Well it looks like you found your problem. But I just went through this with my 77 that I got in September. The speedo did not work I figured a cable was bad. The last time it was on the road was in 1989. Well what I found, and I can't figure out how this happened. I was changing the trans filter and when I dropped the pan the clip that holds the drive gear to the output shaft was in the bottom of the pan. Oh well I removed the tail shaft and the gear had walked it's way foward. Then I found the transducer for the cruise control locked up. I took the apart and it was loaded with carbon. I just kept spraying it with cleaner and then doused it with WD 40. It's nice to know how fast I am going.  

I finally fixed my speedometer and I am happy, it was the melted cable, so I bought a new one.  I put it on yesterday and that fixed it.  I'm glad it wasn't anything more serious.

-Brint

Another one bites the dust!  Problem...that is!

Congrats!  Hope to see you at the CF gathering Ken is putting on!

Is the transducer mounted near the firewall with cable from transmission and then cable going thru firewall to speedo?

My 78 also has erratic speedometer.

Can it be bypassed?

Thanks,

Bick

The transducer is mounted on the drivers side fenderwell area, around the brake booster.
It can be bypassed, but...the speedo will not be accurate.
The transducer also changes the ratios of the speedo, so bypassing it will put the speedo off by quite a bit.

Thanks, Is the transducer only for the speed control? Do not remember on first 78 that did not have speed control. Present speedo is erratic and some times does not indicate at all. The earlier post of flushing with WD-40 may help?

Thanks again,

Bick