Hello everyone...
I recently purchased a 1988 560 SEC and I have an issue and can't figure it out.

The speedo doesn't work and the driver door lock doesn't either. All other locks work. But after driving and hitting a bump the speedo comes alive and the door lock works. But, when I shut the car off they both go dead again.

Any insight would be awesome!!

It’s pretty clear then that you want to hit a bump as soon as possible in your daily drive.

I haven’t had your specific model, but am 90% sure it still had a mechanical speedometer cable connected to the transmission. If the cable is good and connected tightly, next check is the speedo gear at the tranny; if it’s good, then your speedo is bad.

Your door locks are connected to and operated by the pneumatic PSE pump. If the others work, there’s probably a vacuum leak either at the lock itself or in the connecting hose. Take off the door panel to inspect.

__________________
Prost!

Sounds like the vac line to the door lock actuator is a bit loose. You may be lucky and just need to push it back on the actuator nipple. Be careful when you remove the door card. Slide it up first to free up the plastic fingers which hold it to the door frame. Then remove the door card. If you just try to pull the door card off you will break those plastic L shaped fingers. (Ask me how I know.)

The speedometer in all W126s is electric.
The signaling for the electric central locking vacuum pump may be powered thru the same fuse as the speedo.
1) Clean all fuses.
2) Should the situation occur again, try locking the car from the right side door or the trunk. Does the left door also lock?

I will look at removing door card. As the drivers window motor isn't working either.

I will inspect the vacuum line as well. The Passenger door lock and truck lock when engaged locks all doors including the the drivers door.

Do you by chance know the fuse number for the electric speedo and the door locks?

Oh, I also cleaned all the grounds too.

Does the right rear window work? If not, check fuse "C" at the right front of the fuse box; left fr. & right rr. are paired.
Check fuses #6 & #19 for speedo and central locking issues.