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Discussion Starter #1 (Edited)
This issue showed up first when I went to an OHV park and couldn't make it up ordinary inclines that used to give me no trouble at all just the previous weekend on another outing. Since then, I've determined that anything but the lightest throttle in 4 low causes it.

Basically, the engine surges up and down in RPM, really hard, causing a herky-jerky forward movement. On heavy inclines it just won't give any power. The problem gets less severe the less final drive torque there is. So it's worst in 4LO 1st gear, gets progressively less extreme up to 4LO 4th, and when in 4HI it is essentially nonexistent unless under max throttle from 1st gear in sport mode, at which point the faintest whiff shows up when the truck is cold.

Oh, and under heavy throttle in 4LO reverse gear, there is no herky-jerky movement. It just gets up and GOES. That pretty well rules out propshafts, diffs, or anything downstream of the trans, really.

There are no codes, pending or recorded. The transmission has had an oil change recently, with no nasty metal bits stuck to the magnet, discolored oil, or smells. I put in a brand new transfer case because I was originally certain it was a failing chain in the BW transfer case since the truck is at 110K, but that did nothing at all!

I also swapped out the shredded old transmission mounts for a brand new pair from Bearmach, thinking that maybe the transmission was lifting out of alignment and binding the pinions due to high output torque twisting it. No dice, the problem persists.

I can't find ANYTHING even remotely like this being recorded for P38s, or any other 4WD trucks. People talk about surging and irregular RPMs in cars, but the way they describe it sounds like it happens because of throttle position or fuel starvation or ignition failures in just regular operation. Something tells me my issue isn' any of that, because like I said, not even a pending code AND the issue is essentially invisible even under heavy throttle in 4HI sport mode. She'll scream to 5500 with nary a hiccup and shift to second gear just fine.

At this point, my best guess is the tranny control unit is failing and causing a feedback loop to the engine when it shifts its trim readings to 4LO input/output mode. Even that seems like a desperate repair parts bingo play, though.

So does anyone have any ideas on what it could be at this point?
 

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I would look at the diffs. They get the full torque, and you've already ruled out the transfer case. Could be the splines on the pinion flanges slipping. Not seen it on a P38a, but had similar on an L322. The engine seemed to surge because the skipping splined allowed the outputshaft to fluctuate in rpm.
Ring and pinion seems unlikely, skipping teeth would be heard.

Filip
 

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Discussion Starter #3
I don't think that's it. There's no issue giving it the business in reverse. It hauls a truckload of ass, no problem.

I'm suspecting at this point it was a bit of a complicated failure. I hit the trans pan on a rock that I couldn't see while wheeling, and I think that caused oil starvation by bending the pan too close to the pickup tube. I replaced the pan, but the problem persists, so I am guessing oil starvation ate away some of the clutch material. They bite much better with new Dexron VI fluid in high range, especially once warm you can't even tell it's an issue anymore, but in low range I am pretty sure the transmission controller is down-regulating shift pressure to avoid harsh shifting, and it's causing clutch slip. My guess is because you don't hammer the reverse clutch like the forward ones, it escaped unscathed and that's why it's working fine.

Going to try a bottle of Lucas Trans Fix to see if it can revive clutch function. If not, I have a used trans I picked up from a local shop doing some cleanup that I can rebuild. Not looking forward to handing Eriksson $700, though. D:
 

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Sounds plausible if you damaged the transmission.
But, the transmission doesn't see any difference in torque low or high, and shift points shouldn't matter when it's fixed in gear. So doesn't really explain why it's only in the lowest gears.
 
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