Okay so I've spent an entire summer reading forums on many other people who have had this problem so here Is what I have in my car. I own a 99 Civic Si that has a b18c swap and s80 gsr tranny. And a jdm p72 ecu. At first when the speedometer was jumping all around I was like okay not a big problem I'll just get a new vss sensor and install it. (Check engine light would come on for the vss) the new sensor (60 dollar ebay sensor) worked perfectly for about a month and then the speedometer started jumping around again. So I had the ebay company warranty me another one. I installed it and nothing changed. So I figured I would get a vss sensor from O'Reilly auto parts for 155.00 dollars. It worked for about a half a month and then the speedometer started jumping around. So I had O'Reillys warranty it. I installed it and immediately it didn't work. So I figured that the vss sensor wasn't my problem I had other issues. So I read other people's problems were... alternator, mph guage, wiring, electric load detector, vss plug and ecu. So I replaced my alternator and tested the Electric Load Detector they were good. Checked my wiring with a volt meter clear to the back of the gauge cluster. It tested good. Replaced my vss sensor plug with a new one from ebay I also went as far as ordering a brand new speedometer guage from Honda 235.00 dollars later there and it still bounces all over. I'm super frustrated and am about ready to dump gas all over this project and set it on fire. Any help or advice would be greatly appreciated!

My vss sensor signal wire was getting about 5v to 11.4 volt fluctuations to the back of the guage. 12 volts of supply power and the ground was working on the sensor.
speedometer seems to react with bumps on the road. Which makes me think it's still a wiring issue? I thought about hooking up some wires to the vss sensor and running them into the cab to test the wiring with a volt meter while driving to see if the ground or power drops off?

edit: also the 7.5 amp fuse is good.

 

Okay today I decided to triple check everything. And I have confirmed that the two vss sensors that I got off ebay and the two from O'Reilly auto parts were all bad (just my luck)
so I was curious to see what the inside of a hydro speed sensor looked like so I took a hammer and split it in half aNd found a magnet that spins past a pick up sensor. Then I noticed that the shaft that spins looked like it had melted and worn away the plastic carbon housing. And there was not any grease inside. I feel like there should be an electrical resistance grease inside this to prevent wear?

my question is when I get my new vss sensor should I pump some dielectric grease into it? Will that help theses vss sensors last longer?

 

I put in a brand new vss sensor and still had the issue of jumping speedometer needle. I loosened all my ground wores under the hood wiggled them and the speedometer worked for a day. It rained alot today. And its back to jumping around. Would it be okay to try grounding the vss sensor to the negative terminal on the battery?

 

It is likely that the problem is not your VSS sensor at all, but in fact, the speedometer gauge itself. This is a common failure in the EK chassis. I just replaced the one in my Grandson's '97 H/B a week ago... exact same symptom you described. The new one works great !!!

 

Is it just a speedo issue or are there performance issues as well? Mine was doing that and it was a loose ground on the thermostat housing, but it was also bogging down when hard on the gas. Had the tach do some funky things before, no performance issues and it was the actual gauge itself. Before you spend any more money, get yourself a cheap non-si gauge cluster and see if everything works.