I just got a 79 sa automatic and the speedo doesn't work. The odometer works though. When pulling away from a stop it goes to 100, it bounces around and then goes to zero.

 

If the odometer is working properly, then the speedo itself is dirty or damaged. Could just be crud collected on the armature, or could be a spring failure, or something bent.
These are simple magnetic/mechanical units, driven by a rotating cable. No electronics. If the odo is working properly, that means the rotating cable is OK as is the drive gear in the trans. Only other failure point is the speedo mechanism itself.

 

ebay has SA instrument clusters on sale all the time for $20-$80. Or check the FS forum here too. Easy swap in, good to have standing by in case you pull yours apart and discover either it can't be fixed, or you need a part (of which none are available new). Generally these are pretty reliable, so a used one is worth considering...

79 or 80 will fit.

Stu Aull
80GS
Alaska

 

Bryan, I would start by removing the cluster and inspecting the speedometer cable and speedometer unit itself. check the drive gear on the transmission as well. It can be remove easily with one 10mm bolt securing it. The drive gear may be damaged or chipped.

 

Since the odometer is working the cable is turning. The issue is with the instrument itself. The return spring is broken and or the speed cup is galled and binding on the magnet. You need another speedometer.