Where is the vehicle speed sensor on a 1990 Honda accord 4 door?

The vehicle speed sensor (VSS) on a 1990 Honda Accord 4-door is located on the transmission. Specifically, it's usually on the top of the transmission case, near the top of the bellhousing. It's a fairly small sensor, often with a single wire connector.

However, you should consult a repair manual specific to your 1990 Honda Accord to be absolutely certain of its location. The exact placement can vary slightly depending on the transmission type (automatic or manual) and even minor model differences within the 1990 model year. A repair manual will have diagrams and precise instructions.