speedometer won't work

[locdogg]

79 gmc k1500 4x4, 350, np205

speedometer seemed to just jump around from 0-20 until I hit about 40 mph, then it worked.  now it doesn't work at all.  I have the factory cruise control, I replaced both speedo cables and still doesn't work. 

I jacked the back end up and put it in drive to see if the lower speedo cable moved at all.  it didn't.  should I try replacing the speed sensor?  is that what this gray rectangular thing the speedo cable attaches to is called? 

sorry for the bad description, I'm pretty new at this

thanks!

[1976Scottsdale]

That is what is called a ratio adaptor.  It corrects the speedometer due to variations in rear end gear ratios.  If you cannot get any speedo signal, then that is probably stuck or broken.  Remove it and see if it will turn by hand.  If so, make sure both sides turn.  when you turn the pin by hand.

[locdogg]

thanks a lot!  i'll try that and let you know what happens

[locdogg]

the ratio adapter turns fine, I'm gonna try putting it back on and see if it works, maybe it was just stuck or something

if that doesn't work, is it the speedometer gear in the transmission?

[locdogg]

ok, still nothing turning.  the little nub that sticks out (that the ratio adapter goes on to) moves in and out.  is this normal?  i'm betting I have a broken speedometer gear in the transfer case?

here are some pics of it pushed in and pulled out
also a pic of the transfer case

[zieg85]

ok, still nothing turning.  the little nub that sticks out (that the ratio adapter goes on to) moves in and out.  is this normal?  i'm betting I have a broken speedometer gear in the transfer case?

here are some pics of it pushed in and pulled out
also a pic of the transfer case

I believe that nub is part of the gear reduction adapter.  That nub should pull out and have the same end as the speedometer cable.  Can you turn the nub by hand?  If you can't and can pull the nub out, put your speedo cable without the adapter and see if that works then.

[locdogg]

you are right, that nub was part of the ratio adapter.  I opened the ratio adapter and greased it all up, it all moves properly but I can't get the speedo to work with it hooked up. 
I plugged my speedo cable in without the ratio adapter and got it to work.  It took a couple times, it seems like I have to put the speedo cable in just right.  I think if the speedo cable were a half inch longer, it might work better.  It just acts like it can just barely reach far enough into the transfer case to catch and turn. 

Thanks a ton for the help! 

Is this ratio adapter aftermarket?  or do you think it came factory with the truck?

[zieg85]

It came with the truck.  I don't know if there are aftermarket ones out there but I would think it would be still available at the dealer.  I know I have one or 2 laying around in my abyss, if I run across it I will holler out.  See if there is a part # stamped on it anywhere.  I am pretty sure I know which 5 gallon bucket in my shed to look in.

[1973c20]

Haha not to be a nitpick, but it looks like you might need to get a new seal for your rear diff 

[locdogg]

zieg85, it has stamped on it

MADE IN USA
1  3000HX

and on the other side

C.B.I.
MADE IN USA