Speedometer Trouble

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cosgig

cosgig

MoBro Inc.
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Well, I got sick of the wife complaining about the speedometer not working in our 67 Cuda convertible so I went out and bought a new cable and installed it. Now, heres the problem. The car doesn't move 2 feet and the speedo jumps to 20-30mph. I disconnected the cable from the speedo and brought the end out under the dash to watch it spin and it looks to be spinning correctly. What gives??? The speedo is a 150mph that I removed from another cluster and installed in my car, so it's origin is unknown, but I have not had this problem before. Any suggestions? Thanks, Geof
 
I'm wildly guessing, as I've not done anything with a Mopar speedo so far. My best guess is some filth between the drive disc and driven disc that acts to spin the driven disc directly (rather than the normal spinning up gradually) until the filth begins to "roll" across the surfaces. This is assuming that these speedos work on the boundary layer effect and that the cable isn't binding from a weird angle or something. I suppose the same could be true if they use magnets or something of the sort. If it is caused by such a thing, disassembly and a can of air would probably kick it loose.

Crap. I've run out of abstract conditionals. ;-)
 
The one I have (stock square 120mph) I believe uses magnets. It has 2 "fingers" that rotate around a metal cup that connects to the needle and as the fingers spin it counteracts the slight spring tension on the cup and swings the needle. If there is anything in between the fingers and the cup it will cause the needle to jump up then fall as they come in contact with each other. This seems to come up enough.... I'll snap a pic of a speedo that is sitting on the shelf in the garage and try to post tonight.

Now, the speedo of unknown origin????? might work by PFM.. :lol: the gear ratio might be way off, but you should be able to spin it with a drill to see if it's ok internally
 
Oops, I didn't mean that the 150 speedo is of unknown origin, I meant that it's operating condition is unknown. It came from a cluster from a 67 or 68 Cuda, as the speedometer cable screws on instead of the later clip on style. I bought the cluster from the WPC swap meet in Warren, Mi. last year and the seller was selling it for a friend so he could not verify that any of the guages or the speedo worked. Turned out the tach from that same cluster didn't work either, but I fixed that. I will take Doktor X's advise and blow the back with a light blast of air and hook it back up to see if that changes it's attitude though!!! Thanks for any and all replies, Geof
 
You cant kink the cable or shieve. Cant turn a sharp 90 degree corner. The cable cany ne so long that it applys pressure to the instrument.
So make sure its routed a slow curve under the driver seat. Make sure its routed to the instrument correctly under the dash. Leave the collecter nut loosened and try it again. If all the cable bugs are squished yet the needle still bounces ...the insturment is the fault.
 
Yep, gentle bends first. How dry is the cable? Lube cable with powered graphite. Sometimes it binds on the outer portion and it will bounce and/or squeal. Had a problem like this in Wisconsin. Never seen it down south.
 
Well, thanks guys for all the help. I'm going to disassemble the entire operation and make sure the speedo mechanism is clean and the cable is not kinked. I did lube the cable in the sleeve and install it correctly with no kinks or bends, but better to recheck everything to be safe. Just for informational purposes, the speedo does not "bounce" as it were, what it actually does is go right up to between 20 and 30 MPH while the car is barely rolling, and I do mean BARELY!!! Thanks again all for the good advise, Geof
 
Ok that would be in the instrument itself. May just need cleaning ?
The cable may be fine. Easy test is diconnect from speedo and put the core out a couple inches so to disengage from gear and turn with hand. If it turns easy enough and slide out and back easy enough, its probably fine.
 
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