Sorry if this has been covered but I've searched and searched and only found speculation but no proof.

So I have a 96 DX that had a bad automatic transmission. I pulled it out bought a used transmission from a 98 EX and put it in along with a new torque converter and genuine Honda fluid. Car runs good and shifts fine. I've been driving it a couple of weeks and yesterday the CEL came on with the P0700 and P0740. Everything I've searched says the trans is going bad or torque converter is bad. I Tested the TCC and it clicked when power was applied so I believe it to be good. I have not taken it off and cleaned it yet though.

I then came across someone that said they though an EX trans in a DX would cause those codes to come on because the computers were different. So I'm basically curious if anyone else has had this issue or can confirm that an EX auto trans in a DX will cause that issue? I've since reset the CEL and it hasn't come back on yet and the trans fluid looks clean and doesn't smell burnt.

 

different transmission, will not work correctly on ecu (tcu integrated.)

 

So what doesn't work right? The car drives great as is so it's obviously working. Are these codes a sign of it "not working correctly" or should I be experiencing something else?

 

nope

 

It's throwing an incorrect gear ratio code because the ratio for 3rd I think is different than what the PCM excepts to be due to gearing differences between the two transmissions. I did the same thing years ago. It started out working fine, but it starting shifting weird a couple of weeks in. If you cleared the codes it would drive well again for a while then starting shifting weird again. The only fix is to replace the transmission with the correct one, or replace the engine and ECU/PCM to match the transmission.

 

Thank you for the info. So other than throwing the code every now and again does it actually hurt anything?

 

I guess to expand a bit on my previous post the PCM thinks the transmission is slipping due to the incorrect gear code, this is normally why that code shows up. I found it shifted progressively worse to the point that it was slamming into gear at times going from 1st to 2nd IIRC. It may cause internal damage if it shifts this way. If it continues to shift ok and you can live with the CEL being on then you shouldn't have any issues driving it.