How many starters and alternators are replaced unnecessarily every year because of misdiagnosis?
Nobody knows for sure, but suppliers of both new and remanufactured rotating electrical parts tell us more than half of their warranty returns have “no fault found” when the parts are examined.
Many of these returns are from DIYers who lack diagnostic know-how, but some are from professional technicians who apparently lack the same skills.
The point we want to stress here is the importance of diagnosing starting and charging problems before any parts are replaced. If you correctly perform the diagnosis, you’ll fix the fault the first time and won’t have to worry about comebacks, returns or your reputation. You’ll also save your customers money and yourself the hassle of trying to fix the same problem again.
Begin with the Battery
Let’s look at a typical no-start scenario where the engine fails to crank when the motorist tries to start his car. The first thing that usually needs to be determined is whether or not a rundown battery is the reason for the engine not cranking.
Starters pull a lot of amps when they crank an engine, and if the battery is low, there may not be enough current available for the starter to overcome the compression, spring pressure and friction within the engine. If the starter cranks normally with a jump-start, though, you can rule out a starter problem right away. The next thing to investigate would be the battery.
Did the battery run down because the motorist left his lights on? Has the car been sitting a long time? Is it only used infrequently for short trips? Is the charging system putting out enough amps to keep the battery fully charged? Is there a higher-than-normal key-off current drain on the battery that’s running it down? Is the battery no good?
These are all concerns that can be easily answered by asking the vehicle owner a few questions and performing a few simple tests. If the motorist did not leave his lights on, and the vehicle has been driven daily for at least 20 minutes or more, chances are the car has a charging problem, a bad battery, or loose or corroded battery cable connections.
A visual inspection of the battery connections might reveal something if you see signs of corrosion or the cable clamps are loose or damaged. The battery’s state of charge can be tested with a voltmeter. A fully charged battery should read 12.6 volts. Less than 12.4 volts means it’s low and needs to be recharged.
If the charging system is putting out normal voltage, the battery should then be load tested or checked with an electronic conductance tester to determine if it’s good or bad.
Note: A traditional load tester won’t give accurate test results unless the battery is first recharged. This isn’t the case with conductance testers because the test results do not depend on the battery’s level of charge.
Average battery life is only about four to five years at best, and can be as little as three years in a hot climate.
If the battery tests bad, the vehicle obviously needs a new battery. Replacement batteries should always have the same or higher cold cranking amp (CCA) rating as the original, and be fully charged before they are installed in a vehicle. And don’t forget to attach a backup power supply before disconnecting the old battery on cars where loss of keep alive memory data can cause problems.
But what if the battery tests good and the charging system is working normally? Then what? The next check would be to look for a higher-than-normal key-off current drain.
Did You Know…
The charging voltage can be checked at the battery with the engine idling. Most charging systems will put out 13.5 to 14.5 volts at idle, depending on the temperature and the battery’s level of charge. Less than 13.5 volts would indicate a charging problem.
When the key is turned off, the electrical load on the battery doesn’t stop. There is always a small load on the battery to maintain the “keep alive” memories in the powertrain control module (PCM) and other modules. Many modules have internal timers that either turn off the module to put it into a “sleep mode,” or power down the module to a “standby mode” after a certain length of time when the key is turned off. Some of these modules power down in steps and time out at different rates.
The key-off drain can be measured with a milliamp amp probe or an ammeter. A clamp-on amp probe is the better choice here because you don’t have to disconnect a battery cable as you would if you use an ammeter. Disconnecting the battery on many late-model cars without first connecting a battery backup to maintain the keep alive module memories can create additional problems.
Any disruption in power, even for a split second, can cause the PCM and other modules to forget learned values and other key settings. This, in turn, may require using a scan tool to reset or recalibrate certain systems. Don’t make extra work for yourself by forgetting this key bit of information.
When measuring a key-off current drain, keep in mind that opening a door, the trunk or turning anything on may wake up certain modules and increase the key-off current drain reading.
If the key-off current drain is higher than normal, the problem may be a light or module that’s staying on, or a relay that is not switching off. Finding the fault usually means pulling fuses one by one until the current reading drops down to a
Did You Know…
As a rule, the key-off current drain on most late-model vehicles should be less than 50 milliamps (mA), one hour after the vehicle has been shut off and left undisturbed.
Now, let’s say the battery tests good, the charging system is working normally and there are no abnormal current drains on the battery. But the engine won’t crank when you try to start it. Nothing happens when you turn the key to the start position (or press the start button). A no crank with a good battery means there is some kind of problem in the starter circuit.
Voltage may not be reaching the starter relay, the starter solenoid or the starter motor. The cause may be a bad ignition switch, an open park/neutral safety switch (if the vehicle has an automatic transmission) or an open clutch pedal safety switch (if it has a manual transmission).
Diagnosis should begin at the starter relay. If the relay is not receiving battery voltage when the key is turned to start (or the start button is pushed), the fault is upstream of the relay. You would use a voltmeter or test light to find out where the voltage is not getting through. Is there voltage to the ignition switch or start button?
Does voltage flow from the switch or button when you try to start the engine? Is there voltage to the park/neutral or clutch pedal safety switch? Does the switch pass voltage when it should? If the switches are all OK, but there is no voltage to the relay, the fault would be in the wiring or a loose or corroded wiring connector.
If power is reaching the relay, but nothing is happening, you can test the relay with an ohmmeter (refer to the test values and terminal connections provided by the vehicle manufacturer), or you can simply swap or replace the relay to see if that solves the problem.
If the fault is not the relay, check the circuit between the relay and starter solenoid or starter (depending on how the system is wired). Loose or corroded connections can strangle the flow of current to the starter. Using a voltmeter to do some voltage drop tests can help you find bad connections.
If voltage is reaching the starter, but it isn’t cranking, or is cranking too slowly (most engines need to be cranked at least 250 rpm or faster to start), the problem is likely a bad starter.
Starter current draw can be tested on the vehicle or on a starter bench tester, and compared to specifications. A good starter will usually draw a current of 60 to 150 amps, depending on the size or power rating of the starter. Some “high-torque” starters may draw more amps, so always refer to the manufacturers’ test specifications. When a starter draws too much current, it may pull so much power from the battery that the ignition system won’t fire. The engine cranks, but it won’t start because there is no spark (or too weak a spark).
If the starter spins freely (up to several thousand rpm) and the amp draw is within specifications, it should be capable of starting the engine unless there is a mechanical problem with the drive mechanism, drive gear or damaged teeth on the flywheel. If the starter does not spin freely, or draws an unusually high or low number of amps, the starter has reached the end of the road and needs to be replaced.
An unusually high current draw and low free-turning speed typically indicate a shorted armature, grounded armature or field coils (possibly due to overheating from excessive cranking), or too much friction within the starter itself (dirty, worn or binding bearings or bushings, a bent armature shaft or contact between the armature and field coils). The magnets in permanent magnet starters can sometimes break or separate from the housing and drag against the armature.
A starter that does not turn and draws a high current may have a ground in the terminal or field coils, or a frozen armature.
Failure to spin and zero current draw indicates an open field circuit, open armature coils, defective brushes or a defective solenoid.
Low free-turning speed combined with a low current draw indicates high internal resistance (bad connections, bad brushes, open field coils or armature windings).
Testing for Voltage Drop
A voltage drop test is the only effective way to find excessive resistance in high-amperage circuits. As a rule, you should see no more than 0.5-volt drop across a high-current connection, and no more than 0.1 volts across a low-current connection. Ideally, you should see no voltage drop at all.
To perform a voltage drop test, the circuit needs to be on with current flowing through it. You then use a digital voltmeter (DVM) to measure the voltage drop across each connection in the live circuit. Voltage always follows the path of least resistance, so if the connection being tested has too much resistance, some of the voltage will flow through the voltmeter and create a voltage reading.
If a connection is good (clean and tight with no resistance), you should see little or no voltage drop (zero to less than 0.1 volts). But if you find more than a few tenths of a volt drop, it indicates resistance and a need to clean and tighten the connection.
It’s amazing how many times bad connections are overlooked because they “appear” to be OK. But appearances are often deceiving, and the only way to know for sure whether or not you have good connections in an electrical circuit is to measure the voltage drop across those connections. This applies to battery and ground cable connections, starter, solenoid and relay connections, and wiring harness connections.
If the charging system is not putting out enough current to keep the battery charged, the fault may be the alternator, the voltage regulator (or control circuit in the PCM depending on how voltage is regulated), or bad wiring connections.
If you don’t see at least 13.5 volts when you check charging voltage at the battery with the engine idling, check the wiring connections at the alternator not just visually or by wiggling the wires or connectors, but check for excessive resistance by doing a voltage drop test. Many so-called alternator problems turn out to be nothing more than a bad connection at the alternator or a bad wiring harness.
Alternator output can be tested on the vehicle or on a bench-tester. As a rule, an alternator should produce 90% or more of its rated output by 2,500 rpm. The charging curve will vary from one application to another, but if the output is less than specification, the alternator needs to be replaced.
The fact that an alternator is producing voltage doesn’t mean it’s producing enough current to meet the vehicle’s electrical needs and keep the battery recharged. An alternator that is rated at 120 amps at 2,000 rpm, but is putting out only 40 or 50 amps, is a weak alternator that needs to be replaced.
Common alternator problems include worn brushes and slip rings, shorts or opens in the stator or armature windings, broken or melted electrical connections, and bad diodes in the rectifier (diode trio).
Note: When an alternator’s output is displayed on a digital storage oscilloscope, the waveform should look like a series of even humps. Missing humps would indicate bad diodes and a drop in current output.
If an alternator tests bad, make sure the replacement alternator (new or reman) has the same or higher amp
rating as the original.