93 Cobra Speedometer Issues

Discussion in '1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-' started by Baron164, Jul 16, 2014.

  1. So I picked up a 93 Cobra a few years ago, it had seen better days and I've been slowly trying to get it all right. One of the things that has been broken was the speedometer which I know is very common with these. The need was very jumpy up until about 40mph and then it would calm down. But even then it has not been giving an accurate speed. It gradually skews more and more the faster I go. So at 20mph it would say I'm doing 18-19mph. At 75mph it says I'm doing 90mph. I just replaced the speed sensor and the speedometer cable. I lubricated the cable before I installed it but after installing everything the needle is shaking all the time. No matter what speed I'm going it shaking. For example if I'm doing 75mph the needle will be shaking between 80 and 85mph. And of course it's still not showing the proper speed. I examined the speedometer gear when I was replacing everything and the gear seemed fine. That being said I'm not sure what gears are in the rear end. I was told it still had the original 3.08 gears but I have not verified that yet. I'm thinking it would have been more likely for someone to swap the rear gears and not the speedometer gear. My immediate need is to get the needle to stop shaking, and then I can figure out the gear ratio's and get it reading properly. I'm just trying to figure out what else I could do. I'm going to put the old speed sensor back in and see if the shaking stops. I'm just looking for any suggestions from fellow fox body owners.
  2. The jumping speedometer is one of 2 things:
    1.) The speedometer cable (whole cable, not just the inner shaft) needs to be replaced.
    2.) The speedometer gear on the VSS sensor has some bad teeth on it, or it is slipping on the VSS sensor shaft.

    Determine the rear axle ratio:
    Jack up the rear of the car so that both rear wheels are off the ground.
    Put a paint mark on the driveshaft that is visible when the rear of the car is jacked up off the ground.
    Turn one rear wheel 1 turn while counting the number of turns using the paint mark on the driveshaft as a reference point.

    2 3/4 turns = 2.73
    3 turns = 3.08
    3 1/4 turns=3.27
    3 1/2 turns =3.55
    3 3/4 turns = 3.73
    4+ turns = 4.11
    4 1/4 turns = 4.30

    The odds are that the rear gears have been changed to 3.55 and don't have the 18 tooth speedo gear needed for correct reading with 225 -55 17" tires.
    Change the tire size and the speedometer gear needed for correct speedometer indication changes.

    See http://www.latemodelrestoration.com...ium=Speedometer+Calculator&utm_source=YTLinks for the speedometer gear calculator.
    #2 jrichker, Jul 16, 2014
    Last edited: Jul 16, 2014
  3. Yeah, I replaced the entire cable already with a new one and it went from jumping a lot under 40mph and then evening out, to shaking a little all the time. When I pulled the old cable out I found a good sized kink on it which I thought was causing all the problems. I don't have another speedo gear to test with and don't want to order one until I know which size I should have. The tire size on the Cobra is actually 245/45/17, that's the one thing I know that wasn't changed so the tires are the correct size at least for what originally came on the car.

    I've tired the speedometer gear calculator on LMR's site and I can't get it to give me a proper gear. If I input 3.08 for the rear it says I need a 19.4 tooth gear, if I use 3.55 it says I need a gear with 22.3 teeth.
    #3 Baron164, Jul 16, 2014
    Last edited: Jul 16, 2014
  4. Use the nearest whole number of teeth: you won't find an exact match 99% of the time.
  5. ok, well I'll replace the gear and see what that does.
  6. I've been having similar issues with my 91 LX for a while now. Mine does the exact same thing. From 0 to 40 it bounces like crazy and at 40 and above it smooths out. It is however reading accurately at that point. At least according to using my GPS as a way to check it. I also changed out the entire cable assembly as well as the gear on the transmission. No change. In the winter time when the temperature is cold the needle doesn't bounce and actually gets stuck at times. After I drive the car for a little bit the needle will start working correctly. I've pretty much gave up on trying to fix it at this point and just live with it. I would really like to fix it though so I'm interested in following this post to see if you find a solution.:shrug:
    S&B likes this.
  7. I'm going to work on it some more this weekend so I'll post back with how things go. My father has a bunch of ford speedometer gears so I figure I'll try some of them to see if they help. Then I might put the old speed sensor back in to see if that helps as well. Only reason I replaced it in the first place was because the clip holding the cable in was gone.
  8. Some other clues to speedo problems...

    How the speedometer works:
    The indicator pointer has no direct connection to the speedo cable. It uses a drum with magnets on it to couple to the pointer. The drum turns and tries to twist the circular steel disk that is mounted on the pointer spindle. The magnetic force is all that connects the drum to the circular disk. There is very little clearance between the disk and drum, only a few thousands of an inch.

    Lubrication warning
    Use a graphite based lubricant for the speedo cable. It is available at most auto parts stores in a very small tube. Lubricate only the lower half of the cable. The reason for this is that if you use too much lubricant, it works its way up into the speedo head and gets between the rotating magnet and the disk. This causes the speedo to gum up and jam the indicator needle against the stop pin. You may be able to fix things up with non-flammable brake parts cleaner to clean the disk and magnet assembly. Plan on replacing the current cable and housing with a new cable and housing to prevent the excess lubricant from doing it all over again.

    Photos courtesy of Almost Stock


  9. I only put the graphite lubricant on the lower half, once I put it in and found that it was still shaking I put some WD-40 on the upper end to see if that would help. I took the Mustang to work today and strangely enough on the way home I found that the needle would stop shaking for maybe 15-20 seconds, then start shaking again, then calm down again. It did that for the last 10 minutes of my drive home so I'm wondering if it's working something out at either end.
  10. I only used graphite on mine but it made no improvement. :(
  11. I checked last night and confirmed that the rear-end does in fact have 3.08 gears, I'm gonna pull the sensor out probably tomorrow and try re-running the cable from the firewall down to the transmission. I'm wondering if there is just a tight corner somewhere that is causing the metal cable to rub on the sheath and is causing the shaking.
  12. Hopefully you can figure it out. I know for me anyway that things like this drive me crazy until I get them fixed. I tried everything that you have so far and still have the problem. Good luck.
  13. Good idea. Sometimes a small bend in the wrong place can shake up things
  14. Were you able to resolve your speedo issue?
  15. Not yet no, I had to get it inspected and while it was up in the air I checked the speed cable and there were no kinks. The weird thing is that the issue went away for a couple days. It was still bouncing a little under 20mph which I think could be caused by the gear but over 20mph it was fine. Then after a couple days the needle started bouncing again. I haven't had a chance to pull the sensor out and replace the gears yet. I'm going to replace the gear next and if it still has issues I'm going to pull the whole cable out and lubricate it again.
  16. I have a saleen 170 mph cluster in my 90 stang. I dont know who put it in but it sucks, so I took it out and I am looking for a cluster thats suppose to be in my car. The nub that the needle rests on is at 10 mph.. lol so nothing moves till 30 mph? go figure.. any way.. I took the mph and tach black number sheets off and behind that is a copper coil that the needle sits on. Maybe that coil is unwinding and causing the needle to bounce.. Just an observation so no clue if this has any validity to it..