speedo question?

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mopar56

mopar56

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so I just replaced the speedo cable assembly ( new ) and speedometer, ( used one ), in my sons 74 Duster, it has a 904 trans now as soon as the car moves forward a couple feet the speedo starts to move quite a bit, like 50 mph in two or three feet then when I stop the speedo stays put and when I go backwards the same distance the speedo goes back to zero, whats wrong?, maybe a bad speedo?, if so any way to test it? thanks
 
It's the speedo. The internal workings are magnetically driven and it seems that instead, the magnet is touching the spinning drum causing it to be directly driven.
 
mopar56 said:
so I just replaced the speedo cable assembly ( new )
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I had the same problem when I changed mine.

Check the cable FIRST for to tight of a bend etc. I adjusted mine in one spot (right where it came through the firewall) and it cured it. I thought it was the speedo at first, but after chasing the cable I found I had bent it a bit too much in one spot. It would jump to around 50 and stop.

The way I actually found it was I drove the car a few feet and when it stuck at 50, I started wiggling the cable and it dropped to 0.

This may NOT be your issue, but it's worth a shot.
 
And if that works, it kinda indicates that the internal cable is a hair too long inside the sheath. So if the issue shows up again at a later date, I would shave a bit off. Maybe .080 or so to start. You would need to cut it off the head end, because the tranny end is mechanically fixed. It would just require you to unbolt the cable from the tranny and slide that internal cable out, trim it, then slide it back in and engage the head before screwing it back onto the tranny. Easy peezy.Last resort.
But I agree the magnetic drive is probably a little loose in its bearings.
 
It could very well be, but the "new" cable thing caught my attention. I think when I moved the car the cable was bound, and all of a sudden it would release, just enough to make the speedo jump like it did.

I was certainly grateful it was my mistake, rather than changing the speedo.
 
OK, good advice, I don't see any dramatic bends that would cause this and the cable I replaced was an assembly not a core so you would think it would be right?, that being said I have been out in the shop for the last hour or so learning about speedometers, I took the original one almost completely apart, ( a bit of a puzzle ), it appears the odometer gear was binding in the ends of the housing so I drilled the washer out that holds it in and removed, cleaned, and lubed it and did the same with the rest, then put it all back together, just loosely with a tie rap for now and powered it all up with a variable speed 3/8 drill, all seems to work fine so I will epoxy the parts back together that I had to break apart and take out the used speedo I just put in for inspection, if it appears not to work then I guess put the original I just fixed in and see what happens, thanks for the suggestions
 
mopar56 said:
OK, good advice, I don't see any dramatic bends that would cause this and the cable I replaced was an assembly not a core so you would think it would be right?
Click to expand...

Mine was too (brand new assembly), but I had over extended it right inside the firewall, under the dash. It didn't look like a problem, but that was it. I got the stupid thing stuck up on 50, and reached up under there and pulled slightly on the cable and it immediately dropped to 0. :banghead:

Like I said, it might not be the issue, but when I read your OP, it was exactly the same thing I experienced.
 
mopar56 said:
Bad Sport, did yours go backwards with the car in reverse?
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I never tried it, or if it did I never noticed, it just stuck right around fifty and would stay that way for a bit. I would come back to it a day or two later and it had dropped.

That's interesting, I'm wondering if when you back up it releases the pressure on the cable and it drops to 0. IF that's what it is.

It's just too weird that we have/had the exact issue, after doing exactly the same thing to our cars.
 
yeah, agreed, well I backed off the knurled nut at the speedo housing on the trans to help release a bit of pressure so I will try that first then I will tug on the cable like you did, but might wait until tomorrow now as the car just has open headers now, don't want to piss the neighbors off too much, lol
 
Lol, yeah, it only takes a second to do, worth a shot.

Now that I think about it, I'm not sure if it was the bend or if it was cocked where it hooked to the back of the speedo. But it was definitely right up in that area.
 
Ok I couldn't sleep until I knew, so I started it up, ( too bad for the neighbors), lol no joy!!, same problem, I tried pulling on the cable under the dash and the hood, but change, so out with the dash again, arrgghh,!!
 
Engineers did make the marriage of cable to instrument about 2 inches long to insure alignment to a point. Good for 100 thousand miles anyway. The instrument itself cant survive any pressure from the cable in any direction. If the input of the instrument is worn out you will see scaring farther inside where 1 part is hitting the other ( outside surface of what looks like a drum ). If that's the case, replace the instrument. To trim the length of the cable and route to align with the instrument as straight as possible is a hit and miss effort. Good luck
 
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