b2b piston?

[kmaysob]

i was reading posts about something called a b2b pison. could this be why my 83 300sd is starting out in second and not first? if so where is this piece located?

Here's a good link that someone posted a few days ago.

http://business.baylor.edu/Richard_Easley//autofaqs/b2piston.htm

Be sure to check the bowden cable, accelerator linkage, and vacuum switches on top of the valve cover.

I believe the 300SD (most years) had the trans set up to start in second. Said that was supposed to limit those hard takeoffs, or some fool thing.
Has nothing to do with the B2B piston. That is otherwise known as the servo, which when it fails will cause the trans not to shift at all into gear, or at least not without a lot of slippage. It is known to cause a catastrophic failure. I know, just blew the second B2B in my rebuilt trans in two months. Time for a warranty rebuild again. Ok, but I get to pull the trans again.

The transmissions on the 617 engines are set up to start in 2nd gear. BUT, it should shift to first if you floor it. Be sure to check the vacuum switches on top of the valve cover, as these are the switches that control that feature... IIRC.

[adamb]

I thought the valves under the black cover on top of the valve cover had only to do with the EGR? Please correct me if I'm wrong...
Thanks, Adam

Hmmm... I'm pretty sure at least one of them runs to the transmission. car54 and I adjusted the valves on his 300D last night, and we had the whole accelerator linkage and related vacuum things apart. One of them controls the EGR, but there are two more vacuum lines... I'm assuming one vacuum supply, and another one for the transmission.

[kmaysob]

they do not start in second. the 83 300sd does not.it sits in second and shifts down when you take off. it does not have to be floored to the kickdown. this has already been discussed. look under "1stgear" . my vacumm switches need to be replaced as they are leaking. i already adjusted the bowden cable on top of the engine where it meets the linkages. is there an adjustment down on the tranny? i Know that they start in 1st because my dad has the same year same model as mine and his starts in 1st. i found that one of the throttle linkages was bent and when i fixed it, it took off better.

Sorry, I was confusing this with the second gear hill-hold feature. You are right... it has nothing to do with being floored.

[kmaysob]

so it wouldnt have anything to do with my problem?

[jbaj007]

B2 piston failure mode is typically no forward gears, but reverse will work fine; so not a likely B2 piston problem in your case.

I always revisit the fluid level, filter and, yes, even the kick down switch connections and movement, when transmission issues arise; just because they're so easy to do.

Valve body is where the selected start gear is controlled in the 722 trans. The SEC boys do a mod to the valve body to get a first gear start because theirs is made to start in 2nd normally. Yours ('83 300SD) is made to start in 1rst from a dead stop in normal operation, so you may end up looking at your valve body.

Hope it turns out to be something easy.

[kmaysob]

where is that located on the tranny and how do i adjust it?