Since code 9 is NC speed sensor, this suggests you disconnected the NC speed sensor and not the NM speed sensor. This may be why you couldn't fit a NM speed sensor. You may be attempting to install in NC speed sensor opening.

The NC speed sensor is above the dual lockup control solenoids near the distributor.

The NM speed sensor is much lower on trans and further outboard towards the pax side fender. It is roughly level w/ the dual shift control solenoid valves.

Do you have a shop manual? If not, I would suggest the 91 Accord download manual from Online Manuals post in DIY forum. This manual should have the picture of sensor locations I have.

good luck

 

I unplugged the NM sensor. I plugged in another NM sensor and the same thing started happening again. But what I also notice happening when the NM sensor is connected is the following: RPM's idle around 200-300 when i come to a stop and the oil indicator light start to flicker.