My F-350 diesel has 2 alternators. They seem to be okay now, but sooner or later I will have a problem. So my questions is how does the system work? Is one a backup or do they both work at the same time? If one goes bad do they both get replaced? How would I know which is bad?

 

Good question. Here's the primary generator diagram.
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Here's the secondary diagram'

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Looks like the secondary is hooked to the "GEN" light in the dash. I don't know how you would know if the primary has gone bad. As you can see their outputs are just tied together and hooked to the batteries.

You could probably unplug the white/yellow wire from each alternator and then monitor the voltage at the battery. The white/yellow should take the alternator "offline".

 

This is a very good question in my mind too.... and the same one I began asking myself a short while back upon acquiring a 2001 E350 diesel that also has the dual alternator option. -- In my case though, I had charging problems from the very outset, and subsequent testing revealed that both alternators had gone south. -- My immediate concerns were how to go about determining what *might* have caused a catastrophic failure of both alternators at once (even though I have no proof they actually did fail simultaneously).

My 2001 E-series workshop manuals give an operational description that states the secondary is controlled by the PCM. My initial interpretation of that statement was that the output of the secondary is continuously governed by the PCM (independent of the primary alt) in order to help balance the loads between the two during normal operation. But after thinking about it awhile...and with further reading - I'm now convinced I was mistaken about that assumption, and have been attempting to gain a fuller understanding of how things work.

As Franklin2 points out, both outputs are tied together... so it would seem that an isolating procedure (as he suggests) would be one way, and perhaps the simplest, of determining the operating status of each alternator.

But my questions about the operating principles go a little deeper still. The workshop manual only references on/off control of the secondary alternator's output in connection with engine start-up and over-voltage protection of the glow plugs. But that doesn't address the question of how the PCM might control one or the other (or both) of the two alternators. -- And in fact, it doesn't appear to me now that the PCM can actually control one alternator independently of the other very well.... nor does it appear that there is any monitoring of the charging system other than as a whole. In my mind, this implies very little effective control on alternator current outputs other than as a combined quantity. It thus seems to me that control information from the PCM is broadcast under the fixed presumption that both alternators are both capable and do accordingly respond in a balanced and equal fashion. -- But I have to wonder about such logic.

Considering all of the above just provokes even more questions for me. For example; what happens if one alternator responds differently to the PCM's command inputs than the other alternator? -- And how might that affect the stress on either (or both) of them, given that the PCM is apparently calculating it's output commands based upon a combined averaging status between the two. To complicate matters more... the pulley sizes are different between them...and this is apparently by design. -- So somehow there must be some theory behind the control system that intends to cover different engine rpm ranges for purposes of improved idle charging output etc. If that's the case, then it follows that the same design-theory surely must also account for some degree of differing outputs between the two alts; at least at engine idle anyway.

Here's a link from NAPA that offers a bit of discussion on Ford's PCM charging-system control approach. I'm not sure how precisely this approach might (or might not) be implemented in a dual-alternator vehicle.
NAPA - PCM Controlled Charging System

I'm still just beginning to learn about all this myself... so I too would appreciate anyone else's more seasoned and experienced input on the matter.

dave

 

I hope you don't mind, Bob, but I copied and posted your question up in the 6.0 Forum. Me thinks it'll get more people to take a crack at that answer. And as my 05 has dual alternators, I too have an interest in seeing answers!!

 

Thanks for that Napa link. I had heard about the compensation for temperature, but didn't know about the other stuff. But I believe they are solving a problem that never existed. I know that seems a little narrow minded, especially since such smart people came up with such a system, but I have worked with these engineers through the years, and they have to justify their jobs somehow. Something simple that works and doesn't require much engineering does not guarantee them job security. The big wigs pay them big salaries, so they expect big things from them.