Oh boy, more vibrations.
Hey, I'm back. 
Last year, we had this strange problem with vibration throughout the coach when the engine/trans was in a very specific configuration. You all had a bunch of ideas of what it could be, and then, some 3000 miles later... it just went away completely.
We've done several shorter trips since then, including a weekend one a few weeks ago that had 400 miles of perfect driving. Although, we did start out with a dead engine start battery that I revived with a tender and then had working for the drive.
Interestingly, during that trip though, I had this weird retarder problem: the retarder works if I have the switch ON and I press the brake pedal. But if I just let off the gas, the retarder didn't engage. Super annoying, but still safe and comfortable on the downhills, as long as I remembered to at minimum feather the brake pedal. Allison reported a service code SERIAL COMMUNICATION INTERFACE FAULT
Well, that was our shakedown trip for this summer's long one, which we started today. Well, tried to! My engine batteries were this time super struggling, enough that I gave up and went and bought two replacements and swapped them out. Before that, I did manage to get the engine... partially cranked? It was really weird, like it was lugging or something, the whole time. I let it turn itself over for 30 seconds (I let off the key, engine was turning on its own without the starter) before giving up and shutting it down.
Anyway, got two new batteries, and everything spun up just fine. And shortly thereafter, a little sweatier but otherwise none the worse for wear, we started out.
Along the way, shortly, I noticed a lot of voltage fluctuation on the meter I have up front. Like, I was expecting stable 13.5V or more, but it was dropping down into the low 12s and kind of bouncing around. I got out and put my clamp meter on the battery cable and saw strange fluctuation of current, sometimes just draw and then momentary current the other way like it was charging, and then back. Oscillating, at times.
My theory was that we'd burned up the alternator asking it to try to charge this nearly-dead battery, and that I'd need to replace it or something. I did some reading and decided that I would tackle this back at home and with the benefit of all of my tools and the shop, so we turned around and headed 20 miles back.
Along that way, I noticed that the voltage had come back up to something reasonable, and then separately I felt some strange vibration throughout the coach coming from the rear. It reminded me of those adventures last year, and not in a good way!
We got home, and I put more meters on the coach and the alt, and now... it charges just fine, surprise surprise. I took the coach back out for a 20-mile test spin around the "block" and the voltage stayed high the whole way, just like it always had before. The battery seems fine, and the engine starts great. Weird?
Anyway, we got back in to head out, again, and this time... there is a Check Engine light that mostly stays lit but occasionally flickers. AND the Allison no longer shows its SERVICE fault on screen. AND the retarder now works the way it always did before, engaging whenever I let off the pedal!
Except, for sure: now, whenever I ask for more than a nominal amount of torque from the engine, at least in gears above 2, I get a vibration that builds throughout the coach. Holding cruise at 62mph on the flat freeway, it's fine. Ask it to accelerate, it'll vibrate. Set cruise at 65mph, the vibration starts (needs more power because of wind resistance, I bet). Even at 35mph in 4th gear, accelerating creates the shakes. I don't seem to get the shaking in 2nd gear in parking lots, though.
This vibration is not the same as the one last year at all in terms of what seems to trigger it, but it is the same in terms of making the coach feel uncomfortable and like, if it continued, it'd be a tiring ride at minimum and maybe damage some interior parts if it kept going.
So... I'm at a loss (and tired, ha). I don't know if any of these things are related. I'm sort of happy that my retarder is working again, and I like that the alternator looks like it's behaving nominally. I haven't DONE anything, ha ha. I'm not comfortable going on a long trip with the engine behaving like that, but I'm not sure what to do next.
To clarify: two weekends' ago, the trip was perfectly smooth. I've changed essentially nothing in the coach... we have the same the loadout, the same towed car, it's the same tank of diesel, etc. Oil was fine right before we left. With the exception of the fresh chassis batteries, I can't think of anything else that I can point to as a smoking gun.
Any ideas? (First thing to do is find a scan tool to get the code from that check engine, I know. I do have a Bluefire, but the app updated seems to have updated itself into a state where it is incompatible with the hardware, so apparently I have to mail the hardware in to get an update... yeesh.)
Topic: Oh boy, more vibrations.