89 Bonneville (234,000 miles on it) ... Right now, my tranny is slipping practically every time I try to accelerate, but mostly when it'* in 3rd gear or shifting into it. Prior to this, it only slipped when accelerating hard on a left turn. possible cause ---> about a week ago, I was driving (going about 25-30 mph) and I accidentally put the gear selector in 2nd instead of 3rd which caused the engine to stall. Could this have caused my transaxle to start slipping, and if so what can I do about it ...??? Also, I noticed an ATF around the same time, but shouldn't it run fine as long as the fluid level is OK ...?

So, you're saying that I need to put it in each gear for two seconds (with the brake on, I'm guessing), and then recheck?

How many miles on the tranny? My experience is they last around 180-190K miles. If it'* slipping, look at the fluid. If it'* brown or black and smells burned your clutch packs are shot and it'* rebuild time .

Auto trans should never slip. If they do, and it'* not corrected immediately (if it can be), they're toast. Yours sounds like toast.

I parked my gray SSE in the parts bin when the trans started to slip at 200K. I have an 89 trans with a couple thou on it after a rebuild, but that'* my spare. If I remember right, that trans cost me $1800 to rebuild..

good luck

trannys can slip from low fluid pressure due to a clogged filter. Thats the first thing you try if it hasn't been changed.

I checked the fluid level after I warmed it up and put it into each gear for a few seconds, and the level was good. I've continued to notice a spot of ATF each morning when I check underneath, but the fluid level doesn't change a whole lot. When it does get to the point where I need to add some fluid, I have some ATF fluid that'* supposed to recondition the seals. Does anyone know if those actually work? If the problem continues, I'll see about changing the filter - how difficult is that procedure, by the way?

Another question I have is whether I should just drive it in D or OD and let it slip, or if I should put it down in 2nd (where it doesn't slip) when I need to accelerate. It can't be good for the transaxle when it slips and shudders, so I've been doing the latter of the two, and it seems to work pretty well. Are there any problems that will arise from driving it in 2nd (other than poor gas mileage and high revs) ...?

I recommend you invest $20 in a quart of Lucas brand transmission stop slip. It works.
It'* a stopgap measure until you can get it fixed properly, but it should stop the slip.

I'll have to go get some of that stop slip. In the meantime, that still doesn't answer my question about how to drive it when it'* slipping. ???? Lately, I've just been manualing shifting back and forth between drive and 2nd. It seems to prevent it from slipping, and I can easily switch between the two while driving. Are there any negative effects that can arise from driving it that way? I certainly don't want to make the problem any worse!

If it isn't slipping, you drive it normally. Put in the Lucas, and it will stop slipping. That solves both problems.
Best thing to do, call around to parts stores in your area and find which one carries it. Then get a ride over there from a friend or family member. It would be best to not drive it while it'* slipping.