Weird shifting problem

[MikeVA]

I have searched all the post and can't find a description of my problem.

It's a 94 XLT 4.0 with auto trans.
Starts fine and idles ok, seems to have plenty of power.
If I leave it in OD it will upshift through the gears ok then it will drop into OD at the correct speed/rpm, then it will start downshifting and rpms go up to 2000 and hang for a few seconds then it will drop back into drive or OD. This will happen until it reaches 55 mph or so and locks into OD then it will stay until I have stop or slow way dow for some reason then it starts all over again. The trans dosen't feel like it's slipping as the speed/rpm seems ok.

I have replaced or cleaned just about every sensor and replaced the spark plugs and wires and I can't seem to solve the problem. I have checked the codes with a reader and nothing shows up. Trans fluid looks good and is at the correct level.

I'm down to the vacuum modulator or something like it.

Mike

 

[boggs1227]

with mine ,when i start out ( just normal driveing nothing fast) mine shifts normally through the gears till od is next .once my speed is at 45-48 mph thats when my over drive kicks in ...go below that speed or let off the gas a little and the truck down shifts till the rpms even out and od engages again .
but at 45 and above od stays engaged longer till i press the gas down to pass.

once i have od kick in i have had it stay engaged at speeds as low as 35 mph( just barely giveing it gas ...i bet i'm getting good mpg like that to)

so, i thinking that yours is ok .just different wear and tear for the same equipment so there should be some difference in the numbers.

but brooklnbay is the transmission expert ...he may know of something else .

 

[BrooklynBay]

Welcome to this forum! It sounds like there is slippage when it engages the overdrive band. I would recomend a band adjustment on the first (overdrive) band. It's possible that the servo piston is dried out, and is leaking. The symptom for that is very similar to what you're describing. It engages the band, then slowly pinches off causing the band to slip over the overdrive drum. You could adjust both bands as well as the kickdown cable before you start to replace any parts. Did you check the modulator to see if it's leaking? One sensor you might want to examine is the VSS. Do you have any problems with the cruise control or the ABS?

 

[MikeVA]

Maybe I didn't explain this correctly, once it's in OD it will stay there until I stop or dramatically slow down. When it's going through the upshifts it will "bounce" between second and drive and drive and overdrive. It will rev up about 500 rpm for a few seconds and then upshift back into the next higher gear. The cruse control works ok and the ABS functions on start up like it should. I'm not sure what a VSS is. The reason I suspect the vacuum modulator is a couple of weeks ago I started the truck up and apparently it blew off one of the connectors to the vacuum tree at the back of the intake, it could have been dried up or just loose. I replaced it with a new one and the shifting problem is still there.

Thanks,
Mike

 

[BrooklynBay]

Did you use a wire brush to clean the bore where the pin from the modulator goes into? Sometimes debris from old fluid which collected particles could prevent the pin from moving properly, and cause it to stick. Many people have complained about similar symptoms after replacing their modulator. The VSS is the vehicle speed sensor. Did you buy the adjustable or non adjustable modulator? The adjustable model has to be calibrated.

 

[MikeVA]

I have not replaced the modulator yet, just the 90 degree rubber fitting for the vacuum line. Do you recommend the adjustable type or non-adjustable modulator if I need to replace it?

 

[BrooklynBay]

The regular one is fine, but the adjustable one lets you fine tune the shift points, and is a little higher in price.

 

[boggs1227]

hey BrooklynBay...the adjustable modulator ...(since were on the subject) when i got my unit back from being rebuilt it shifted really soft ..took it back then shifted more firmer ( nothing like thowing your head back stuff) ...
would it be safe to say they installed a adjustable one in mine ?


thanks

 

[BrooklynBay]

One way to find out is to see if there is a small dial on the back of it.

 

[MikeVA]

After a little more investigation it looks like it is only happening between drive and overdrive.
I drove to work the last two days in with the transmission in drive, and it shifted through the lower gears fine. As soon as I shifted into OD the problem came back. It may rule out the vacuum modulator and point to the band adjustment?

Thanks,
Mike

 

[BrooklynBay]

It won't hurt to adjust the band to see if it will make a difference. I saw that you had an accidental double post, so it was deleted. Sometimes the site's software gets a little slow, and will double post if you hit the post reply 2x.

 

[MikeVA]

Replaced the vacuum modulator, that did the trick.
For anyone attempting to do the same, I highly recommend a 10 mm ratcheting box end wrench, it made removing the heat shield and and clamp bolt much easier. For reference the Auto Zone replacement number is MV214 ($12.99) and is adjustable with an allen wrench, the adjustable version with thumb wheel from NAPA is ATP17787 and it was $27. It was a bear to reach from the top, but doable, I would have gone in from the bottom if I had a pit or lift.

Thanks for the help and pictures, it made it a lot easier.

Mike

 

[BrooklynBay]

Thanks for the update!