educate me on auto transmissions

Forgive me. I have always been proud of my ignorance of 'Merican auto-tragic transmissions but I am now potentially encountering one.

I went and test drove an E39 wagon last week. Loved everything about it (obviously, I love my E39 sedan) but that GM sourced 5L40-E transmission just killed the overall experience.

It felt like every old GM auto I have ever had the misfortune to drive. It had all the slush of a malaise era Chevy Caprice.  Sluggish from a stop and a bit rubber-bandy. The same shifting feeling as my old Avalanche back to every TH350 I have felt. The car was easily 3 seconds slower to 60 than my 5 speed E39. To be fair the car had sat for months before I drove it. 

But I can't imagine it was supposed to be like this. It is in a BMW after all. Is this possibly the result of wear or damage after 20+ years? I have heard that a bad torque converter can cause the rubber band effect.

I know there are upgraded "tunes" that are available from the aftermarket.  Not looking for a hard shifting drag racer nonsense, just not sluggish and slushy.

Anyway, might this be a problem with mechanical bits or with my expectations?

I have not found a later touring with the more highly regarded ZF box in my searches.

The missus loves my car so much she wants one too. I am all in if I can find the right one.

I am a big fan of shift kits.

You don't have to create a tire-chirping neck-snapping monster, but you can really crisp up the shifts with not much effort.

There are also many updated valves and whatnot from Sonnax and TransGo that "fix" issues of most transmission designs.

Big fan of shift kits.

Regulators might be getting old. We replace a (literal) E36 M3-ton at work every week. I work for ZF.

These are from the BMW "lifetime fill" era of transmission fluid (read: no dipstick, BMW just assumed you would drive it until it blew up), and it has probably never been changed.

My dad used to have an E39 sedan with the GM slushbox. Dropping the pan, replacing the trans filter, and filling it up with fresh new fluid made a huuuuuge difference in the shifting.

As a former transmission repair shop chain manager, I can't recommend the 5L40.  Steer very clear of it's mother, the 4L30.  Both were not known for their longevity.  Fortunately they're pretty easy to rebuild which means they won't need quite as big a wallet compared to a ZF or MB.

If I were getting a low-mileage 5L40, I would definitely do a mild shift kit that includes a means of upping the line pressure potential (more on that in a bit), additional cooler that you install before the radiator, and upgrade the 1-2 servo.

GM uses a pressure solenoid like most automatics.  It raises pressure based on your right foot to prevent slippage (well... they at least keep the slippage the same regardless of where your right foot is).  The problem is, the port that the solenoid uncovers is too small.  It's ok for up to about half or 3/4 throttle, but at WOT it doesn't matter that the solenoid is wide open, there isn't enough flow to maintain pressures needed.  It's not apocalyptic, but it does mean that wailing on the car will cause more wear faster.  Using a shift kit that increases that pressure means that the solenoid can actually flow enough to make pressure happen.

You can also get a tune that opens the solenoid faster, but again... if you don't have more flow, it will only firm up part-throttle shifts and leave you hanging at WOT.

GM transmissions are actually pretty simple, cheap to build, and can be capable of great things, but GM neutered them to cater to old people who wanted a nice smooth slushbox.  Fixing what GM screwed up makes them pretty not-terrible.

If that E39 is under let's say 75k miles, I'd risk it and do a shift kit and then tailor the shifts with a tune.  Just be aware that the firmer the shift, the more often you'll replace guibos.

In reply to Curtis73 (Forum Supporter) :

Any input/advice on the latter 6l40? My wife's e91 has one that developed a bit of torque converter shutter when hot & under medium load.  I changed the fluid twice & put some shutter stop in that has helped.

In reply to Caperix :

Wish I could help.  I got out of the business before any of those hit my bench.

If they're like other GM lockups, they use a PWM solenoid to vary the apply pressure.  It's always a good mod to make it an on/off.  I'm gearing up to do that with my 4L60E, along with a corvette 1-2 servo.  My TCC isn't shuddering yet, but my 1-2 shift is about as soft as grandma's tushy.