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Thread: Those with Dual Alternators Reply to Thread
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  Topic Review (Newest First)
02-23-2019 07:54 AM
clev I did the high idle mod quite a few years ago. Thanks for the recommendation.
02-21-2019 09:53 AM
Nickelplated5s Yes, but if he wants to turn his truck into a generator go all in. That mod is fairly cheap and would add flexibility.
02-20-2019 04:18 PM
Originally Posted by Nickelplated5s View Post
I would say do the high idle mod at the same time if that's the route you want to go.

From my impression he is only concerned a this time with charging thee batteries while in transit.
02-20-2019 12:09 PM
Originally Posted by Nickelplated5s View Post
I would say do the high idle mod at the same time if that's the route you want to go.
That's a great idea. Not much juice generated at idle.

Originally Posted by 1SapperAirborne View Post
Is my battery light coming on because one alternator is faulty and when the other kicks in it picks up the slack? Or is this a normal design operation for the dual operator setup.
I think you are on the right track thinking your primary alternator has quit functioning correctly. Definitely not designed to have the light on with an alternator code. Easy to pop it off and run it down to be tested.
02-20-2019 10:45 AM
1SapperAirborne Dual alternator setup, not operator...😂
02-20-2019 10:40 AM
1SapperAirborne So, I’m getting conflicting information concerning the dual alternator setup. I have a 2005 F350 and have owned it for 5 years. When I start it cold, my battery light has come on consistently for 20 to 30 seconds while the GPCM is active. I’m aware that the PCM turns off one alternator to keep from overpowering the glow plugs. I can hear the relay click when second alternator kicks in and the battery light goes out.
So my question is:
Is my battery light coming on because one alternator is faulty and when the other kicks in it picks up the slack?
Or is this a normal design operation for the dual operator setup.
When this occurs, I do get the generator fault code, but I’m able to clear the code and it doesn’t return until the cold start cycle repeats itself.
02-20-2019 10:02 AM
Nickelplated5s I would say do the high idle mod at the same time if that's the route you want to go.
02-19-2019 08:17 PM
RT On the alternator, there is a lug usually on the back that is the output. Thatís the one you will run to the batteries to be charged. Then, on the side, there is usually a plug with one or two connectors. One of those is the field terminal. It is used by an internally regulated alternator to determine the required output. If your cable to the batteries is large enough, you can just run it over to the back terminal as the voltage difference between each end of that large cable will be negligible.

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02-19-2019 08:05 PM
clev Perhaps I wasn't clear, but I do not want it PCM driven. That's why I mentioned running the cable straight from the alternator to a plug at the back of the truck for a cable running to the batteries. The 370 amp alternator shop, builds per order. I'll give them a call. By 'field terminal', I assume you're referring to a plug-in assembly for a hookup between the truck and trailer, or am I totally off base?
02-19-2019 07:53 PM
Originally Posted by clev View Post
Question: how would you regulate the charge from the second alternator to the rv batteries? Or are these newer alternators self-regulated to kick on and off depending on demand for charge?
So, that goes back to my original post about needing the field coil being fed from the batteries in the camper. On a self-exciting alternator, you would simply run the output of the alternator to the batteries using an appropriately sized cable - probably 1-0 or 2-0. The field terminal would be attached to that cable. On your fancy new alternators with the "better idea by Ford" technology, the alternator is regulated by the PCM. I have no idea how you would control it. I would suggest - if you insist on going that route - of getting an alternator shop to build you a self-exciting alternator that fits into the case size of the second alternator. For example, if the alternator is a Ford 3G case, just call one of the shops and tell them what you want to do and need an appropriate alternator in that case size. Then, it won't be PCM driven - it will only be driven by the voltage of the batteries you want to charge.
02-19-2019 02:03 PM
clev RT, Larrgh, thank you both; very good ideas. I have a bachelors degree in math and science and fully understand everything each of you explained. I, too, had wondered how to separate the charge from the truck batteries and the rv batteries. Keeping them on the same charging circuit is not conducive to either one. So, yes, I agree; a second alternator is best solution, and keeping the second one solely for the RV batteries would be my choice. Having the dealer program my vehicle, even if it's only for alternators gives me cause for concern. The 370 amp charger probably will not fit the second charger position; it calls for 157 amp charger.

Question: how would you regulate the charge from the second alternator to the rv batteries? Or are these newer alternators self regulated to kick on and off depending on demand for charge?
02-19-2019 11:50 AM
larrgh This is what I contemplate doing to solve this problem.

I would see if the high amp alternator, you are considering, will fit a truck that has dual alternators.

If so,

Then, I would contact an auto electric shop to see if you can configure it as I have described below, without any safety hazards.

If they confirm that it won't pose any safety problems, I would go ahead and convert the configuration of the truck to have dual alternators. I would change the water pump and add any brackets required to add the second alternator.

Instead of changing the PCM to see two alternators, I would leave it as configured for one alternator to keep the truck batteries charged.

Next, I would add the high amp alternator as the second alternator, Then add any necessary fused wiring and regulator to have this high amp alternator feed the truck's 7 pin connector or separate connector, to feed 12vdc to the trailer batteries.

I would make sure the high amp alternator would sense the trailer batteries and output proper voltage/amps to charge them while you are in transit. When the trailer is not attached, this alternator should not put much load on the engine nor reduce your mileage.

Will this work? No clue, but sounds feasible and practical in theory.

Appears these are the parts you would need beside the high amp alternator and associted wiring, etc

Alternator Belt for dual alternators: BC3Q-8620-GA

Alternator Fasteners:
W715092-S437: M10 x 92 mm (lower alt bolt) Qty 2
W715094-S437: M10 x 40 mm (upper alt bolt) Qty 2 Secondary

Water Pump*: BC3Q-8501-BB

02-19-2019 08:49 AM
RT I think that was Chuck's way of saying, "Tag, you're it..."

So - things to think about. Electronics are scary to a lot of people. Mainly because electrons behaving like particles some times and waveforms the next. For DC current, think of electricity flow and water flow as the same. Voltage is pressure. Current is flow. Your alternator has a certain flow capability, but the flow isn't what determines the rate of movement from one body to another - it's pressure.

Think of this example. You've got two tanks next to each other. You put a line between the two tanks. One tank has 12.54 feet of water in it, the other has 14 feet of water in it. Water will flow from the higher tank into the lower tank at a certain rate. Now, if the first tank only had 6 feet of water in it, the pressure difference would be greater and the flow would be faster.

In our vehicle, there is a voltage regulator built into the charging circuit that determines that pressure. If the desired battery voltage is 12.54 volts, but the voltage regulator senses there is only 11.5 volts, the output of the alternator is increased. The VOLTAGE is increased - up to a predetermined maximum.

Actual current is (flow) is also fixed - by whatever rate limiting factor is in the system. Yes, a larger wire can carry more current, but the actual rate-limiting factor in battery charging is the chemical reaction taking place inside the battery. You could run 10 1-0 cables to the battery and it couldn't accept any more charge than if you did just one. The rate of charge CAN be influenced by the voltage (pressure) presented to it. Charge a battery with 13 volts from a 5000 amp alternator and it will charge more slowly than if you charge another battery with a 15 volt current from a 90 amp alternator.

So - why are your batteries in the trailer charging slowly? Because your alternator is set up to see the voltage of the two batteries under the hood. If those batteries are essentially fully charged, the alternator is only going to put out enough voltage to cover the operating needs of the motor and just a tad more to "float charge" the vehicle's batteries. The fact that you have 6 batteries downstream hooked up with an 18 gauge wire through your trailer plug isn't going to move the needle much when those under hood batteries are fully charged.

So - solutions.

One would be to let the alternator "see" those discharged batteries directly and then have a cable large enough to carry the current to them. The larger cable because once that alternator senses 10 volts, it's going to start putting out its maximum current and that current is going to go down the easiest path - which right now is those batteries under the hood wired with finger sized cables - not so much the ones wired with 18 gauge cable in the back. In fact, that cable will probably just melt. Still, that's probably going to cook your under the hood batteries.

Two would be to have an independent charging system. I can see two ways of doing this. I had a friend that was into those death bass trucks back in the 90s. He had 6 amplifiers and 8 monster bass speakers. 4 marine batteries running the sound system. Same problem you had. So, I put a second alternator dedicated to those batteries. Same ground, but the output of the second alternator went through a 3-0 cable directly to the other batteries. That worked perfectly. Probably not so easy in your case. The second, slightly round the bend way of doing this would be to have a different type of self-regulating charging device in place. The easiest way I can see to do that is to have an inverter wired into your vehicle batteries and run that 120-volt ac current back to the trailer where you had a decent sized battery charger installed. That's essentially what your ONAN generator is doing when it's running. You'd have to size the inverter to the charger's maximum draw with a good size safety margin built in. After thinking about your problem for a bit, I think that would be the best, cheapest solution.
02-18-2019 07:37 PM
clev RT, chuckster said that you were the wizard for wiring. Can you elaborate on "switch that moves the field wire of the alternator to the discharged batteries."
02-18-2019 01:50 PM
clev Thank you, RT. I'll shoot chuck a PM
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