Diagnosing Computer-Controlled Charging Systems | MOTOR (2022)

Sometimes we goof by making a job more complicated than it really is. Later, we look back and realize we cost ourselves time, money and grief by making the task tougher than it needed to be. Whoa! We have enough aggravation already, don't we?

Charging system diagnosis fits into that category, and I believe it should be kept as simple as possible. The way to simplify always has been to divide and conquer. In other words, divide a vehicle's charging system by carefully isolating a suspect alternator. Then replace the alternator if it still doesn't work or work correctly.

Diagnostic Premises

The single, most common charging system symptom is a dead battery. First, always recharge or replace the battery as needed, then test the charging system. Suppose the system fails the routine volts and amps tests —especially an alternator that won't charge at all. At this point, the diagnostic choices used to be a bad alternator or wiring failure. Naturally, a thorough voltage-drop test will identify any wiring problems and/or bad connections.

Unfortunately, many modern charging systems have a third failure possibility—a computer! This combination of possible problems is one convincing reason why isolating the alternator is more appropriate than ever before. Another compelling reason is the time required to R&R some of these alternators. If you're spending that time, you want to know the effort is justified.

es, you must access the alternator and connect something to it in order to isolate it. But you probably were heading toward that suspect alternator anyway. And once you make the proper hookup, this approach completely separates the alternator from the computer and its wiring. Now if that alternator still doesn't charge normally, you're confident that you must replace it and retest.

However, if a so-called isolated alternator charges normally, the trouble must be somewhere outside the alternator. This means you need troubleshooting time to methodically track down the problem. Politely explain your diagnostic format to the customer. If he or she won't authorize even a minimal amount of diagnostic time, collect for what you've done thus far and move on to the next vehicle. Why entangle yourself in a potentially lengthy troubleshooting procedure if you won't get paid for it?

Let me give the ol' scan tool its due here before I proceed. Many scan tools have an extremely helpful selection of active or output tests. These may include the ability to command the alternator to charge. But suppose you command the alternator to charge and it doesn't respond. In a pinch, is the root problem the scan tool or its software? Or is it an ECM failure, a wiring problem or a bad alternator?

Here I'll concentrate on nailing no-charge conditions on popular General Motors and Chrysler systems. Fords are a topic for another time. I'm working on the assumption that you'll also consult the appropriate shop manual and wiring schematic for the vehicle in question.

(Video) ECM Controlled Charging Diagnosis

GM's Terminal L

The overwhelming majority of General Motors products have used alternators with the same basic terminals since 1986. The most popular alternators have either terminals P-L-I-S or P-L-F-S. Remember that a harness connector for a P-L-I-S alternator physically interchanges with one for a P-L-F-S unit. For the purposes of our isolation tactic, we're concerned only with terminals P and L.

Earlier alternators with these terminal layouts have rectangular harness connectors; later ones have oval-shaped connectors. The lion's share of GM products coming into your bays have the oval alternator connector, which was used until the mid-2000s.

The neat thing is that regardless of connector shape, the first pin is always terminal P. I'll discuss P momentarily. The second pin is always L, which stands for lamp or charge indicator light. The key to operating these alternators is energizing or exciting terminal L. A typical P-L-I-S or P-L-F-S alternator will operate on terminal L alone. Indeed, zillions of the GM vehicles that have passed through your shop had a single-wire alternator connector—a lone wire connected to terminal L.

The electrical systems on the most common GM vehicles apply either 5.00 or 12.00 volts to terminal L after the engine starts up. Remember that these alternators shut off immediately if L loses its power supply! Equally important, it takes only a very small amount of constant current to energize L and keep the alternator operating. I've measured current flow on many L circuits; typically, it's only 5 to 8 milliamps (.005 to .008 amp) with the alternator operating normally.

GM's Terminal P

Terminal P, which stands for phase, is a stator terminal. The typical P-L-I-S or P-L-F-S alternators have functioning P terminals, but it's rare to see a wire connected to P on the vehicle. Suppose there's a good circuit from the alternator output terminal (BAT) over to the positive battery terminal and the alternator is properly grounded. In that case, start the engine and carefully backprobe P. Stator voltage on a healthy alternator is one-half charging voltage, plus or minus about .50 volt. Therefore, if voltage at BAT is 14.00, P should be 7.00 volts or extremely close to it. Experience shows that when a stator or diode failure occurs, stator voltage is way above or below the one-half value.

Isolation Test Procedure

Suppose you're working on a GM charging system with the common terminals I described earlier (P-L-I-S or P-L-F-S). The alternator isn't charging and you've already checked the ground and output circuits. Shut off the ignition switch and unplug the harness connector from the alternator. You can isolate this style alternator by exciting or energizing terminal L independently of the electrical system. The alternator is good if it charges properly when you excite terminal L.

(Video) How to properly test a Ford non-Smart charge PCM controlled alternator.

You can buy the proper excitation tool or fabricate one. Keep these points in mind if you make your own: First, many alternators are mounted in awkward locations, where you can just barely get your fingers on the harness connector. Second, the terminals (including L) are recessed deep inside the connector; securely hooking up to L can be a challenge even when the alternator is easy to reach. Third, a secure connection to L is vital for an accurate test.

I've found that using a regular alternator harness connector is the easiest, most reliable way to make your own hookup to terminal L. Source a P-L-I-S/P-L-F-S style connector from one of your suppliers or from a salvage yard. Basically, the idea is to jumper L directly to the positive battery terminal but to limit current flow through this circuit with a small light bulb. A simple, effective approach is to splice a No. 194 light bulb between the battery and the L terminal of your harness connector. A bulb socket with pigtail leads for a No. 194 bulb is readily available in parts stores and salvage yards. For your information, this bulb is rated at 270mA (.270 amp).

Never route a jumper wire directly from battery positive to terminal L because it will almost certainly toast the voltage regulator.

Okay, the ignition switch is off and you're energizing terminal L through a low-current bulb. Now the bulb should be illuminated. Replace the alternator if the bulb is off. But if the bulb is on, start the engine. A healthy alternator will turn out the bulb and then pass the routine volts/amps charging tests.

Note that you may encounter a few perfectly good voltage regulators that can't turn on a No. 194 bulb to normal brightness. Really, your concern is whether the bulb turns on or not, period. But if you're fussy, avoid this potential annoyance by using a lower-current bulb such as a No. 53 instrument panel bulb. It's rated at 120mA (.120 amp).

What’s more, you can increase the utility of your homemade isolation harness by installing a terminal and wire in the P position so you can check stator voltage. If the alternator turns out the low-current bulb and produces normal stator voltage, it’s yet another measurement telling you the alternator is good.

Remote-Mounted Batteries

You may have to diagnose a no-charge condition on a GM car with a remote-mounted battery. The alternator isolation technique in such a case is exactly the same as the one I just described, but it may require an extra step. Here’s the info you need.

GM’s G-body cars of the 1990s, which include the Buick Riviera, Olds Aurora and Cadillac Allanté, have a battery mounted under the rear seat. The charging system on these cars also may have a thermistor that monitors battery temperature. If so, the thermistor is located at the positive battery terminal and is wired in series between the battery and the alternator’s S terminal. Note that on the alternators discussed in this article, the S terminal is always at the opposite end of the harness connector from the P terminal.

First, check the wiring schematic for the particular car, or else visually inspect the positive battery terminal area. If the car is equipped with a thermistor, you must recreate the thermistor-style sense circuit in order to test the alternator accurately. Go to the electronics store and get a resistor or combination of resistors totaling approximately 1800 ohms (1 ⁄2- watt resistors are adequate for this task).

(Video) Comptuter Charging Systems - Ford

Make a jumper wire that’s long enough to go from the alternator’s S terminal all the way back to the positive battery terminal. Carefully splice the 1800 ohms of resistance into this jumper wire. Now, the alternator should operate normally with a low-current bulb in series to terminal L and an 1800-ohm resistor in series to terminal S.

Chrysler Charging Systems

The more some things change, the more they stay the same. For instance, look closely at the most common Chrysler systems in service since 1989- 90. First of all, these systems are basically the same. Second, they’re really just variations of the first-generationChrysler systems of the 1960s. Third, they’re only as complicated as you choose to make them.

On the earliest Chrysler systems, there are two field terminals on the back of the alternator. Battery power is applied to one terminal—technically, it doesn’t matter which one. An external voltage regulator controls the alternator by toggling the other field terminal; that is, it switches the other field terminal on and off to ground. The longer this field terminal is grounded, the longer the alternator charges, and vice versa.

Suppose this earlier charging system doesn’t charge. First, verify that there’s battery power supplied to one field terminal. If there’s no power, trace that leg of the field circuit, fix the problem and retest. Second, if battery power is present at the one field terminal, check for battery voltage at the other field terminal with the ignition on, engine off. Replace the alternator if voltage at the other terminal is 0. This means the field circuit opened up somewhere inside the alternator; the brushes probably wore out. (By the way, worn-out brushes are a common cause of alternator failure on computer-controlled Chrysler systems.) But if the other field terminal measures battery voltage, continue testing.

At this point, you can isolate the alternator from the regulator simply by grounding the ground-side field terminal—the one wired to the regulator. Does the alternator charge? If it does, then either the ground circuit is open or the voltage regulator failed.

You can make another simple system check by connecting a low-current test light, duty-cycle meter or oscilloscope to the ground-side field terminal. Then restart the engine. If the regulator is toggling the ground side of the field on and off, then the test light should blink. Or the duty-cycle meter should show aduty reading. A scope should show the field being switched between battery voltage and approximately 0 volts. If none of these techniques shows activity on the ground side of the field, it’s another clue that there’s an open ground circuit or a bad voltage regulator.

Okay, let’s flash forward to modern Chrysler charging systems. The first big difference is that Chrysler hasn’t produced its own alternators for years. Second, the alternators are often much more difficult to reach. Third, the ECM controls the field circuit, so now a computer has replaced the relatively inexpensive, stand-alone voltage regulator. Fourth, Chrysler gradually changed over to B-circuit control; this means the ECM switches the hot side of the field circuit instead of the ground side.

The test tricks I just described for the old Chrysler charging systems also apply to the computer-controlled versions. But there are important precautions you should take when isolating an alternator on one of the modern systems. Some techs isolate a suspect alternator from the electrical system by grounding the appropriate field terminal with a jumper wire. Obviously, this approach bypasses the entire ground side of the field, including the ECM. If the alternator charges, the tech knows the problem is in that side of the field circuit.

On the other hand, you isolate a Bcircuit alternator by applying battery voltage to its hot field terminal. Remember, B-circuit charging systems began appearing on Chryslers during the 2002 model year. So when in doubt, check the appropriate schematic before hanging a jumper wire anywhere on such a vehicle!

(Video) COURSE UPDATE! New Alternator Testing module added to Understanding Charging Systems!

It’s also possible to isolate the alternator on a Chrysler vehicle by disconnecting both field wires from the back of the alternator. Jumper battery power to one field terminal and then ground the other one. The advantage here is that bypassing both sides of the field circuit completely separates the field circuit from the rest of the electrical system. What’s more, the alternator rotor doesn’t care which direction field current flows through it—as long as it flows. So for the moment, you needn’t worry about which field terminal is hot or which is ground.

You can speed up this isolation technique even more by making a set of Chrysler-specific jumper wires with female terminals. The vast majority of Chrysler products you service have either Nippondenso or Mitsubishi alternators with male spade field terminals. Each male field terminal usually measures .097 in. wide, so a .110-in.-wide female terminal fits fine.

Perhaps the most pertinent precaution for this discussion is a reminder that bypassing the ECM full-fields the alternator. That means the alternator operates uncontrolled on a vehicle laden with electronics, so exercise appropriate caution. For example, be sure all electrical accessories are shut off beforehand. Then connect a digital voltmeter to the battery. Have your jumper wire(s) securely connected to the alternator field terminal(s) before you start the engine. Then start the engine and connect the jumper(s) to power and/or ground as needed—but only long enough to see if the alternator charges. Disconnect the jumper wire the moment you see a reaction on that digital voltmeter.

Experience shows that these alternators usually charge or don’t charge. Revving the engine won’t turn a bad alternator into a good one during your isolation test. The only thing that revving the engine does for an uncontrolled charging system is potentially send alternator output into the stratosphere!

Aftermarket Isolation Kit

Finally, don’t overlook the practicality of using an alternator isolation kit such as the I-60 from JIMCO, Inc. (sales @jimcotest.com; 816-331-1917). I first mentioned this back in the summer of 1999, and it’s still the only standalone test kit of its kind I’ve found. Typically, this kit bypasses all external wiring save for the alternator output wire.

JIMCO has been building test equipment for remanufacturers and auto-electric shops for more than 30 years. One of its specialties has been adapter harnesses that connect a wide variety of alternators to its test benches. Effective adapters save time by minimizing the use of jumper wires and improve accuracy by making an alternator on a test bench think it’s operating in a vehicle.

The I-60 test kit puts this extensive array of adapter harnesses to use in the vehicle instead of on the test bench, allowing you to isolate a suspect alternator from the electrical system. The heart of the kit is a little box with two leads going directly to the vehicle battery. The vehicle-specific adapter harnesses, which link the I-60 to the alternator, plug into the tester’s own connector.

Common alternators have internal voltage regulators. This kit lets you safely test charging systems under normal operating conditions because it bypasses only the external wiring, not the voltage regulator. In fact, the JIMCO setup for testing common Chrysler systems substitutes its own solid-state voltage regular in place of the ECM. This enables you to do a safe and thorough alternator test while bypassing the vehicle wiring and ECM. Where necessary, the adapter harness for a specific application includes features such as a low-current excitation light bulb and a test point for measuring stator voltage.

No, a kit such as JIMCO’s won’t do your thinking for you, nor does it eliminate the need for methodical voltage-drop testing. But it does give you a big head start on dividing and conquering those challenging charging systems. Good luck out there, and keep your cool!

(Video) Computer Charging Systems - GM

Download PDF


How do you test an ECM controlled alternator? ›

ECM Controlled Charging Diagnosis - YouTube

How do you diagnose a charging system problem? ›

Charging system voltage is simple to test. With a voltmeter connected to the battery, load the charging system by applying an electrical load such as the headlights, blower motor, or windshield wipers. The charging voltage should fall within the manufacturer's specification; generally in the 13- to 15-volt range.

What is the first step in diagnosing a charging system? ›

The first step to diagnosing any problem in the starting and charging system is to make sure the battery is fully charged, and in good condition. 1) How many volts does a fully charged battery have?

Does the ECM control the charging system? ›

ECM Controlled Charging Systems Training Module Trailer - YouTube

How do you diagnose an alternator problem? ›

7 Signs of a Failing Alternator
  1. Dim or Overly Bright Lights. ...
  2. Dead Battery. ...
  3. Slow or Malfunctioning Accessories. ...
  4. Trouble Starting or Frequent Stalling. ...
  5. Growling or Whining Noises. ...
  6. Smell of Burning Rubber or Wires. ...
  7. Battery Warning Light on Dash.
Aug 24, 2020

What module controls the alternator? ›

The voltage regulator controls the alternator's output, while the rectifier converts the 3-phase output of the alternator into dc. That dc power then charges the battery and powers the automobile's electrical loads.

How do you test a charging system? ›

AutoZone How-To: Check Your Starting and Charging System

What component should you test first when troubleshooting a charging system issue? ›

Answer: Check the voltage regulator and the battery. An auto parts store may check the battery for you. Also, may sure the circuit connections for the charging system are clean and tight and check the engine body grounds.

What are the common problems in the charging system? ›

3 Common Charging System Problems Your Car May Have
  • Dead Battery. One of the most common problems that indicates a charging system problem is a dead battery. ...
  • Bad Alternator. ...
  • Worn or Broken Belts. ...
  • 5 Dos and Don'ts for Better Gas Mileage.
May 6, 2019

What five steps should you follow while inspecting a charging system? ›

If any of these symptoms are evident, then follow these five steps to pinpoint the issue:
  1. Perform a visual inspection under the hood. Look at the belt tension andcondition. ...
  2. Visually inspect and test the batteries. ...
  3. Measure system voltage. ...
  4. Test alternator output. ...
  5. Troubleshoot using the service manual.
Aug 1, 2013

How do you test a charging system with a load tester? ›

Charging System Test (Using a Carbon Pile Load Tester) - YouTube

What controls the charging system output? ›

The BCM is the brains of the operation, but the PCM is the module that actually controls charging system operation. The PCM controls the signal to terminal L of the alternator to control system output.

Will a ECM drain your battery? ›

Battery drain or dead battery

A failed ECM power relay can also cause a battery drain or dead battery. If the relay shorts it can leave power on to the computer, even when the vehicle is turned off. This will place a parasitic drain on the battery, which will eventually cause it to go dead.

Where is the ECM power relay? ›

The ECM-power relay is verified that it needs to be replaced. The relay is located in the engine main fuse relay control box. The defective ECM-power relay is removed from the control box by pulling it straight up and out of socket. The new ECM-power relay is installed into the socket and the engine scanned for codes.

How do you test a smart alternator? ›

During the drive, have the other person monitor the meter. Should the voltage go up and down between around 12-15V then the vehicle has a smart alternator and you will need to use a DC-DC charger. If it stays steady & consistent at about 14V or so then you will have a fixed voltage alternator and be able to use a VSR.

Can you bypass a battery current sensor? ›

Electronic Battery Sensor - YouTube

How do you turn off a smart alternator? ›

You won't believe this simple trick to disable a Smart Alternator! Nissan ...

What is a full field alternator test? ›

Voltage Regulator: Full Field Test

Test the alternators regulator with a full field test when the alternator has low output. This test bypasses the regulator energizing the rotor with unregulated voltage. A Type-A circuit has the voltage regulator on the ground side of the field coil.


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