ugh. ok

so i replaced the cable finally after getting sick of the speedo not working. the cable itself got pulled out of the housing which caused it to not spin the gear in the trans. simple, replaced cable and put on a new purple speedometer gear.

car reads 10mph in park and off. - it seems WAY off when driving. i accelerated to at least 45 mph and it says im going like 70.. o..k.. wtf?

what's going on with that?

i have the purple gear due to the fact i have 3.73s in the back (yes, 3.73.. not 3.70, 8.8" swap) but i'm baffled as to why it reads 10mph when parked.. ?

 

What does the speedometer read when the car is idling? If it's reading 10mph with the engine off then it sounds like the speedometer is miscalibrated. Check what it reads with the cable unplugged, if it still reads 10 then you know it's the speedo itself, otherwise I would havea frienddrive infront of me at a predetermined speed so I could check what the speedo is giving me at that speed.

 

It's your speedo head unit. The cable turns a little bar shaped bracket inside the speedo head. The bracket has a magnets in the end of it that drag a pivot wheel attached to the speedo needle by a shaft. As the magnets try drag the wheel/needle assembly along with them a light coil spring tries to turn it back. Before it can turn back the magnet on the other side comes around and lifts it back up. The faster the cable spins the more the magnet bar spins the wheel/needle while the spring keeps it from snapping around to 120mph. Where they balance out is what your speedo reads.

The "back pressure" spring in your speedo head is most likely "sprung". It won't push the needle back down to 0 and does not provide enough resistance to keep the speedo accurate. That is why it reads faster than you are really going and reads 10 mph at a stand still.

This is kind of hard to explain in a post as you can tell, but I tried to "rebuild" my speedo head to try and get it to work. It didn't work but I DID learn how the dang thing operates.

I would bet you need a "new" speedo head from one of the Mustang parts houses...or you could try a bone-yard speedo head and put your face on it if yours is in better shape.

 

If you live in a bigger city like Phoenix you should be able to find a speedo and tach repair and calibration shop.
I don't know if it would be cheaper than buying new but it would be accurate.

I had an old sentra with the same symptoms.I tried to fix it by spraying it with WD-40. Made it worse.
Like Montanas said, there is a fine balance between the friction/spring pressure and the magnets.
I believe they bend the magnet bar to adjust.

 

As said above, I'm pretty sure a lubricant (such as WD-40) won't help since it won't go back to zero when the car is stopped but still over-speeds when the car is moving. If the wheel/needle were "sticking" and needed lube it would read slower than actual speed and would resist moving from whatever speed it was stuck at.

That being said...65Coupster brings up something that I never had access to in the barren wilds of Montana...a shop that actually repairs things like this. Like I said, I tried to fix it but it was a "it don't work anyway so...what the hell let's tear it apart" type of thing. If you can find someplace that that knows what they're doing and can get the parts...who knows!

If you find someplace that can fix it...let us know...mine STILL needs to be fixed, including all the stuff I dorked up when I took it apart!

 

well, it's fishy and pissing me off. it worked perfectly before until the end of the cable broke off and i drove around for weeks with no end on it to spin it, now that i replace everything it works all crappy :|

 

I guess the next question I have is...what caused the cable to break? It may have just been old and ready to move on to the great auto parts store in the sky...but...if something went wrong and caused it to over-wind the spring in the speedo head it would explain why the cable broke AND why the speedo head is toast. The spring in the speedo head could never break the cable, but the friction in the cable housing when the cable spun too fast certainly could. Did the old speedo gear look torn up?

I still think the speedo head is the problem since you have replaced the cable and gear and the speedo head DOES show speed (even if it's wrong). If the drive gear in the tranny was bad your speedo would never show speed, just sit at zero. Since your speedo does "work" it means the drive gear in the tranny is turning the cable gear and the cable is intact. Your head unit is just out of calibration. But as I said above...it ain't easy to fix and I wouldn't know how to calibrate it even if I knew how to fix the "10 mph at stand still" problem.

 

nah, it wasn't broke broke what happened was i think the cable internally got pulled out of the other end - which is when i took the old one out i could pull the innard out and the end was perfect on the other side. needed to be replaced anyway - the original gear looked in perfect condition.

 

It's almost gotta be the speedo head unit. The tranny gear, cable gear, and cable are good or else you speedo wouldn't turn at all. All I can say is buy a new speedo, find somplace to fix the one you have, or take your chances with a bone-yard unit. At last yours turns the speedo. Mine doesn't work because the tranny gear isn't turning...and I wrecked my factory original head unit to figure it out! Oh well...that's life with a 40+ year old car!

p.s. If you REALLY want to make sure it's the head unit...take your head unit out of the dash, put the dash back in place and drive down the road. You should be able to watch the end of the speedo cable spinning as you drive. If it is spinning...the head unit is the problem. The problem can't be a wrong gear on the tranny end of the cable because then the speedo would just read the wrong speed, not hang at 10mph or over-speed like you describe.