The 'E' Type Forum

Has anyone got any advice please on a speedometer that moves around plus or minus 5-10mph when driving at a constant speed?

Either the cable is binding, or the instrument needs attention. There is an excellent article available by a guy named Anthony Rhodes, you will be able to find it with google, here is one place where you can download it.

The quick diagnostic is to remove the speedo, take a bit of solid copper wire with the end bent over so it fits nicely into the square drive on the instrument, and chuck that into your cordless drill.

If the needle wavers at the same speeds while you're driving it with the cordless, then it is your instrument. If not, then it is your drive cable.

Sometimes it is both.

Hi Bernie.....be very carefull connecting to your cordless drill......i think from memory that you have to run the drill in reverse to operate the speedo.....so if you run in the wrong direction you could/will damage the speedo.....someone should come on soon to confirm.....i may be wrong but best get confirmation befor you start
Edit...
Haveing just looked at the download posted above the diagram shows the rotation as anticlockwise when viewed from rear of speedo

mgcjag wrote:Hi Bernie.....be very carefull connecting to your cordless drill......i think from memory that you have to run the drill in reverse to operate the speedo.....so if you run in the wrong direction you could/will damage the speedo.....someone should come on soon to confirm.....i may be wrong but best get confirmation befor you start
Edit...
Haveing just looked at the download posted above tbe diagram shiws the rotation as anticlockwise when viewed from rear of speedo

Steve,

The cable turns the other way when you engage reverse. I start very slow with the drill and change direction of the needle is going the wrong way. (I'm crap at remembering stuff like which way the cable turns when you're going forward.)

Thanks that's great advice

It might be the Speedo itself but 9 times out of ten it is the cable. A small around of oil dribbled into the top end often effects a cure. Better still remove the cable from the car, withdraw the inner cable that rotates, clean using e.g. A cloth and WD40. Rinse the inside of the outer with WD40 or isopropanol. Apply a small amount of 20/50 oil or similar over the whole of the inner. Pop the inner back into the outer and refit ensuring no sharp bends or kinks. If it still wobbles then it is the Speedo itself.

Will do thanks

I'm sorry, I must respectfully disagree with part of Julian's advice. It is a recipe for a.getting oil all over your trousers and b. wrecking the speedo head. His advice to lubricate is spot on. But what you do it with is critical : grease is the proper lubricant for the cable, and it should be applied to the bottom two-thirds of the length only. The grease will then work its way gradually up the cable but will not escape the top connection and will not penetrate the speedo head

Bernie2015 wrote:Has anyone got any advice please on a speedometer that moves around plus or minus 5-10mph when driving at a constant speed?

My car had this issue. It was mostly resolved by (a) replacing the speedo cable and (b) re-routing it to avoid kinks i.e. making sure the cable bent on one clear and continuous arc down to the gearbox. I said mostly, because there was a residual wobble. This was caused by the trip meter, which clicks the tenth of a mile counter six times per tenth of a mile. Watch the speedo needle and the tenths of a mile and if they click and wobble in time with each other then you have your cause. Lubricant is the solution.

I would not use grease...it hardens and gums things up. I would stress a small amount of oil. If you remove and clean the cable then you just need a film on the inner. A few drops from an oil can is sufficient. Wipe along the length of the inner with fingers not a cloth or the turned cable will collect fluff. If you are trying the simple solution again just a drop or two at the top. Wait till it disappears then a drop or two more. No more. Let it drain down the cable before driving. You don't need vast amounts or, yes, it will get into the Speedo and gum it up. I would also say that the head construction does prevent ingress of oil to a point anyway. Remember that gravity will mean oil naturally flows down hill away from the head. Guess me and Chris differ in opinion.

I am about to publish an article by an ex-Smiths quality supervisor and factory rebuild dept worker. Commonest causes of failure:

1) screwing the cable to the head too hard (finger-tight only). if inner protrudes even a millimetre or two deeper into the head, due to using locking pliers etc., the end thrust will damage the head unit PDQ

2) overlubing the top foot or so of cable, especially with grease that clogs the head as others have stated.