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Wavering speedo needle

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57_BN4

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I have here a 57 and a 62 Healey that both have the same wavering speedo needle. Always have had so the stories go.

The needle will stay on speed say 60mph then dip to 50, rise to 70 and swing back to 60 for a second and repeat. It is perfectly in time with the odometer advancing, as if the increased load of the odometer(s) operating is enough to temporarily slow the spinning magnet, then as the cable unwinds it flicks the needle higher momentarily.

The speedo on the (my) 57 has been lightly lubricated inside but not disassembled, the 62 has never been apart. Putting a battery drill on the stub end of the angle drive gives the same result but the wavering is much smaller, only about 1-2mph change. Speedo cables are both visually as-new but of unknown age.

Anyone else have this situation? Is there a cure?

Andy.
 
57_BN4 said:
I have here a 57 and a 62 Healey that both have the same wavering speedo needle. Always have had so the stories go.

The needle will stay on speed say 60mph then dip to 50, rise to 70 and swing back to 60 for a second and repeat. It is perfectly in time with the odometer advancing, as if the increased load of the odometer(s) operating is enough to temporarily slow the spinning magnet, then as the cable unwinds it flicks the needle higher momentarily.

The speedo on the (my) 57 has been lightly lubricated inside but not disassembled, the 62 has never been apart. Putting a battery drill on the stub end of the angle drive gives the same result but the wavering is much smaller, only about 1-2mph change. Speedo cables are both visually as-new but of unknown age.

Anyone else have this situation? Is there a cure?

Andy.
Click to expand...

Yup. There is a (relatively) large worm gear driven directly by the cable that drives two smaller gears; one drives the odometer and one drives the tripmeter. They're about the diameter of an (American) dime. It is not uncommon for one of these gears to develop a radial crack from the center to the perimiter (usually, the crack ends in the trough between two teeth). When these gears rotate near the crack I suspect the crack opens a little, causing 'slack' in the mechanism then, as the gears rotate past the crack the slack is suddenly removed, causing a varying load on the cable--as you suspected--and the needle to 'pop' as you describe. I tried repairing one of these gears with glue with no success; however, I assisted another chap who claimed success gluing his.

The early driven gears were made out of a pressed fiber material, the later ones out of plastic; I suspect but can't prove the fiber gears were more robust. The mechanism that advances the tripmeter and odometer is a ratchet-and-pawl driven by an offset cam driven by the (suspect) gears; generally, they work smoothly and don't put much of a load on the cable.

Unfortunately, there is no source that I know of for these gears. I bought a used speedo for parts.

Info here: https://home.comcast.net/~rhodes/speedo.html
 
Could be the bearing inside the speedo needs cleaning and lubrication. Remove the speedo from the dash. Remove the trip odometer knob by undoing the small screw in the side of the knob. (Might want to hit it with some pb blaster first) Carefully lever the tabs on the reverse of the chrome trim ring out just enough to allow the ring to move and rotate the ring to the slots in the case and remove the ring, rubber gasket and glass. Take a dinner fork and placing some cloth to protect the speedo face, insert the tines of the fork under the speedo needle, lever it off (it may take some force). Undo the two small screws and remove the speedo face. Undo the two screws on the back of the case and remove the speedo mechanism. Remove the screws holding the trip and odometers in place and remove. You have now reached the bearing. Clean the bearing with some aerosol brake cleaner and let dry. Lubricate the bearing with ONE DROP of light machine oil. Reassemble the unit.

OR

Just learn to live with the **** thing.
 
I recommend silicone lubricant for speedo internals and cable. Won't collect dust and gunk up like petroleum oils. I lubricated a starter bendix with silicone lube 100K miles and (at least) 20 years ago and the starter has worked flawlessly.
 
The two small gears mentioned should also be pulled out and the shafts and holes they go through cleaned as over the years they can start to bind and eventually even seize which causes the worm drive to chew them up. Easy to do if you have the speedos opened up.
 
Thanks for the replies. I pulled my speedo apart yesterday and found the brass worm gear is almost worn right through after only 162000 miles. I could feel a strong binding when the pawls engaged and perhaps some of the problem is that they are synchronised so both pull together. Upon reassembly I'll put them 180deg apart. I suspect the problem is due to the pressure angle of the worn gear being wrong so that instead of trying to drive the small gear around, the force is directed more towards the axle line and tries to force the gears apart instead.

Everything else looks in pretty good condition in there so I'm hunting down another worm gear/magnet wheel. Fortunately we have a vast supply of old Brit cars with similar speedos here.

speedo gear worn.jpg speedo gear worn2.jpg

Next thing the calibration is out because of the 3.54 diff so I'll try and replace the odometer ratchet wheels with some closer to the new spec. The calibration for 4.11 diff is 1325 turns per mile and the cable now measures at 1100/mile so I'm on the hunt for a similarly marked unit.

Andy.
 
Wow. I wouldn't have imagined the worm gear could be worn down like that. Maybe THAT'S why they changed to plastic gears.

Note the (apparent) radial stress cracks evident on the cam gear--it's possible one of them will crack through eventually.
 
I had a similar problem, I tried re routing the cable drive and it sorted it out at certain speeds but it would still waver at other speeds. A new speedo cable sorted it out and the needle is steady now through all speeds.

:cheers:

Bob
 
Hi Bob,

did you replace just the inner drive cable or the outer conduit as well?

Today I dismantled, cleaned and lubed the tricarb speedo and it was like new inside having only done half the mileage mine has. It still wavered the same upon reassembly while running a drill on the gearbox end of the cable. Not bad but annoying enough to fix. I'm thinking the cable is part of the issue too.

Andy.
 
Just a quick update- fitted a new speedo inner cable and the wavering is completely gone. I hadn't really considered that a speedo cable wears out, more a case of if it isn't broken then it is ok.

They wear internally within the twisted strands so that eventually you can turn one end nearly two turns without the other end having to move. The new one would only turn maybe an eighth of a turn before becoming solid.

Now just the odometer to recalibrate and it'll be good as new.

Andy.
 
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