My cav has some issues,thaht have popped up recently.When shifting in to high gear,at road speed it instantly goes to OD(auto tranny).I let off and give it gas it immediatley shifts back into OD.Heres prob. #2- Occasionally the check engine light comes on,when it does,it starts running rough,and will not shift into OD.If the light comes on going down the highway,it will immediatley shift down out of OD and will not come out until the light goes out.I've checked the codes on the engine light,and it comes up #51,which according to my code list,means it a ecm failure,so is the computer going bad?I'd love to keep the old car,gets good milage,and I'm kinda attached to it,would love to fixer up and pait it,some ground effects maybe,engine mods,but don't want to throw money into it if it's about to die.Any help from anyone would be greatly appreciated.

IF its a 90 its a 3spd auto- no overdrive. 1,2, drive. Sounds like your torque converter is messing up. If you have the mechanical knowledge fairly simple job. I belive the 125 is whats in there, very common and hard to kill transmission. Im almost 100% that the transmission is all mechanical, so the computer dosen't control anything. Is your car a 2.0, 2.8, 3.1 ? IS the shift linkage that connects to the back of the throttle body intact, the part that you can use to rev the car under the hood? I have a 91 2.2 so it should be very similar to your car

It's a 1990 cav cl with a 2.2 L.Yes the shift linkage going to the TB is intact,what effect does it have?I know it controls passing gear,and downshifting.Some locals have told that the tranny needs new fluid/filter that is whats causing my weird highgear shifting..Do you know where i might find a key /legend for j-body VIN# so I can check my vin to see what tranny,n such are in my ride.

Well the shift linkage would cause the transmission to do weird things.
Type your VIN into www.compnine.com and it will tell you what RPO codes the car was built with- reposted via another thread. Theres a whole subsection in transmissions under the go go go, maybe try searching in there.

TH125C is the transmission you have, and as Andrew pointed out it's a 3spd, but it also has a locking torque converter. Pretty much as soon as you're in third/high gear and reach 40mph the converter will lock up. Letting off the gas or hitting the breaks will unlock it, but the converter pretty much locks back up after getting back on the throttle. On my '94 you have to be in the throttle almost to the point of downshifting for the converter to unlock. As far as your code goes I'd say go to a junkyard and look for some 2nd gen Cavaliers to snag the computer out of. IIRC it's not uncommon for them to go out, but luckily you just have to swap the PROM chip. I'd TRY to get one for the same year/options, but it might not be critical considering the meat and potatoes is the PROM chip. Had to do it with my '88 Celebrity when the computer went out.

Well,bad new and then good new.....Tues. the cav,or the beast as I call her, quit running completely.She cranked right up that morning as usual,and after about a minute,she stalled and would'nt restart.Yesterday I installed a new ECM,and still nothing.Checked the codes again,and it told me to change the crankshaft sensor,which I did today,and walaaa she's back running like her ole self again.Think next week,just for good measure I'll change the fuel filter,and the plugs.Thanks for everyones help,really appreciate it.

I just want to make sure I understand something as my 1st gen is having some weird issues (under "the saga continues") in that thread.

Am I correct in this statement: A crankshaft sensor would not exist on my 1985 1.8L motor because I have a distributor

From what I can tell, this sensor would only come on a car with a "black box". I just have to ask because I didn't realize the 1990 2.0L would be without a distributor.

^ ^ ^ Ima say no to the crankshaft sensor on a '85. BTW it's usually one of the first things I change on a used car when I buy one cause they will leave you stranded. Sorta easy to troubleshoot though, as the car will crank and run fine till hot and then die and won't re-crank until it has cooled back off.

my 1990 2.2 L has the crankshaft sensor.I also changed the ignition control module,runs like a new one again.