MG Car Repair: Speedometer, screw driver, speedometer

Question
Dear Mr Fitzcharles--1975 MGB--Re Speedo--Starting with the back of the speedometer,the cable is attached to the speedo, when loosened, the cable will turn freely by hand, so we know that cable is free--------Then the problem arises, the cable goes thru the firewall,(thru its housing), and down to a metal box and it appears that the cable comes out the bottom of the box and straight down to the transmission--The cable under the dash does not spin while driving------Please advise my answer--What is the metal box?  Thank You

Answer
Hi Mick,

With the cable housing removed from the speedometer pull the inner cable out of the housing and examine the end to see that the square is on both ends. If it is good on the ends then go to the connection on the transmission and put a small screw driver into the hole in the center where the cable goes (or use the end of the cable) and try to rotate the transmission drive. If you can rotate the drive then the drive gear is probably broken so you need to remove it from the transmission and examine the small gear to see if the teeth are stripped or the small gear slips on it's shaft. This is where most of the problems are other then a broken cable.

Howard