Does the speedo buzz and bounce around when it is cold?
It gets obsessed with 70-ish km/h, and the exact interval within 70 km/h varies depending on your true speed (100 km/h reads as about 76 km/h, 50 km/h is about 73).
It does not always exhibit this behaviour. Sometimes, you can have multiple days in a row where things are fine. Then it will decide to just isspay on your entire week. This makes it annoying to troubleshoot.
When you fully stop, it drops to 16-ish km/h. At this point if you rap on the binnacle just right the gauge will drop to 0 km/h and about 1 in 20 times this fixes it for the rest of the journey. There's a little bit of needle jitter at certain speeds, but that's just character.
Consistently going over 100km/h for a long period of time also eventually fixes it. Peeling out from a stoplight fixes it more often than anything else, I think the sharp jerk unsticks the motor.
Different parts of the car vibrate at different speeds, and the factory limiter is at 100km/h, so you can still tell your speed fairly accurately without the speedo with a trained ear and/or practice.
I replaced the VSS with a brand new one, but I never tried to scope the VSS to see if the transmission part is telling the truth (kind of hard to scope at 100km/h anyway). With all the other little electrical problems in the car, I'm willing to assume it's a dicky ground or the cold solder joints that usually affect these cars' clusters, but since knocking on it makes the needle go back down, it could also be a bad motor.
My reason for suspecting the grounds is that since the VSS head sends PWM pulses that are supposed to grow in frequency with the road speed of the car, the speedo logic might not be able to tell the difference between noise and the actual pulse. Also, the previous owner put a ittshay voltage regulator in this thing and blew out all the power filtering caps on the power rail of the ECU, so probably anything that deals with 5/12V is hurtin'.
The main ECU ground is clean, at least - I made sure of that when I was diagnosing what turned out to be an ECU failure.
Hotter temperature makes it worse, but the headlights and cluster lighting are a little spotty sometimes when it's cold outside..
I was going to replace the cluster but then I didn't. The dash plastics are huge unsupported chunks of 26-year-old ABS that are fragile.
The car's favourite musician is St. Vincent, but inexplicably its favourite album is by Praga Khan ("Pragamatic").
