Sierra at the local Chevy garage.

[DickWells]

Hauled in from Fair Haven, VT on Sunday, with Jean and me in the cab of the flat-bed. Just 5 miles over the AAA 100 mile max. at $3/mile. Whew! Trailer came up here on Tues. night at 10:30, at just 1 mile over the 100. Whew, again. Glad to give both drivers a tip.

A vibration started a little below Washington, DC. Felt like engine, with ~crankshaft frequency. Stopped at first opportunity and checked all 8 wheels, and looked for anything else that was obvious. Continued on to Harpersville, MD. Next morning, started with no problems and ran to Onionta, NY. Again, started and ran fine all day, with the vibration. Most noticeable under load. Stopped at convenience stop in Fair Haven. Got back in, started instantly, turned 4-5 revolutions and shut down. Starter won't turn! All dash lights glow until you turn to start, then go out, just like normal. Thinking bad batteries. Nope. Batteries full-up. Jumping the starter terminals results in fast turn-over and instant start, but, instant shut-down. Edge monitor says can't connect to ECU. So, I dis-connect Edge and re-hook to stock. Same result.
Check all fuzzes. All okay. Swap starter relay under hood with identical relay. Same result. Hook up my new code reader. No codes set, none pending. Call AAA.

Have had a "Service 4 Wheel Drive" message (intermittent) for a year or more. Have changed the dash switch, and freshened up the big ground at the sub-frame under the driver. Both turned off the message for a couple of days. The message came and went all the way from Texas. I've sort of learned to ignore it. My bad, maybe, cuzz it occurs to me that maybe the encoder, or whatever it's called, on the rear of the transfer case may have just given my ECU/PCU a reason to turn off the system; like maybe the vibration was a partial in/out of gear condition?

Anyhow, it sits in McMahon.s yard, they, having been told by me that there's no hurry. Probably shouldn't have said that.

Like to know what Y'all have to say about this one. And thanks for listening.

[Kennedy]

Did you try it with the Juice removed? That was my first thought until the no crank=bad.

[DickWells]

Yes, John. Removing the Juice was my first action. I did read the message on the Juice monitor that it could not connect with the ECU before I swapped the connectors, though.

[DickWells]

Truck is back home! Apparently, the young DM driver who used my ohm meter to bug all the fuses didn't use his younger eyes to good advantage, because our Chevy guys found a blown 40 A ignition fuse under the hood! Truck started right up and, according to them, ran smoothly. No codes found and non to delete. All stored memory was lost, including the Service 4WD message. When I went to pick it up, though, the message was back on. The service manager told me that the Edge still wouldn't connect to the ECU. Well, duh! The dang thing was still dis-connected! When I got home, I re-connected it and it came back on immediately, with the last parameters still set, and works just fine.
The garage couldn't sense any unusual vibration, and I'm assuming that, that was when the 4WD message wasn't showing, because as soon as I left their yard, I could feel it, just as before.
My son drove it and gave me his "take" on the vibration thing. Seems if you downshift on a hill and run the engine up at a higher rpm, the vibration stays the same. Good, enough, that's one test I hadn't done. So, it's apparently not crankshaft frequency. Now, I'm back to wondering about that encoder in the output end of the transfer case. ?????
They had 1.5 hours in it when they found the blown fuse. So, I have to look at the $148 as cheap, compared to a new ECU, or whatever else could have been wrong.
Kicking my own butt that I didn't just get back with my VOM after the young guy left, and double check the fuses. Live and learn, I guess. Fun, getting old!
Where to go from here? Will get it up on the Bend Pak and check for visuals, and look to getting that 4WD message to go away. Thoughts on how to prove that it's the encoder (or whatever it's called) that needs changing, bearing in mind that the usual codes are not there, and never have been?
Thanks, guys and gals.

[jbplock]

Dick, I had a Service 4wd message when my truck was young at 18k mi but no vibration was present at the time. Dealer replaced the encoder and it hasn't ever returned - now at 113 k mi. Have you checked your universal joints? They may be a source of vibration if worn. ??

[DickWells]

I'll be checking everything, hoping to find something as simple as U-joints, Bill. Since I have a very slight leak at the rear of the transfer case, I intend to replace that encoder and the rear seal anyway. Ashamed that I haven't seen fit to tear into this thing for a year or more that the message has been popping up.
But, with no codes set, it just hasn't seemed that urgent.