Speedometer not working

the speedo meter is bad if ,odom works.
finding a spare that old is not easy
ebay used.

the actual speed is a Voltmeter , classical.
so the PCm sends speed this meter in voltage levels or pulses.
so if no VSS errors. send scanned then PCM (ECU) is ok
and the meter is bad or cable to it is loose or damaged.
a tech can even remove the meter and clean it. in the magnet inside
and the core my have metal dust stuck to it and jam it up.
i have repaired many and some are just warn out.

Vehicle speed sensor may be loose or going bad.

sounds like the coil spring for the speedometer needle is bad. you can replace it or buy a new instrument cluster. Look at local salvage yards for cheaper price.

Phylspilot, my 86 Spider never displayed less than 10 mph. If you look closely, you should see a tiny pin that the needle touches at about 10 mph. That prevents it from ever falling to zero. The needle will not move up until the speed exceeds 10. From my experience, that is normal. In your case, not displaying >20 is NOT normal.

The problem could be a few different things. Since the odometer appears to be working ok (I assume you checked that it racks up the miles at a rate commensurate with your approx speed), the speed needle may be sticking. I had the opposite problem with mine - speed needle okay, and odometer messed up.

I recommend pulling the speedometer housing out of the dash and checking to see if anything is causing the needle to bind. If it isn't, then check the fine spring where the needle is attached to ensure the spring is okay. You should be able to move the needle with your finger smoothly and have it return gently to the start position (in your case 12 mph).

This is a tricky operation. If you remove the needle from the stem, carefully place it back on the stem. Do this without moving the stem. Just reinsert the speed needle on the stem exactly at the small pin that holds the pin at about 10 mph.

Check the entire speedometer housing for cracks. Any crack could be enough to prevent proper operation. (That was the source of my odometer issue.)

Have fun putting it all back together.

Have you ever had the transmission fluid and filter replaced, do that and see if that cures the problem.

Try this. Turn the key on, do not start the vehicle. Absence the needle on the speedo. It should move down and then back up. Before it moves back up turn off the key. Repeat this until the needle is at zero. Then start the vehicle. If the needle goes back to a spot other than zero, you may have to take it for a quick drive, than the speedo head is likely bad. This is a pretty common an known issue for gm's. Let me know if you have any questions and I'll do my best to answer them. Thank you for using FixYa.com!

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

2005 BUICK LESABRE TACHOMETER DOES NOT REGISTER WHEN ENGINE IS RUNNING. SOMETIMES IT BARELY MOVES, THEN GOES BACK TO 0
SAME CAR HAD SPEEDOMETER QUIT, REPLACED WITH NEW SENSOR, DROVE ABOUT 1300 MILES AND IT QUIT AGAIN. CLEANED THE SENSOR, WORKS OK NOW.

Try disconnecting the battery again, only the + side. Wait for 3-5 min before hooking back up. Turn the switch on but not over, then start the car after a few seconds.

with the needles off and the scanner set to calibrate the speedometer and all the rest of the needles need to be installed at 1/2 full range with the system on unless otherwise noticed by the scanner.

About the only way is to diassemble it, but it sounds like it's not wrth doing. Your speedometer is having some real malfunction issues anyway. Might as well get a new one (or used). If you did get the needle to the right place, it's just going to end up in the wrong one again.