The speedometer had stopped working, although the odometer was still
running.  Replaced the instrument cluster with a used one. The
speedometer then worked for the first test drive, but on  the next,
the speedo needle started becoming erratic, and then died altogether.
THis time, the odometer is not working, either.  I spliced test
pigtails into the pos. and negative signal wires from the
sender---right at the speedo connector.  At 60 mph  (more or less)
with a digital multimeter reading rms AC volts, the signal is 2.0
volts. Not much of a signal...but there may be an amplifier.  I am
sure it is a pulse train, and so the indicated voltage is probably not
accurate. I don't have a scope to read the p-p voltage, to see if it
is still normal.  I would like to know for sure that the signal
amplitude is still high enough to drive the speedometer OK before
telling the vendor that the speedo died the same day installed. It is
maybe not likely, but I could have had some problem with the sender
circuit (and still do)  combined with a failed  speedometer MPH
needle.  (Boy...the hits just keep a'comin!)  

I'd just as soon forget the OEM speedo altogether and install an
aftermarket one.....but they don't seem to be common and are
expensive.  I found one speedo repair shop that will repair Volvo
speedos, but its very expensive.  But then "pot luck" with a much
cheaper used instrument cluster sure isn't working out.

Regards, geronimo