I own sc400/93 and have replaced a variety of parts over the years
after the odometer- began to work only on even days- the speedometer on odd days
I had the cluster replaced-
the mechanic found that my old cluster- seemed to work- just not in my car-
the rebuilt one is working fine
so now i have two-

there is a small computer board for each, the speedometer and odometer-
this can be replaced - and purchased separately
you might try this

 

This video explains the 3 simultaneous symptoms others have had and shows the process of how it’s fixed. Hope this helps everyone who may run into this issue.
3 issues are:

1. 1. speedometer not moving

2. 2. odometer digital display is blank

3. 3. shifting is erratic while trying to accelerate

 

In the mean time, you can use this device below as an alternative method:

 

Unfortunately your method is not applicable to our L400s/Celsiors because our cars don't use a ceramic oscillating resonator.

 

What needs to be done if the whole speedometer is blank?.. replace it?

 

It's all up to your skill. The symptom blank often means no power to CCFLs.

It's not very difficult to fix for those who are skilled to handle printed circuit boards since the no power to CCFLs is mostly caused by the worn electrolytic capacitors used at the DC-DC converter circuit. Sometimes a switching semiconductor there is dead though. I think I can fix it due to my experience. Those who are not fond of troubleshootings would need to buy a used working unit or simply ask a professional to fix it.

 

If you're not an e-tech/DIY'er, you can take the instrument cluster to a local instrument repair shop.

Or you can send the cluster to a place that will repair them - you send them yours, they troubleshoot, repair what's needed, test and ship it back to you. I can't recommend any particular repair service. However, I did have Circuit Board Medics (https://circuitboardmedics.com/) repair an obsolete Whirlpool oven control board for me. Super tech support after the fact as well. If you go this route and the place you choose offers shipping through them, go that route. Often, it will save you a few dollars.