I've been noticing a strange behavior with the way my transmission shifts.

Background: 2000 SW2 at 188K with the MP7 TAAT tranny.

Previously had the input shaft nut come loose due to an apparent funky solenoid action and wrecked the side cover and clutch pack from driving it anyway for several thousand miles thus allowing rain and dirt to get in the tranny through the wide open crack. Replaced valve body with working used, replaced side cover and clutch pack with used and used red locktite on nut when reinstalled. Fluid and filter change. After a couple days run-in it was working flawlessly, wonderfully so.

Fast forward 6 months and after having to replace the radiator due to being "front-ended" I discovered that it was losing tranny fluid due to a cooler line not being tight at the rad. I had loaned the car to my daughter for about a month and she started complaining it wasn't shifting right. Yeah, cause the fluid had gotten that low. She never thought to check and I never thought to tell her to check. SMH! Anyway, topped it off and tightened the fitting but now there's a lingering problem with how it shifts.

From start up to any speed the upshifts are smooth, timely and normal.
If I get on the gas and the tranny needs to downshift for power I get this strange behavior (it doesn't matter if I'm going from 4th to 3rd or 3rd to 2nd or 2nd to 1st): The rpms will go up as expected for the lower gear, there will be a micro second burst of acceleration, then a half second where the rpms drop and there's no acceleration at all, then the rpms go up again and the acceleration resumes. It's really hard for me to describe as you can see but the result is this strange go-wait-yeah go sense from the car. If I ease up on the gas as it starts to downshift I can make it happen more gently but that's about it.

Transmission doesn't feel like it's slipping and there's none of that classic flaring behavior where the rpms go up but you're just not accelerating accordingly. I've also noticed that it doesn't matter if I've get it close to redline when it downshifts or if it's just above idle, the behavior is the same.

It almost feels like it downshifts, then tries to upshift, gives up and downshifts again, all in about 1 second.

The only code I'm getting is P0133 (HO2S Slow Response Bank 1 Sensor 1) which may play a part but I really don't think is the cause.

I've searched this site and several others but haven't found anything relating to this specific problem.

Any thoughts or suggestions?

 

Well, off the top of my head,

If your front O2 is not keeping up with reality, it will be equally sluggish if you mash the pedal to get the car to downshift. If the air/fuel mix requested at the time you are downshifting is not in sync with the air/fuel ratio you NEED to get that acceleration, then it stands to reason that there will be a delay ---after you mash the gas pedal and it down shifts but before the front O2 sensor reading catches up and straightens out the A/F mix.

Might be true. Might not. Plausible I would think....

 

yeah, that does at least sound plausible so this morning I watched how it would downshift before it went into closed loop operation and it still misbehaved.

 

As far as the symptoms already described I have to say that they are not 100% consistent, sometimes things work as they are supposed to but I can't find any common thread for when it does.

Additionally, sometimes, like rarely, when downshifting under power, it appears to shift into the lower gear but there's no burst of acceleration, rather there is a creeping acceleration and the engine sounds like it's laboring more than it should. There's no smoke behind me like I would expect if it was running over-rich. Also, when this happens the rpms don't go up in proportion to the acceleration. It really seems like it's in two gears at once. Once it reaches the normal upshift point everything seems to be fine from there on. It's frustrating and sometimes a little dangerous since I'm used to this thing being able to take off like a scalded cat when I floor it at about 30-45mph and will use that to be able to merge with fast moving traffic at times. Several times it has pulled this stunt and I've caused traffic behind me to have to dodge or slam on the brakes while I creep up to speed with the pedal to the floor.

 

I have to point out that from a standing start or any time I start from 1st gear I can floor it and it will shift perfectly every time although it's frequently a VERY firm shift into second (reminds me of my brother's '69 Chevelle that we put a B&M shift kit in the tranny). I don't want to change that but I don't think that's normal behavior for this transmission. Under part throttle acceleration (same starting conditions just mentioned) the tranny shifts normally and smoothly. Haven't paid enough attention to precisely when converter lockup occurs except that it appears normal.

 

It almost sounds like a single gear downshift is not enough b/c the RPMs are still so far below the power band (starts right around 2k rpm) that the car is bogging down. Maybe it downshifts 1 gear, then the hesitation is really from it realizing it's bogging and downshifting a 2nd gear which brings up the rpms into a range where the car can actually accelerate,

Sounds stupid as I re-read it but that, along with the Front 02 issue combined, maybe causing confusion as the transmission downshift MIGHT BE (no evidence on this whatsoever) influenced by the slope of the front O2 sensor V output when deciding the gear to downshift into and the timing of that downshift. This would explain the A - OK behavior under more or less steady state driving, but if there is a radical change requested, the behavior you see is, well....seen

 

I think you have some damaged parts in your transmission. Seals get hard due to heat and no lube when the transmission got run low on oil. Part of your description of being in two gears at once is very possible. It may have spiked line pressure and pushed a gasket out on the valve body. If it were mine I would go visit my tranny buddy and have a chat.

 

Since I've always worked on my own transmissions and have not had money or occasion for a mechanic to do anything other than alignments I've no "tranny buddy" to go to. I plan to use some info I found in a TAAT manual online and hook up some guages to watch the pressures as I drive (eventually). I figure before I'm done I'll have the valve body back off for cleaning and checkout, maybe even pull the whole thing and split the case for some inspection, clean and repair. In the meantime I'm sharing my experience here and looking for ideas and experiences by others that may help.

 

Update:
A few more thousand miles and I've noticed a couple more things.
First, the turbo whine that thing does under acceleration (there's no turbo) only happens when the torque converter is not locked. Once it locks the whine is gone. Must be some damage to the torque converter.
Second, on sub-freezing mornings it has occasionally been slow to engage both forward and reverse. A little rev and in gear it goes but once I'm moving there's no more problems. Once it's warmed up it's fine.
Still doing that down shift, no wait, ok let's go thing.
Still from time to time doing that slooow acceleration. In fact, doing that more often.
I'm going to check if there's any new codes tomorrow.
btw, I found my tranny leak today when changing the oil. The axle tube extension on the passenger side is leaking where the axle goes in.
Up to 191K and I really need it to keep going.

 

Finally got the codes yesterday.
P0133
P0420
P0731

I cleared the codes and drove all day, multiple trips and no new codes until late last night when it put out a pending P0133.
Never did the SES light come on.
This morning I backed up out of the driveway and then it didn't want to go in first again. A rev and it engaged and everything was fine after that. Wasn't very cold at all this morning. I think I've got a valve sticking. Oh, and immediately after it did that I got the SES light. Don't have my scanner with me but I'm sure it's the P0731 again.