Speedometer goes crazy sometimes

[nfldfordltd]

Every now and then the speedometer needle in my 88 Town Car goes nuts. Like at 30 mph it registers 100+ mph, and bounces/hunts all over the place. Odometer seems to be stable all the same.
After I bring the car to a dead stop, it still reads around 20 mph. It can do this for only a moment, or sometimes acts up for a full day or more. Very erratic how and when it happens. Can go as long as a year between crazy fits.
I haven't done anything to remedy this yet, some input would be just grand. I'm thinking about starting by lubricating the speedometer cable just to see if it helps.

[RyPow]

Every now and then the speedometer needle in my 88 Town Car goes nuts. Like at 30 mph it registers 100+ mph, and bounces/hunts all over the place. Odometer seems to be stable all the same.
After I bring the car to a dead stop, it still reads around 20 mph. It can do this for only a moment, or sometimes acts up for a full day or more. Very erratic how and when it happens. Can go as long as a year between crazy fits.
I haven't done anything to remedy this yet, some input would be just grand. I'm thinking about starting by lubricating the speedometer cable just to see if it helps.

Lubricating the cable usually only helps a "twitchy" or "bouncy" speedo (the characteristic slight wobble many old cable-driven speedos have, especially at low speeds), since it's caused by the cable binding up and lurching. The speedo needle is attached to a shaft with a round magnet at the back, and the speedo cable drives a U-shaped bar with a magnet on each end which rotates around the round magnet. The "pull" of the outer magnets on the inner one as they rotate is what causes the needle to move upwards against slight spring tension (obviously very finely calibrated). So, if your speedo is reading very high and bouncing all over the place, then either the needle spring tension is too low (broken/weak spring) or there is dust or other matter getting stuck between the outer and inner magnets and making the inner one turn much further than it is supposed to.

The odometer is gear-driven off the speedo cable input shaft, so it will always read smoothly unless the cable is binding up really bad (as in, close-to-breakage bad) or the speedo gear in the trans is so worn out it is skipping (although, I am only theorizing on that one, I've never seen a worn-out speedo gear before).

I would pull the speedo and make sure there is nothing affecting the clearance between the outer rotor and inner drum (dust, etc). Otherwise, a replacement is probably in order since you will likely jeopardize the calibration if you pulled it apart to remedy the spring.

[miamibob]

Lubricating the cable usually only helps a "twitchy" or "bouncy" speedo (the characteristic slight wobble many old cable-driven speedos have, especially at low speeds), since it's caused by the cable binding up and lurching. The speedo needle is attached to a shaft with a round magnet at the back, and the speedo cable drives a U-shaped bar with a magnet on each end which rotates around the round magnet. The "pull" of the outer magnets on the inner one as they rotate is what causes the needle to move upwards against slight spring tension (obviously very finely calibrated). So, if your speedo is reading very high and bouncing all over the place, then either the needle spring tension is too low (broken/weak spring) or there is dust or other matter getting stuck between the outer and inner magnets and making the inner one turn much further than it is supposed to.

The odometer is gear-driven off the speedo cable input shaft, so it will always read smoothly unless the cable is binding up really bad (as in, close-to-breakage bad) or the speedo gear in the trans is so worn out it is skipping (although, I am only theorizing on that one, I've never seen a worn-out speedo gear before).

I would pull the speedo and make sure there is nothing affecting the clearance between the outer rotor and inner drum (dust, etc). Otherwise, a replacement is probably in order since you will likely jeopardize the calibration if you pulled it apart to remedy the spring.

Hope it helps and GREAT explanation!!

[gadget73]

Sounds like seized lube inside the speedometer. Mine does that kind of thing when its very cold out. I've never attempted to re-lube the speedo but I expect thats the real fix. The concern would be getting oil where it doesn't belong, or using the wrong kind. I might get my boss who does clock repair to do that job for me.

[87gtVIC]

Sounds like seized lube inside the speedometer. Mine does that kind of thing when its very cold out. I've never attempted to re-lube the speedo but I expect thats the real fix. The concern would be getting oil where it doesn't belong, or using the wrong kind. I might get my boss who does clock repair to do that job for me.

The escort I previously had did that this past winter. Super cold days you could expect that speedometer to be reading all sorts of crazy numbers.

[nfldfordltd]

Thank for the words of wisdom, guys.
Temperature doesn't seem to have an impact on my speedometer in particular. There could be dirt in my speedometer head unit as I have needed to travel dusty gravel roads more than I like to. I'll check to make sure the gear is fully seated in the side of the transmission as well, as I had to put a new gear in there when I changed the transmission in 2009.
And it might be one of those quirks that never seems to go away for the life of the car. It's one of those things that's annoying when it's happening but it's easy to forget when it's not happening...haha
After I finish unpacking (again, ugh) I'll open it all up and have a look at it. I think the service manual tells what type of lube to use for the speedometer parts, I'll have to check that out too.

[sly]

check the cable itself to make sure it's still intact. my 88 and 82 needed that replaced as the cables were worn through from either having come free and sat on the exhaust or rotted out.

[nfldfordltd]

I know my cable isn't on the exhaust because I have it zip-tied to the transmission crossmember away from the exhaust. The cable may be fooked up in one way or another. I'll get into it someday when it's not so mother-trucking hot out...well into the 90's here today. Not bad for being in the sub-arctic lol.

[sly]

wow... and it's only in the 80s here in Texas. What up with that?

[nfldfordltd]

I'll be lamenting for the 90 degree temps when it's sub-zero here from December to April haha.
The biggest thing I have to get done this year is the work done on the trunk and the A/C fixed and operational again. The fam-jam would rather go places in the reg cab truck than the Lincoln because of the climate-controlled interior...

[nfldfordltd]

Fixed with a brand new Motorcraft speed sensor.

[gadget73]

Interesting, but doesn't make any sense. The speedo in an 88 Townie is cable driven, even the electronic ones. Actually the electronic ones don't even have a VSS down at the transmission, its cable straight into the cluster where an internal VSS lives, and output from that goes to the computer. The only thing the VSS does is operate the cruise control and it has something to do with the EGR functions.

[sly]

if the VSS was binding and causing the cable to whip, it would explain it.