Speedometer Needle Jumps

[Mikeb79]

So, I have had my 1979 Mark V since January and have driven it sporadically, but now that the weather is getting nicer I will be driving her much more. One thing I noticed on my drive tonight is the speedometer has suddenly become "noisy" and the needle bounces around a little bit when accelerating. This was the first time I noticed this and I do not think that it had done this previously. I imagine it must be the speedometer cable (needs lube)? My question is, with my limited abilities working on older cars like this, what can I check to see what the issue is? Hopefully the dash doesn't have to come out to troubleshoot. Thanks!

[elcad70]

The bounce is usually caused by the inner cable rubbing against the outer casing. Check for any kinks or tight bends along the length. Failing that you can withdraw the inner cable and ensure no strands have broken and are rubbing, or that dirt is causing the friction. The inner cable can be lightly oiled but grease will effect the accuracy of the gauge.

[Lee]

It’s been a few years, but if you locate the cruise control speed sensor by the firewall, you should be able to unscrew the upper and lower cable attachments and withdraw the cables from there, and not have to pull the dash (but it’s not really that hard). Clean the cables well with gasoline, or brake cleaner, and then lubricate with graphite based lube, before stuffing back in the sheaths. I’ve used this stuff in the past, now owned by Dorman in their HELP! Product line:
https://www.amazon.com/Dorman-03344-Gra ... _ci_mcx_mi

[Dan Szwarc]

One thing I noticed on my drive tonight is the speedometer has suddenly become "noisy" and the needle bounces around a little bit when accelerating

Please provide a little more information regarding this. Driving it more may "fix itself." That, or you didn't notice it last year.

The speedo cables run inside a plastic sheath which has probably worn out in a bend causing a high-friction spot, hence the jumpiness. It probably needs to be replaced. But, try whatever you want. This is a pretty basic debug. The hardest part will be removing the cable from the back of the speedo due to access issues.

You can pull the cable insert out of the sheath and inspect it. It may show you where it is binding. If you can't get the insert back in, then you will have to replace the whole cable.

[Mikeb79]

Although I haven't driven the car much, I am certain that this just started yesterday. The speedo needle, when it jumps, makes a noise in the gauge that is audible. Also, it is especially noticeable in reverse slowly backing up. You can hear the "click, click" and the speedo jumps a bit from zero backing up. When driving, it is more faint as I get up to speed, but you can still see the needle flutter a bit and then sometimes it jumps. I am going to try to take a closer look today. Thanks!

[Dan Szwarc]

Sounds like the cable is binding. It should be silent.

[TonyC]

Remembering the problems I had with speedo cables in my old Electra, I too say the cable is drying up and needs servicing. I don't know the design of Ford speedo cables of the '70s; but if they have the same design as GM cables, then the core is removable once the cable is unscrewed from one of two ends. If you can get the core out, then apply grease (like white-lithium spray) to it and down into the outer cable. Reassemble, and you should be done. On my Electra, it was easy to spot the failed cable, as it was the one that went from the rudimentary cruise-control servo to the speed gauge that had the tendency to fail. I must have gone through about four cores, all of them shearing their plastic heads off at the servo, before I finally cured the issue by treating the inner cable with Dexron before inserting.

Don't just ignore the problem, presuming it will fix itself like the hole in my radiator did yesterday. If a dried-up cable is worked long enough, it will shear itself at its weakest spot, usually the shaped plastic end that acts as a "key" to the mechanism.

---Tony

[Lee]

In my limited experience, it’s that cable that runs from the transmission to the cruise speed sensor that needs lubed.

[Mikeb79]

Thanks everyone, I have located the cruise control speed sensor and I will disconnect the speedo cable from there and lubricate it (graphite no grease). We'll see how it goes. I appreciate all the help!

[TonyC]

Let us know what you find out, either way.

---Tony

[Mikeb79]

Will do, I am planning on looking at it this weekend. Thanks to everyone for all the great advice.

[jtheye]

I have had this problem a couple of times. Even had the shop take out the speedo in the parts car and replace in the original. That didn't fix it. Ended up being the cable not being inserted correctly/loose in the cluster. Was experiencing not only the tick tick tick that increased with speed but also the jumping of the needle until the needle would just get pegged all the way to the right. It is surprising how easy it is to not have the cable not all the way seated where it belongs.