I have a 79 speedometer in a 79 rx7. Sometimes it will pin itself passing 130mph. if the gauge was marked the pin on the other side of 0 would be about 180mph.
All well and good but I am only doing 20mph? Sometimes maybe 35 mph. then sometimes the speedometer reads correctly.

What is going on and how do I fix it?

 

I had this happen a couple of times in my RX2, the needle would suddenly wind around to the deep end of the numbers then go normal.
.... In my case it was a broken speedo cable.

 

mine stays pinned for the whole drive, not just for a moment or two. If I stop at a light it might act normal from there but usually it takes the car to sit overnight and maybe it will act normal. most of the time it acts up in the pined stage thought.
If my cable was broken or binding I guess it would be 0 of flutter erratically.

 

I havent seen a FB speedo that can go 180 mph.

 

^try reading what he typed?

 

What mods do you have. ****, that sounds fast. You must have put a Type R stickers and maybe a giant windshield decal to go that fast!!!!!!!!!!!!

 

hahgahah, that a good one lol

 

Turn your hat backward, roll ethe window down and holler vroom vroom and pretend. Then pull the speedo cable and if its not kinked ,lubricate it or replace it , maybe replace it anyway. have fun. Bob

 

I had that problem and checked the speedo cable to no avail (though that's the usual cause as I understand)
My problem turned out to be the speedo itself.

 

Ok, for bob holton, how could a binding cable rotate faster ( as compared to a good cable ) to cause the speedometer to read higher than it should. doesnt something have to makew the cable turn faster in some unit time to cause the speedometer to read a higher mph...after all thats what happends in this system if the car is moving 10mph vs 60 mph...the speddometer reacts accodingly.

if it is the speedometer itself how can it be fixed?

 

If the cable binds , it twists and causes greater torque , pushing the needle further ahead . Try it with a piece of cable of any kind , sort of like winding a rubber band.

 

ok for bob holton,
I understand the speedometer will advance to higher speed as the cable unwinds from binding.
but once that hapends and it then binds just after,,, the speedometer will read too low for that moment in time. then I would expect the process too keep repeating.

my speedometer neddle stays at the pin ~180mph all the time for the whole ride generally. though there are occasions where during a different ride it reads normal.

today it was so bad the all I had to do was move at about 10 mph and it was already at the pin. I think I could have walked faster.

I guess you might be right about the binding causing this but I just dont see the complete detail of what is going on from binding cable to what is happening in the speedometer to seeing the needle stay at the pin ( the 180 side )

 

I had one that if I backed up and did not stop before putting it in first it would do that. Pin at about 35. I just made sure I stopped before putting it in first.

 

bump

hope bob holton will see this and reply
or anyone else who can explain the mechanism of why this is happening

 

I had this problem with the Gauge cluster I tried to install in my truck.

Remove the gauge cluster. Carefully disconnect the speedometer cable. I'm willing to wager that the nubbin that the speedo cable connects to is broken off.

Get back to me if that's the case.

 

pele,,,,what is a nubbin?

 

A nubbin is a real small potatoe, Think! once it is wound up, it stays that way till there is no reason to, my toyota speedo drove me nuts, so I pulled the cable made sure the sheath was not kinked or bent , it was so I put in a whole new cable and sheath and it worked fine till the frame broke at 350k miles.

 

bump


any other ideas?