1991 560sel Speedometer test
 

Can anyone tell me if there is any easy test to tell if the speedometer is receiving the correct input? My speedo (electronic) and odo died suddenly. I'd like to be able to see if the correct voltage is coming in to the cluster. I can pull the cluster, and know which plug feeds the speedometer. I'm assuming I can use a multimeter to determine if it's getting the right (or any) input.

My car has ASR and used clusters are hard to come by. I know I can have it rebuilt but I don't want to send it somewhere and have them tell me it's in the harness or worse, the tranny.

Is there a way to see if the right info is making it to the dash?