Cable VSS Signal Noise

[RyanG]

Hi,

Trying to sort out a reliable VSS source on my 1988 Honda Civic ED which has a B18C turbo swap.

It uses a cable VSS drive as the speedometer is cable driven. There is an electronic variant dash in a JDM model, but unfortunately they don't quite fit my pre-facelift surround. I do have an electronic VSS which I tried to test, but had no luck getting that to read. Ultimately I need a solution which leaves my factory cable driven speedo working so didn't spend too much time on it.

I originally tapped into the VSS output from the reed switch in the back of the dash. With appropriate filtering or averaging, it would read great until 80+km/h. After that it would get large noise spikes, meaning I couldn't use it to calibrate gear position for boost by gear.

I pulled the dash out and replaced the reed switch, as well as tested it using a drill. Unfortunately, still the same issue.

Does anyone have any advice for things I can try? I'm going to modify the reed switch mount and experiment with adjusting the distance between the switch and the magnet. I'm not very electronically experienced, but have seen things like Zener diodes or low pass filters being suggested - anything like that worth trying? I can also look at rewiring the switch to go to an ecu signal ground, if that would help?

Or is something like this just never going to work? My last resort will be to wire in aftermarket hall effect wheel speed sensors which is doable, but would require some custom work. I've seen some pass through cable VSS boxes which have a hall effect sensor, but they're not cheap and would require work to get them to fit with my VSS and cable.

I've attached some images, tune file and 2 logs that were taken directly after each other. The first the VSS was working fine, then on the way home it had noise spikes (and reads 6000+km/h). I'd think these large spikes could be filtered out with a median filter, but I don't think that's possible within the Link itself.

https://drive.google.com/drive/folders/1r0GfJJ8OPbbY3yK17nv4TpFiGwVZ8N20?usp=sharing

[essb00]

You can try to enable the DI Freq Filter in the GP Speed input... It might help, but if not enough try below:

I made this RC low-pass filter --- and I have not seen similar fluctuations since I made it. Check out the link below. 
*You might need to do some trial & error with the capacitor value so that higher speed signals would still be useable. 
 

 

[RyanG]

5 minutes ago, essb00 said:

You can try to enable the DI Freq Filter in the GP Speed input... It might help, but if not enough try below:

I made this RC low-pass filter --- and I have not seen similar fluctuations since I made it. Check out the link below. 
*You might need to do some trial & error with the capacitor value so that higher speed signals would still be useable. 
 

 

Exactly what I was looking for, thanks! I was just researching low pass RC filters then. I also should've mentioned I had tried various DI freq filter values, I'd just happened to leave it on "none" after giving up. Looks like in the latest update the max has increased from 7 to 10. Regardless, it didn't help the huge noise spikes, but it did significantly reduce the amount of averaging required so I'll take another look at that once I wire in a filter.

At max speed of ~250km/h my frequency should be 180hz, so I was looking at an RC filter to suit. I downloaded your log from the post and your frequency values aren't too dissimilar, so I'll replicate what you've got and see how it goes. Hopefully I can sort it out as the car desperately needs boost by gear now haha.