83 Speedometer

[Landon730]

Not sure if I have this in the right category but my 83 c10 is off my about 10-15 mph. Everything on it is stock, tranny, tires, rear end. I think its the gear in the tranny but what the the other little thing that plugs into the back of the speedometer do? I found it listed as speed sensor. Could this be messed up? Heres a picture to help

 

[ak diesel driver]

I would have thought an 83 would be all mechanical

 

[THEFERMANATOR]

The speedometer is all mechanical, but if it had a 700r4 it had an optic sensor in the speedo itself that read the gear turning. It has no effect on the speedo.

 

[ak diesel driver]

So what does it do?

 

[Will L.]

It should be off by percentage, not exact amount. Drive a measured mile at low speed like 25 mph and check odometer. Then do it at 50 mph checking odometer.
The ratio should be the same. If not, I would suspect something like speedo cable damaged and creating drag and slipping.

 

[THEFERMANATOR]

So what does it do?

It tells the ecm how fadt you're going. Before GM went to all electric speedo's, they had cable speedo's and an optic sensor that was triggered by a hole drilled into the drum that the cable spins. For his truck it was probably there to control lockup for a 700r4 trans.