I bought a 1986 240DL station wagon recently and I can't get the speedometer/odometer to work. More than one person told me it was most likely the speedometer that needed to be replaced. I bought a replacement cluster from a junk yard and plugged everything in - nothing. Then I took the car to a Volvo dealership and they said I fried the speedo unit because I plugged in the tachometer wire (this cluster doesn't have a tachometer). I finally bought a new speedometer and it doesn't work either. Other than plugging everything in, is there something else that needs to be done? I'm out of ideas and my temporary registration will run out soon. - Speedo model is K9800.

I havn't noted what model my insrtrument panel is, ( while I have had it out a few times). It is electronic, which I believe yours is also (later than '84, and your dealer said it fried?).
The speedo gets a signal from a sensor on the differential (look for an electrically wired device). I think either the circuit to the speedo is broken or the sensor is defective. (This pick-up sensor must be adjusted on installation, that has an effect on its ability to pick up the signal. )
I am not sure which connector on the back of the instrument panel is the speedo, but i don't think you can attach the white and red tach wire (you do have one) to it and thus fry the speedo.
The only connector that you could attach the female tach lead to is smaller than the tach wire is made for. The wire that belongs on this bayonet connection is
-(female),
yellow and red,
will only fit here,
and belongs to the overdrive indicator light.

Hope this helps,