speedo reads ~3.5x the speed...
 

im having a weird issue with my speedometer; it occaisianally reads ~3.5 times the normal speed (cruising down my street at about 25 it reads 80). the car is a 87 gxl, and i just finished swapping in a s5 motor and transmission, but the previous transmission was also s5 and i never encountered the problem. During the swap i also completely removed the speedo cable, washed it and greased it (a few months before the original motor died the speedo cable came loose and fell out, and i had to replace it). Any ideas why it would read so fast intermittently?

bump

If it happens ocasionally and not all the time, I would blame the speedometer gauge itself. It would be very difficult for the gearing in the transmission and the cable to all of a sudden speed up out of nowhere.

sounds like the tension spring is slipping inside the speedo.

mines starting to do the same thing, but i notice its only after ive drove the car a while and it is warm inside . speedo will read 1.2x the speed, then other times 2,3, or 4x the speed.

the transmission speedo or the dash speedo?

Dash. The spring is attached to the needle, and provides specific resistance, while a magnetic disk attached to the speedo cable, turns near a similar disk attached to the needle's shaft and pulls the needle along with it against the spring resistance. If the spring is broken or not properly anchored, no resistance, and the needle swings much further than it should.

awesome. i was hoping i didn't have to pull the cluster apart...

Could it be caused by too much grease or oil? Just pulled the speedo unit out and the spring looks fine but the back plastic peice and magnet peice is oily. Im debating on whether or not i want to pull th whole unit apart to clean it...

I posted this issue just a couple months ago because I found no information on here about this problem. Everybody was suggesting the cable but that didn't make sense to me so I removed the cluster and took the speedometer apart. I went carefully through the speedometer and found nothing amiss except for an excessive amount of sticky oil between the magnetic plate and the copper disc. Since the needle movement depends on a phenomena where drag is introduced between the magnet and copper disc by inducing a current in the copper disc, the system is EXTREMELY sensitive to any excessive drag.

Clean the oil out thoroughly, also clean the needle shaft where it goes through near the spring to reduce the friction there and re-assemble. Once re-assembled you can spin the input by hand and you should see the needle jump up and fall back down smoothly, if it sticks at all keep on cleaning. Once you are done you should be able to apply a tiny (TINY) amount of oil to the input shaft to keep it moving smoothly.

My best guess on why this happened is one of 2 things:
1) The grease inside the assembly slowly ran out from its normal position to interfere with the disc assembly
2) Extra oil/grease worked its way into the speedometer through the speedometer cable. Both seals on my speedometer cable had failed completely and oil was leaking out at the cluster end of the cable, although the oil inside the speedometer did not appear to be gear oil.

Thanks for posting up your fix. I'm going to add this to the FAQ, since I know I've seen a handful of threads where someone had the same problem.

will the botom plastic peice/input shaft separate from the piece holding the needle/spring? i do NOT want to remove the need from the spring or touch any part of that, my fat clumsy fingers will break something...

I took a couple pictures along the way, I'll see if I can round them up today for you. You have to disassemble the big plastic multi-piece housing in the back of the speedometer, then the input half stays in one housing while the needle half stays in the front housing. You can clean it by only disassembling it this far. I used non-residue electronics cleaner and rubbing alcohol to clean the needle bushing to reduce the drag there, and just tissues and the same chemicals to clean the grease off the magnet / copper disc assembly.

*EDIT*
Looks like I don't have any pics of the inside of the speedometer assembly, sorry. Here is a link to my thread from when I fixed it, however it got merged with another thread of mine so it's a bit of a pain to follow:
https://www.rx7club.com/new-member-rx-7-technical-256/fc-speedometer-reads-3x-real-speed-sometimes-951494/

yeah, got it apart and cleaned all the old grease. suprisingly there was hardly any grease on the magnet/copper disc but it still caused the needle to spin when i spun it by hand. after cleaning it spinning the input shaft by hand does nothing to the needle so it should work normally when i get it back in the car...