I picked up a 2000 Accord from a neighbor who said the transmission would "lock up". I did a bit of work to it and flushed the tranny, it drove just fine in the summer. When it started to get cold the transmission would do this thing where sometimes when leaving a stop light the trans wouldn't shift out of first, so you would have to pull over and shut the car off, then it would be fine again. This went on for a month or so, then the transmission started loosing gears. First reverse stopped working, then i think 5th (this was a while ago).

I purchased a JDM transmission and installed it, but it never worked right. It would clunk into every gear no matter the fluid level, and when putting it down to second manually it would kill the engine. I took it to a shop and they did a little bit of wiring fix up and replaced the distributor, but didn't know how to proceed. I took it to Honda and they put a battery in and told me the battery was dead and the trans needed to be replaced. Ugh.

I had no warranty or anything so I was stuck on that one, so I bought another trans from that same company (JDM NewYork I think) and installed that. It was still clunking into gear, but as I filled the trans it started feeling better in reverse. Drive is still wonky, but you can't win them all. I drove it about 1/2 a block and it wouldn't shift, then finally started flashing D4 and threw a CEL. P0730 and P0780

I transferred all of the USDM solenoids and sensors over. They were all replaced during the troubleshooting of the last transmission, so (in theory) that shouldn't be a problem. It should also be noted that the car has a new (?) cardone distributor and an eBay PCM that was flashed at Honda.

Has anyone dealt with this issue before? It seems the flashing D4 is a common thing, but everyone seems to say replace the trans. That cannot be the answer in every case, I have to be missing something here.

EDIT: this car is a 4-cyl A/T, in case that wasn't clear.

 

A lot of the stuff I found for code P0780 has to deal with the "dual linear" solenoid on the passenger side of the transmission. I had some leftover pigtails from the JDM transmission so I built a little tester harness and one of the solenoids on that unit sounds very weak, the other sounds good and strong (the black one, in case it matters). I have another one of those units that was with the car originally (the mechanic shop I had it at didn't give me back the new one I bought, it was a cheap-o one from Amazon), and it sounds about the same. Wondering if I take my little harness to Autozone if they will bring up a battery and let me test it on the bench? Would hate to spend $250+ down another rabbit hole for nothing.

I did notice a HUGE disparity in pricing between like Amazon and RockAuto/Autozone/O'Reilly. The most expensive unit on Amazon is some knockoff Chinese unit thats about $90. RockAuto starts at about $250. Wondering if this has been my problem the whole time? Maybe I need to buy the more expensive unit and see how that does. Might try and find a junk yard local that I can pull one from.

 

have you tried other USDM solenoids to see if that changes anything. I mean multiple transmissions but similar problems sounds like somthing else beside the mechanical. Meaning the solenoids or wiring. I don't know the difference, but if you put the JDM solenoids on does it behave any better?

 

For anyone interested, it seems the fix in my case was a combination of another JDM transmission (unknown if needed), a new A/B solenoid (the bigger one on the far passenger side of the trans), and a different "lock-up" solenoid, and potentially getting a different PCM ($40 on eBay, $75 at the dealer to flash it). When I put that different lock-up solenoid on the car shifted just as expected, and I nearly had a stroke. Though I had changed it before, and the one I had on seemingly tested good, changing it did the trick. Here is the text of the service manual that led me to changing it again.