Speedometer bounce

[38plymouth]

1938 plymouth p6, coming home from a car show today and the speedometer started to jump from 40 to 50. It kept bouncing up and down about 10-15 mph over what I think was the correct speed limit. Any thoughts on where I should look first? The car is mostly stock, transmission is stock.

[Jakub]

Probably You need to clean and lubricate speedometer cable. Remove it from car, disassemble, clean with kerosene, lubricate generously  with motor oil, assemble, mount back in the car, avoiding sharp bends. I'm sure that it would help.

[Dan Hiebert]

Yep, remove, clean, lubricate, reinstall.  But, don't use motor oil, you'll just have to do it again in a year or so.  Use graphite cable lube, the stuff is made to lubricate, primarily, speedometer cables.  It can be quite a messy endeavor, but worth the effort. 

[38plymouth]

That was quick, thank you.

[greg g]

Careful! Remove the worm at the speedo end by unscrewing it from the back of the instrument cluster and pulling the worm from the casing.  It will be dirty with old lubricant so you will want to protect carpet and upholstery with some sort of drop cloth or similar. As mentioned clean it with a general solvent, brake clean, wd 40, acetone, etc.  Then apply new lube in short sections as you reinsert it.  While you are under the dash look for an oil cup on the back of the speedometer, and add a couple drops of light machine oil.  Used to use 3 in 1, don't know if the still make that.  You might use Marvell mystery oil or a bit of atf.

 

This is an easy job but can be messy.

[Philthy Phil]

I just thought this was an old car thing.

I've owned numerous classic scooters (Mainly Lambretta) and even with new cables and worm wheels etc they still bounced all over the place.

[normanpitkin]

I did exactly this for my 41 ,it was quite easy ,i actually bought a NOS cable ,be careful the length is correct ,the old cable was so gunked up ! I also connected it to the speedo end and ran it using an electric drill  from the unattached end just to check it worked before complete reassembly. .A tiny amount of liquid graphite on the ends just to make sure!

 

[38plymouth]

So I only need to unscrew the cable at the speedo and not at the transmission? I was thinking that I needed to pull the whole thing out of the car.

[Dan Hiebert]

I would say that depends on just how dirty the cable is.  If it hasn't been serviced in a long time, or if it was serviced with the wrong lubricant, even recently, it may pay in the long run to remove the whole thing.  You will be able to tell once you get the worm out of the casing, (unscrewed from the speedo), if it was lubricated with anything other than lube made for those types of cables (IMHO graphite is best), or if it has dirt/grime/gunk, etc., on it, it may be best to go ahead and remove the casing for a good cleaning as well.  A bit more work, but always worth the effort to do a thorough job.

Edited November 3, 2019 by Dan Hiebert

[kencombs]

Lots of ways to do this.  I like simple.   So I leave the housing in the car but unhooked at both ends.  Pull the drive cable out, use rags to handle and prevent dropping stuff in the car.  Wash cable in solvent and dry well.  cover the bottom end of the open housing with rags, lots of rags.  Use your favorite spray can solvent with an extension tube,  I like brake cleaners for this, to flush the housing. Be sure to keep the tube inserted in the housing and wrap a rag around the speedo end to catch any bounce back.   Use at least a full can to get the most out of there.  That's what that lots of rags is for.  Now lube the cable with stuff made for the purpose as you reinstall.

[38plymouth]

Thank you all, I'm stopping at the auto parts store tomorrow for brake cleaner and cable lube.

[ozzmonaut]

Mine is pretty rough. The cable seems to be turning fine. The speedo was at at 20-40mph when sitting still, and went up from there. Lots of bouncing as well. I pulled it all apart and cleaned it, getting it to sit at 0. But it still shoots up to 60+ mph when going 15-20mph. It seems to me that the speed cup might be getting contacted by the piece spinning inside it. I am a little confused as to what exactly causes a correct speedometer to operate properly. There is no adjustments apparent. Speed cups were a regularly sold item, so maybe they wear quickly? I have 3 speedos to work with but they are all reading incorrectly. What can I do to get one consistent operational speedometer?

[Ernie Baily]

I had the same problem with my 48 Plymouth. I did the same thing, removed the cable cleaned soaked the cable housing and then soaked the cable in WD-40. I cleaned the gear end and reinstalled in the trans and then used some WD-40 in the back of the speedo. I worked for a while, however the clicking noise came back and the needle started jumping around. I decided to remove the speedo and send it out for Repair/restoration. I sent it to Williamsons Instruments. He contacted me and said the inside was all gummed up and the old lube was hard. So after 60+ years it needed servicing. I just received it back and have not installed it yet. It was a little more costly than I thought it would be. However I trust them because he has rebuilt some other gauges for my brother Studebakers, and they work great.

 

Good Luck,

Ernie Baily