Wire from speedometer?
 

So i was gonna hook up my speedometer today bc when i bought the car it wasent working and then we swapped engine and tranny for new ones and replaced the cable. They put the cable in and hooked it up while i was gone but i just thought it was disconnected from the actualy speedometer cus it didnt work and i went under to hook it up and it was. wtf? so i did some investigating and found the 3 wires comming from the speedometer went into a blue boxish thing and then one wire came out but the one wire was cut about 4 inches later... what does this wire connect to?
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The speedometer only has a cable to the trans. The rpm gauge has 3 wires that are connected to the printed circuit. Is that what your talking about?

You sure that's coming from the speedo/gauge cluster? Green wires were commonly used for the horn, dash lights, and the interior lights down by the kick panel areas. Hook up a test meter or light to the open wire, and start playin with some switches.

That makes sense now. But y wouldnt my speedo work? Its hooked up on both ends but dosent move when i drive.

Did you pull the gear on the side of the new trans to make sure that cable is comparable?

Could be the drive gear inside the trans tail slid back on the output shaft, or the drive or driven gears are stripped, or your speedo is shot. Your speedo is mechanical, there is no wiring that makes it work. Pull the cable off the back of the speedo, see if you can get it to hang down so you can reach it, then go for a drive. You should be able to see/feel the end of the cable spinning. If not, the problem is at the trans, if so, it's your speedo.

Looks like maybe someone has swapped the speedo out at some point.That looks to be the little wiring box that is on the back of an 81 only speedo for the speed sensor that goes to the computer.It wouldn't stop the speedo from working,but figured i'd let ya know what it was for.Like mentioned,try feeling or watching the cable move inside the sheath,also try to grab the back of the speedo with the cable off,and spin it to see if the needle moves any.

Hey sorry ive been busy lately but i have one last question. we pulled the speedo cable from the trans and it was all good and dandy and when i put it back in and took it for a drive it worked for awhile but was inaccurate. it got more inaccurate the faster i went. at 55mph it was at like 80. and then i went to a stop sign, stopped (obviously) and then drove and it stopped working again. any ideas?

Yeah, your driven and/or drive gear for the speedo (at the trans) isn't matched to your rearend gears.

Sounds like the wrong speedo gear, and possibly worn speedo gear in the tailshaft also. Either way you need to pull the tailhousing to fix it. A new correct ratio gear will probably fix both problems.