machine's purpose. Some common safety interlock devices include: transmission-neutral switches, brake switches, PTO switches, seat switches, shroud switches, etc. (see Figure 5, at left). You can use these devices alone or in combination with each other. They usually are tied into the cranking, the ignition and the machine's PTO systems but are seldom tied into the charging or accessory systems.
For the most part, all of these devices are just switches. That means that they just open or close to allow the flow of electricity into a circuit. An electrical switch is very easy to test with an ohm meter or continuity light. With your VOA meter set to the ohm scale, simply touch your two test leads to the two terminals of the switch. Then operate the switch (open and close it). You should have continuity in one position and no continuity in the other. If you do not, then the switch is defective, and you should replace it. Bad connections or wiring also could be a problem in the safety-interlock system. You can usually find these problems visually or by using a continuity light to check for shorts, opens or grounds.
Because safety-interlock systems offer so much diversity, it is impossible to cover them all here. One helpful suggestion when troubleshooting safety-interlock systems, however, is to think of the system as the "brains" of the machine trying to keep the operator safe. For example, on a riding lawn mower, would you want the blade to be engaged (PTO on) when you are off the seat? Obviously not. Therefore, the PTO switch is tied into the seat switch, and you cannot engage the PTO unless you activate the seat switch (when you are sitting on the seat). So if you have a machine that runs great but shuts down every time you engage the PTO, it probably isn't the PTO switch that is defective but rather the seat switch. In other words, every time you engage the PTO, the brain of the machine thinks you are off the seat and shuts the machine down. This is an example of a common interlock problem and illustrates how you can think your way through an interlock-system-troubleshooting procedure. Many times, cranking or ignition problems are not in the cranking or ignition systems but rather in the interlock system. So don't forget to perform this troubleshooting procedure when trying to repair a machine that will not crank or run.
Accessory systems Accessory systems include things such as oil-pressure switches, coolant-temperature switches, lights, sprayer monitors, tachometers, electric motors and radios(see Figure 6, above left). Because so many different possibilities exist, it would be impossible to cover them all in this article. However, if you understand the basics we have already discussed (such as using a VOA meter and continuity light, how a switch functions, when battery voltage should be present, etc.), you should be well on your way to troubleshooting these problems. They are usually not as critical a problem as some of the others that we have covered and will seldom result in your machine not running. An operator's or technical manual with wiring diagrams and a good VOA meter will be helpful in troubleshooting these accessory systems.
Larry Van De Valk is assistant professor in the Agricultural Engineering Technology Department at the State University of New York--Cobleskill.
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