Quote:
Originally Posted by tyl604
Clemson - are you saying this is different from my 1981? I thought they were all the same. If they changed and made it backwards in 1984, that is news to me. I have a 1981 300SD and mine functions the way I described it; the upshift is designed to happen with no vac and it bleeds off immediately when you go WOT. I cannot fathom why they would have reversed the operation.
Can anyone confirm? Kerry - are you out there; do you know?
Thx.
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What we have here is "Failure to communicate," to coin an old phrase.
The concept of 'vacuum as a supply,' was my biggest hurdle to overcoming my misunderstanding about MB transmissions of this era.
Let us consider your statement '...the upshift is designed to happen with no vac and it bleeds off immediately when you go WOT...' I agree.
The VAC (vacuum control valve) mechanically bleeds vacuum in relationship to the accelerator linkages' position. The more pedal the less vacuum. That is probably confusing until one breaches the understanding that vacuum is a commodity which is generated for smooth low RPM shifts.
Thus, when the pedal is slightly depressed the less vacuum is drained from the transmission. It's affect on transmission vacuum is realized by the modulator valve on the tranny immediately since the VAC is a part of the supply (suck) of the transmission vacuum line. In fact, on my 84 SD it is a branch off the transmission vacuum line.
We have the exact same position on the affect vacuum has to the shift harshness but we are failing to use the same terms and advance the concept to the soft shifts needed when acceleration is light.
Several respected member here recommend a vac supply of 12 mg at the VAC. If less was needed for transmission vacuum why require a such a supply?
If vac bleeds off as the accelerator is pushed (which is the recommended test for the VAC) why should we think a soft acceleration would not allow for more vacuum to the transmission?