Help! Weird Shift Problems

[jKsled]

Hey, i have a 1986 2.9L with the A4LD. tranny has been working fine for the past couple months, but recently it started acting funny.

The shift points seem to have changed dramatically, changed so that it wont upshift until your revving around 3,500, and you let off the gas for a second. Then, it is very prone to jump back down a gear with even the slightest gas, revving the engine very high. Once you manage to get it in the OD gear, then it wants to stay there, so that i have to floor the pedal, and it will jump back into third, or usually second. D is worse, it wont shift until the revs are very high. Fluid is fine, just changed it three months ago, and i hve never had much experience fixing trannys.

I have no idea where to start, so if someone could give me some detailed help on diagnosing this problem, that would be great. I just don't have the experience with automatic trannys, and don't know where to start. Thanks!!

 

[JamesC]

Check the vacuum modulator or for a vacuum leak in the line going to it.

 

[jKsled]

where is the vacuum modulator, and how can i check it??? thanks

 

[ForOffRoadDriving]

i believe its on the passenger side of the trans and has a vaccum hose that runs from the valve to the back of the intake manifold. mine went bad and it shifted like crap and would actually suck trans fluid up the tube and burn it through the engine.

 

[jKsled]

ok. how would i check to make sure its functioning correctly?

 

[Jason]

ok. how would i check to make sure its functioning correctly?

Usually just check the line for leaks and use a vacuum pump like a Mityvac to see if the modulator holds vacuum.

 

[jKsled]

ok, i don't have a vacuum pump, but i tried something else... by sucking on the end of the vacuum line, i could create pressure on the modulator. after about 5-6 seconds, the pressure would go away, and i would have to suck more to keep the same pressure. does this sound right? should it hold a constant pressure then?

 

[JamesC]

It could have a split diaphragm I would replace it usually fairly easy to do.

 

[jKsled]

ok. Where could i find info on how to remove it, etc... the hose seems to lead somewhere that is hard to reach.

 

[pianoman2]

It is a pain to reach the modualtor on that truck. If yours still has the cat converter, wouold be easier to remove it to get to the modulator valve. Be easier too to make sure you don'tloose the little pin that will come out with the modulator.

Otherwise you have to thread your arm up and around the cat. converter to get to the bolt that holds the modulator in. Not impossible to do, but can be a pain.