Diesel charging

Jan. 1, 2020
First and foremost, the SAE J1930 regulation dictates that if the PCM controls the alternator field, the alternator is supposed to be dubbed a generator, but if it has its own internal regulator, it’s still OK to call it an alternator.

First and foremost, the SAE J1930 regulation dictates that if the Powertrain Control Module (PCM) controls the alternator field. The alternator is supposed to be dubbed a generator, but if it has its own internal regulator, it’s still okay to call it an alternator.

The magnetic field on a generator is created in the windings of the spinning rotor. Since the strength or weakness of that field controls the charging output, the spinning rotor receives a controlled field current from the voltage regulator, or the PCM through some small sintered metal brushes sliding on copper rings connected to each end of the field winding. The magnetism in this spinning rotor field induces current in the stator windings, which are the stationary ones around the outside. These are wrapped through and around a laminated steel core, and there are three of them, with each winding either tied together at the ends in a Delta configuration or tied together in the middle in a "Y" configuration.

The three output ends of these stator windings are connected to the center of a very robust rectifier bridge, usually consisting of six diodes. The rectifier is necessary because the current that is created in the stator is a three phase alternating current (AC) and has to be converted to direct current (DC) in order to charge the battery and feed the electrical system. The charge makes its way to the battery through a heavy gauge wire connected to a 6 mm or 1/4" post securely married to the positive side of that rectifier and connected to a positive junction point, and the negative side of the rectifier is connected to the generator’s aluminum case, which is grounded to the engine.

Since an alternator on full field can destroy a battery in short order, voltage regulation always has been a big deal. In the early days, it was handled by an external voltage regulator that typically had four wires connected to it and was grounded to the body and the engine. As for the four wires, there is a field wire that controls the generator’s charging output. This wire is a switched feed that can either be negative or positive, depending on which side of the stator is hard wired.

Then, there is a battery wire to sense system voltage. Next, there is an input to the regulator from one of the three legs of the generator stator (GM called this the R terminal for years) to tell the regulator if the generator is actually charging. Finally, there is an ignition terminal that turns the regulator on. This turn-on voltage flows through the charge light and a parallel resistor to make the feed constant, even if the bulb blows.

Generators with internal voltage regulators work pretty much the same way, except the stator feed is typically done inside the generator. Internally regulated generators typically have the heavier gauge current carrying wire to feed the brushes and sense system voltage unless that feed is an internal one. The one wire that’s always fed externally is the turn-on voltage that comes through the light or from the PCM, and the third one, if present, will be a stator output if the system needs to use that voltage for the PCM to determine if the charging system is operational.

Diesel Differences

On some (not all) diesels, two fairly normal size generators are appropriated for packaging and control purposes, but gas burners typically have more sophisticated generator systems than diesels. The two generators will collectively produce from 170 to around 240A, depending the platform, and it would take a very large and expensive generator to make that happen with a single unit. Some of the earlier diesel pickups and vans I serviced at the Ford dealer had massive generators that were expensive.

I remember replacing one large unit on a diesel ambulance that cost $1,200. That monster obviously is the exception rather than the rule, and most people haven’t even seen one of those. A single normal sized generator usually costs between $250 and $400.

The cool thing about having dual charging units is that one generator can be shut down during the times when it’s not needed. General Motors and Ford have dual generators on Power Stroke and Duramax, but the Dodge RAM has yet to use a second generator.

Most light vehicle diesels either have two strong fairly normal sized batteries or one very strong larger battery, and the charging system has its work cut out for it, especially with the glow plugs and manifold heaters diesels need. The battery charge has to be recovered quickly after juicing up all that stuff and then spinning a heavy starter to squeeze the air in a bunch of 18:1 compression ratio cylinders, and while one generator can do the job, two are better in my humble opinion.

When a diesel comes with a pair of generators, pulley sizes and output ratings are different from one to the other, and the two units aren’t usually interchangeable. Here’s an overview of the domestic three.

Dodge RAM Diesel

The Dodge Diesel charging system works exactly the way it has for decades, except the field is hard wired to the ground, and the PCM sends pulse width modulated signals to control the field. The ECM receives a voltage reading from the generator, and a battery voltage input from the Totally Integrated Power Module (TIPM), and it compares that measurement to the voltage required by the generator’s Electronic Voltage Regulator (EVR), which increases or decreases output accordingly using all-too familiar pulse width modulation to switch the field on and off.

When engine speeds are high, the pulse width delivered to the alternator field can be as low as 10 percent, and when the engine is idling, the pulse width can be as high as 90 percent, which is a fairly consistent principle in just about any modern charging system.


GM’s Duramax is equipped with two generators, namely the AD-230 and AD-244. The "AD" stands for “air-cooled dual internal fan", the "2" designates the electrical design, and the "30/44" part of the tag denotes the outside diameter of the stator laminations in millimeters.

Gas burners have a rather complex battery/generator control module that monitors the battery’s health and mitigates the generator’s load on the engine, but that system isn’t available on the Duramax.The two generators are rated at 102 and 130A respectively, and the voltage regulator in each unit controls the field current of the rotor, which is par for any charge system regulator. The regulator switches the current on and off at a rate of 400 cycles per second for radio noise control and, more obviously, to provide control of charge current.

Power Stroke

Ford diesels come with either a single or a dual generator setup, and 6.0L platforms outfitted with a dual system have an upper and a lower generator, with the upper generator a 140A unit. That one is fitted with a 2.79:1 pulley ratio. The lower generator, a 120A unit, spins at a slightly different speed, with a 3.00:1 pulley and is not interchangeable with the upper generator.

The 6.4L, when equipped with a pair, has its two generators mounted together on the driver side of the engine and aren’t as powerful as the generators on the 6.0L. The pulley ratios are the same, but the primary generator on a 6.4L boasts only 95A, and the secondary unit pushes only 75A. Even more anemic, the standard single generator on a 6.4L is a 65A unit.

The I terminals on both generators is fed by the PCM, which in turn illuminates the charge indicator light if the generator grounds that terminal due to an absent 7V stator feed. Single generator I terminals are fed through the charge indicator light.

The I terminal feed

Key troubleshooting point: Voltage regulators have to be turned on in order to work, but it only takes a whisper of voltage to the I terminal to wake up the regulator. If that voltage is absent when the engine is started, the generator won’t work. This circuit is important and often overlooked during diagnostics.

If the turn-on wire that feeds the I terminal is shunted to ground, the charge indicator light will be on, and the generator won’t charge. In cases like that, the generator is just about always replaced (unnecessarily) before the problem is found because in most cases, a charging problem is generator related.

In cases where the PCM sends the turn on power to the I terminal (through a current-limiting resistor internal to the PCM itself), this condition will typically throw a DTC. If, however, the I terminal’s turn-on feed travels through the charge indicator light, it’s important to check that light and see if it illuminates with the key in the on position (This is the smartest first step when addressing a charging problem anyway). If the light doesn’t work on a system like this one, find out why, and you’ve usually found your fix.

If the indicator always illuminated at key on, disconnect the generator and see if it goes out. If it does, that means the ground is coming from the I terminal rather than from a pinched or chafed wire. Even if the PCM feeds turn-on voltage to the I terminal, that generator won’t work if that circuit is shorted to ground somewhere.

For testing, connect a low impedance test light to the hot side of the battery and touch the I terminal to see if the charging system comes online. This is actually bypassing the existing circuit to turn on the regulator. If a dead charging system comes online when you do this, troubleshoot the I terminal circuit but make sure you know where it comes from.

Battery sense/brush feed

It goes without saying that a voltage drop test should be done on the charging circuit (the big wire that actually charges the battery). Connect your meter between the big post on the alternator, and the positive battery terminal and look for less than 0.5V with the generator spinning and current flowing. The charge circuit has traditionally been fed through a fusible link, a two to four gauge size smaller wire than the wire it’s protecting. It’s designed to handle current surges but will burn out if a dead short occurs.

This kind of fusible link has always been a special platinum alloy wire coated with heat resistant DuPont Hypalon insulation that usually remains intact even if the link blows, but DuPont stopped manufacturing that elastomer in 2010, and aftermarket replacement fusible link wire has heat resistant insulation, but it’s plain old copper. If you don’t have power to the big post on the alternator, find the fusible link and see if you can stretch it. If you can, it’s blown, and you need to find out why.

The voltage regulator is totally in control of the field strength on light vehicle diesel generator systems, and the rotor windings usually have from 4 to 6 ohms. The battery feed to the alternator is either appropriated internally from the big charge wire’s circuit, or it’s separately fed through an external connector plugged into the generator’s internal regulator. This voltage provides system voltage information to the regulator, and it also feeds the hot side of the brushes on most units.

There are exceptions so make sure you check your schematic before down and dirty diagnosis. Because this feed is supposed to be providing accurate system voltage info, a relatively minor voltage drop situation on this circuit can feed enough power to the field to make it really strong while under-reporting system voltage. For that reason, voltage drop on the battery sense circuit can cause overcharging. I’ve seen that a time or two, but usually on one of these, a faulty voltage regulator causes overcharging.

When there are two

Because dual alternator systems like the Power Stroke don’t use both alternators all the time, you can verify each alternator’s willingness to push amps by isolating first one, and then the other with your charge tester connected. Disconnect one alternator and feed the I terminal on the opposite unit with your positively charged test light probe to wake it up.

If the charge numbers look good, and there’s a notable amount of current flowing on each one, you’re beyond a reasonable doubt as to whether both are capable. All in all, diesel charging systems are simpler than the ones on gas-burning platforms and, quite frankly, they don’t give much trouble at all.

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About the Author

Richard McCuistian

Richard McCuistian is an ASE certified Master Auto Technician and was a professional mechanic for more than 25 years, followed by 18 years as an automotive instructor at LBW Community College in Opp, AL. Richard is now retired from teaching and still works as a freelance writer for Motor Age and various Automotive Training groups.

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