Wildly erratic Ford speedo, looking for forum wisdom

[1969Mk3Spitfire]

Not related to my Triumph, this is a Ford issue.  I'm hoping to tap into the wonderful breadth and depth of knowledge contained within the Forum.  Plus, to be honest, I've already failed miserably within any Ford forums.

My 1980s Escort has a mechanical speedo.  Per the picture, within the instrument panel the instrument yoke rotates by cable from gearbox.  The outer ring of the yoke is slightly magnetic and as it rotates it attracts a tang connected to the spindle.  The faster it rotates, there's a bit more torque to rotate the spindle, hence show a higher speed.

The car only does a couple of hundred miles per year.  A couple of years ago, after driving on a motorway for about an hour, the rock-solid speed suddenly became erratic. For example, say I was driving at 70, the speedo would swing wildly and show between 30-50mph.  After stopping at a service station, the speedo returned to normal.  A few weeks ago, I was travelling on a motorway and it did it again.  Stopped at service station and afterwards it performed normally.  

Some people have said that it could be the cable sticking so I've lubricated it.  I don't think that this is the issue.  Yesterday, it began its wild swinging again after only a short journey and now it does it continuously.

Via an internet search, North American Muscle Car Forum, replying to what looked like similar symptoms, someone suggested that it may be friction within the mechanical odometer and trip mechanism causing the yoke to stall.  That seemed a reasonable explanation so today I have very carefully lubricated all visible plastic gears, worms etc.  I think that I've made it worse.  The erratic movement from 20-40 mph now appears "more free".

Anyone experienced anything similar and/or has suggestions of root cause and a fix?

 

I'm hoping that you guys like a good mystery.

Thanks for any help

 

 

 

[johny]

If the odometer/milometer sticks it wont do for long as its a direct drive from the cable via gears! No, think the problem will be from the spinning magnetic field onwards and the possibilites are dirt or wear in the magnetic transfer mechanism so that theres intermittent contact between the two parts. Problem with the bearings or clock spring of the needle part of the mechanism - have a close look at the spring as obviously this is what offers resistance to the magnetic torque and returns the needle to zero plus get some thin oil into the bearings. Finally the needle itself mustnt make contact with anything and should be able to bounce around freely as you shake the body of the speedo...

[1969Mk3Spitfire]

Many thanks, @johny

the spinning yoke makes no contact, it's a proximity magnetic field creating a torque on the spindle, so I can't see it being dirt. I'd be surprised if it's wear as the car has low miles, 25,000 since 1987. The needle/spindle does bounce freely if shaken. I have put thin oil around the shaft/bearing surfaces.

What am I looking at with the spring?

Earlier, the wild swing, say 20-50mph, appeared somewhat "laboured" if you know what I mean.  Since lubricating the mechanism, it now dances around quite freely.  I'm not 100% sure but I don't think that the speedo now reaches true speed. It starts swinging prior to getting there.

[johny]

Oh in smiths/jaeger instruments its a disc which runs quite close to the spining magnet and you can get stuff between the two... Must admit Im basing all my comments on our gauges but yours must have some sort of spring to zero the needle and offer resistance to the magnetic torque. The lubing wasnt just the incoming side was it - the yoke and needle bearings also got done?

Can you make up a square drive that fits in place of the cable and give it a spin on a drill? This would discount possible external problems🤔

[1969Mk3Spitfire]

Thanks again @johny

I’m happy with having lubed everything but not at all convinced that I’m close to finding cause. Manually spinning it is a good call.

On a Ford forum, some suggest a worn gearbox worm gear. I don’t understand how a worn gear would cause the needle swinging. 

[johny]

No, the cable sticking, winding up and unsticking I can see but not the angle drive... Find that spring😁

[Pete Lewis]

its whats commonly called a hair spring  that loads the needle return

fitted to the needle spindle 

Pete

[1969Mk3Spitfire]

Update.

I removed the (2-piece) speedo cable from the gearbox end and put the square into a drill. The speedo was steady, not at all erratic / swinging.

After carefully reassembly of the cable into the gearbox, disappointed to find no improvement, if anything it continues to deteriorate. Now, up to about 50mph (by tachometer), speedo will not exceed 20 and swings from 10 to 20, per short video clip.

I don’t understand how a failing speedo pinion gear would cause this. Any ideas?

 

 

[1969Mk3Spitfire]

Workshop manual says that pinion gear can be changed without having to remove gearbox.

It says “using a pair of side cutting pliers, lever out roll pin……”

I can’t imagine that it will not give up its hold without a fight. Access, as you’d expect, isn’t great.

Ideas for removing the pin, please?

[johny]

Dont know the Ford gearbox but suppose at least one speedo drive gear is plastic so if partially mashed it could give intermittent operation? The speedo cable spins free does it even when in its normal bent routing?

[1969Mk3Spitfire]

28 minutes ago, johny said:

Dont know the Ford gearbox but suppose at least one speedo drive gear is plastic so if partially mashed it could give intermittent operation? The speedo cable spins free does it even when in its normal bent routing?

Yes, speedo cable spins freely.  That said, I may try to find a replacement as, presumably, it's an easier job than removing a roll pin. All very strange. I do appreciate your time, thanks.