Thanks for the quick replies, man,

No, it is not shifting any differently with the modulator unplugged... it still slips/flares with 0 vacuum. Whether there is vacuum or no vacuum it's the same thing, it feels like it did when there was too much vacuum and the VCV was not working properly. But, as I said, even when I unplug the modulator and feed it 0 vacuum, its the same as having it plugged in and feeding it too much vacuum.

The funny thing is that it had the opposite problem when I first got it, then it ran fine as I removed the vacuum leak and adjusted the VCV, then after a few days it started misbehaving this way, slipping/flaring all the time.

My only experience is with my '81 300SD but, as much as I hate to say it, I think you may have tranny trouble. You need to get the tranny guys interested for help.

I would suggest you start a complete new thread and call it something like "Need help with slipping transmission" or something like that. There are a big number of guys here who can help you diagnose what may be happening inside the tranny and unfortunately I am not one of them.

Gut feel though - it is not vac problem; it is a tranny problem. Especially since it ran better for a while with new thicker transmission fluid.

Good luck and I will continue to follow.

'81D - wondering why it does not have a Bowden cable; maybe the D's just did not. Not sure.

I solved and replied to the new thread I created.. i'll copy and paste here for record's stake..

"So last night I crawled underneath and adjusted the modulator, tightened it 1.5 turns, drove around, felt it shifted better immediately, so turned it another .75 turns, a little better... so today I guess I'll tighten it maybe another 1.5-2 turns!

The way it is now, it shifts all right with the VCV and modulator getting fed zero vacuum / unplugged. So I'll tighten the modulator until it's a bit on the harder side, then plug the vacuum system back in to get it back to normal.. should work and would feel pretty satisfied.

Is there any limit to how much it can be tightened?

I would do the whole precise adjustment with a pressure gauge / banjo bolt thing, but probably I'll try that some other time when I have more time/resources. I feel it's so far off now that even just tighten to test drive and tighten some more will be sufficient. "

My guess is that your clutches are slipping and the major adjustment to the trans modulator will make it shift better for a while. That means that adjusting it to book specs probably would not work.

But I am really glad that you reached a combination of fixes that made it driveable.

When the tranny finally goes, do not give up. You have a great, lifetime car and you can locate a used tranny pretty easily. My guess is that you could find one and an installer for a total of $1000-1500. Just a guess. That's two or three modern car payments.

Yep, it is a great forum.

I must admit though.. such a sudden and abrupt change in trans pressure makes me believe that the clutches are fine.

I mean, it was working one way one moment, then after adjustments made to the controls, it started working another way (the correct way). Then after a few miles of operation, it crapped out again, but in a different way.

I don't know the details of how an auto transmission works, but I'm thinking after years of an inactive modulator, suddenly reactivating the modulator may have, after a few miles, suddenly dislodged or unclogged something?, releasing a buildup in pressure, resulting then in really low pressure, resulting in slipping/flaring. Now just have to compensate for that released pressure and bring it back to spec.

This also makes sense, because if I understand how clutches work, they wouldn't crap out completely from one moment to the next, they would slowly begin misbehaving and gradually get worst. Or at least, if they did crap out instantly, I feel it would have been a lot more dramatic, perhaps having even more dire consequences, grinding gears, etc? This was a sudden and extreme change, and after years of it shifting hard, abruptly starting to flare in such a way so soon after a repair, makes failing clutch seem unlikely?

Anybody with more knowledge could surely correct my way of thinking if it sounds off, please do. I guess I'll have to get the modulator pressure to specs to really find out, but I suspect that it'll work perfectly with spec pressure. Hopefully I will try this soon and let you guys know how it goes.