Hi all, i'm wondering about the vacum control on an older automatic transmission. All the automatic transmissions i've ever delt with have been electronically controlled, so i'm wondering what exactly the vaccum does for an automatic transmission?
I'm wondering this because i'm converting an older vehicle that used to be gas to a turbo diesel engine. As we all know turbo diesels don't produce vaccum, so i'm trying to figure out if i can even use this transmission in the swap.
Thanks!

 

Not likely....Old autos used vacuum (or lack of) to determine shift points. Low vacuum usually coincides with torque peak. That's why they used a modulator. There may be some aftermarket company using a computer to simulate the modulator, but I have not heard of any.

Tony

 

Well yes diesels dont produce vacuum but thats why they have vacuum pumps on the engines so that would solve your tranny problem......I think....

 

If Tonyb is right, then that would mean my setup would be pretty much backwards. As you rev a gas engine up, the vacum goes down and as i would rev the diesel up with the vacum pump i would think it would produce more.
Then again i could be wayyyy off and that's what i'm trying to figure out by posting.
Thanks

 

One last thought: how did dodge get around this issue with the automatic transmissions ( provided they're the same between gas and diesel)?

 

They now use a Throttle valve cable to adjust the pressures and shift points, (at least on the 47rh's) It hooks to throttle linkage and opens and closes a valve inside the tranny as you mash the throttle.

 

chrysler has always used mechanical linkage for throttle pressure in the trans. even on some of the e-models.

 

What about the 4bt's used in Frito-lay trucks with a GM turbo 400 trans.
Aren't those trans vacuum controlled? How did they overcome the problem?

 

on the gm diesels in the 80's they used a vac valve on the side of the pump,as the throttle was opened it would alter the flow of vac to the modulator.they had to do this because the vac pump supplies a constant vac,unlike a gasser that loses vac when you are under power.

 

Thanks for the replies. My truck (an 87 mazda with 3 spd auto) has the stomp it switch under the acclerator pedal to manage downshifts for passing.
So what i'm understanding from everyone's post is that the auto trans uses the amount of vaccum to control normal shifts (upshifts) right? So what would happen if i were to hook this tranny up? Would it just shift like crap all the time and be erratic or would it just not shift at all?
I did do some research on the transmission and it's a nissan (Jatco) built L3N71B. I looked up a parts sheet and it does list several vaccum modulators one of which is a diesel application. So what i'm wondering is would just the modulator fix my problem here or would i need other parts, too?
Thanks

 

You may want to research the nissan diesel modulator, and other parts associated with it....May be your way out.