Changed out both batts three weeks ago, but truck still died. Had it taken to dealer (due to warrintee) and they changed out just the upper, main, alternator. Wife unit picked up the truck so I couldn't talk to service folks. Called this AM and was told that secondary alternator has nothing to do with charging system, and mechanic tested system and is charging normally.
Is this the way a dual alternator set up works? What exactly is the secondary alt. for? Just want to make sure I'm correct and need something concreate to dispute with them. Thanks.

 

Good question, I allways thought it was for charging also.

 

I guess I am not sure how the factory did it. On my truck, I added one and use/control it to charge a second set of batteries. If I was you, I would unhook the upper alternators control wiring, and check battery voltage. If it 13.5 or higher, the secondary works. If it is about 12.9 or lower, its not doing anything.

 

Guess I could disconnect the main and do that check, but I really want some ammo to take this issue back to the dealer, cause they are pi__ing me off today. Ya know, load both barrels!

 

If you do like Mike says and the second alt is charging your batts you will unequivocally prove them wrong on that point.

 

What would be the purpose of the 2nd alternator if it didnt help charge? Isnt that what an alternator does?

 

i was told it is for a almbalance package.

 

Me thinks that ALL systems, to include trailers, fifth wheels and such run off the batteries and the alternator(s) recharge the batts, true? If thats the case, why wouldnt the secondary alt. be tied into the charging system for heavier draws when connected to and running lights or accessories? I don't know much 'bout electricty, thats why all the questions.

 

The batteries start the car, and run things while the motor is off. Once the car is running, the alternator charges the batteries, and runs whatever you have running that requires electrical power. The batteries do power things that have high current surges, that happen faster then the alternator can compensate for. For example, car stereo amplifiers, when they hit a high current note, or the initial inrush current/ surge from turning on a couple high power lights, or initial trailer brake surges. You can see this on a real voltmeter. The voltage may dip for a second, using the battery power, untill the alternator can compensate for the load, then the voltage goes back up. For example, when you first turn on two 100 watt lights, the initial inrush current may actually be 70 amps, but this tapers very fast, and they may only draw 70 amps for 20 milli-seconds. While they are running at operational load (elements both lit up, and stable) two 100 watt lights, should draw approximately 14.8 amps, at 13.5 volts. Hope this helps some.

 

I didn't ask the question very well, so; what role does the secondary alternator play? and if the batteries die due to charging problems do both alternators need changing?
Thanks

 

You would only have to replace the bad alternator. But you should find out if just one charges your battery or if both do. I would what to know that just for peace of mind. If I had a dual alt system and only one charges the battery and I had to replace it I would go with a 200 amp alt and do away with the second one. The second alt would only be necessary if you were running a bunch of DC accessories or an inverter for AC use. That is why it is sometimes called the ambulance package. They run all kinds of extra lights and DC accessories and maybe even an inverter.

 

Thanks, for the answer. Guess the steelership kinda told the truth.
Ya'll have a great weekend

 

I made the mistake of trying to add the second alternator to the main system. I tied it in to the main battery terminals. It worked fine at idle but any RPMs would shoot the voltage up over 14V and threw the charging system warning lights. No matter what I tried It just wouldn't work. Even tried one high amp up top and a stock one on bottom. Same thing happened each time. I think I burned up 4-5 alternators trying to get it to work. I finally gave up and wired the lower one up to charge a triple set of batteries (and fifth wheel camper) in a box on the frame. I use a dash computer (gallery) to turn the lower alternator on & off as well as throw a relay to the main system just in case it goes dead.

If you think you need any extra power simply add a high amp to the upper mount and you'll do fine. I wasted over $2K on that adventure.

However, if you search the forum you'll find that a couple people have been successful retrofitting a makeshift ambulance package using the two stock alternators. But no one that I know of has both permanently wired together.

 

FWIW, with the oem single alternator, in the winter, your voltage runs upto about 14.7-14.8 even at idle. My system is similar to yours, in the way of having a separate battery system to charge, and getting its own alternator. If one alternator were to fail though, I have a large relay, to join the two systems, and work until I can replace the bad alternator.

 

Here's a quick rundown of how the dual alternator set up works.

The optional dual generator package consists of an upper/lower dual generator system. The upper generator is a 6G 50/110 generator with a 2.49:1 pulley ratio. The lower generator is a 6G 50/110 generator with a 2.32:1 pulley ratio. Thus the upper and lower generators are not interchangeable.

This system is also monitored and controlled by the Powertrain Control Module (PCM) . The PCM monitors both the upper and lower generator
I circuits to determine both generators output and set possible fault codes (Diagnostic Trouble Codes [DTCs] ).

The I (ignition) circuit is used to turn on the voltage regulator. This circuit is powered up with the ignition switch in the RUN position. This circuit is also used to turn the charging system warning indicator on if there is a fault in the charging system operation.

The PCM only controls the lower generator by turning it on or off by opening or closing the lower generator off only when the glow plugs are commanded on by the PCM to avoid possible damage (excessive voltage) to the glow plugs.

Both generators are independent of one another and only control themselves (through their own internal voltage regulator). If one generator fails, the other generator can maintain system performance under light load conditions. Under a heavy load, the PCM should detect a problem, set a DTC and illuminate the warning indicator.

The PCM controls the charging system warning indicator and will command the light on if the PCM detects a problem on the monitored circuits.

Austin