1999 Pontiac Grand Prix • 129,000 miles

I have a transmission problem its a bit drawn out but I have to explain. Just bought this 99 grand prix gt v6 3.8l, the guy said he'd dropped an engine with 80,000 miles in it. Which is great the engine sounds beautiful but this might be my problem. The engine was from a 97 grand prix I believe. I know he said 97. Tomorrow I will search the engine and find out for sure. I noticed after driving it awhile it shifted hard going up-shift and down-shift. I'm hoping it's just trash and a new filter and fluid will fix it, but I was searching on eBay for transmissions in case I need one and according to that the 97 engine is not compatible with the 99 transmission. Tonight it got stuck in 2nd gear for about 2 blocks at least and finally kicked into third and drove fine the rest of the way other than the hard shifting. So I need to know a few things. 1) is this 97 engine really not compatible with the 99 trans and is it even possible that it would still work just not work right? 2) do I need to get the 97 trans or another compatible trans and if so will I need to replace anything on the engine side that might have been damaged from this?
March 15, 2013.

You need to have engine and trans compatible with your computer. It seems tha tit wold be easier to put all the correct sensors in engine. Check tosee if they are all hooked up and scan for codes first. It may be the harness isn't plugged in correctly

Mar 15, 2013.
TY today I had diagnostics ran on it and kind of figured that out it came up with one code only. PO502 (low input speed sensor) showed twice one old and the other pending then we looked up the computers and for 97 it was 4T60E for 99 4T65E
So now I'm just stumped on where to go from here is there a way to adapt it where it will get the right voltage to the speed sensor? Will it be as simple as just changing the speed sensor? I'm lost when it comes to this electrical stuff. This is the newest car I've ever owned or worked on. I believe there is a trans shop here that does free diagnostics so im gonna see what their ideas are tomorrow hopefully. Mechanically I think the trans is still good the fluid I drained out was pretty clean. Not alot of metal shavings or any large shavings. There were small pebbles in the pan? Lol no idea how that happened but I dont think they ever made it through the filter or thered have been more metal shavings. Changed the filter and put lucas and new trans fluid it shifts right now not hard like b4 but just every now and then it doesnt want to upshift from first until 3.5 rpms when normally it will shift a little past 2. Thats another thing throwin me for a loop why would it get the right voltage and shift when its supposed to sometimes and others not until its at higher rpm?

Mar 17, 2013.
The 4t60 and 4t65 may not be tht much different like maybe a another clutch or so in it. They did have a 4t65 in 97 just not for the 3.1L so it shouldn't be that much different however after looking at wiring diagram it looks as if the 4t65 has a pressure control solenoid in it which the 4t60 does not. I'm sure a trans guy can figure this out alot better than me as they deal in them all the time.

Mar 20, 2013.