The 1976 T140V’s speedometer was fluctuation about 30 mph and was reading 40 mph high. I checked the condition of the speedometer cable system and it was good. I re-lubed that part of the speedometer system. I found when the speedometer input was rotated slowly by hand it would the needle would immediately rotate to 30 mph and fluctuate. Armed with a couple of helpful articles on Smith’s Speedometers I opened up the speedometer head. I found the input magnet had a loose retaining plate rivet and allowing the rotating magnet disc to rub the aluminum needle disc. That seems to be the source of the fluctuation and the instrument reading high. I inspected cleaned and lube all rotating parts with a very light tool oil. I reassemble the input drive and replaced the four retaining rivets for the input drive retaining plate and odometer retaining plate with screws for future disassemble. The bike has stock tire size and final sprocket sizes. I located a formula to calculate rpm gauge reading to MPH in 5[SUP]th[/SUP] gear to calibrate the speedometer. Tomorrow afternoon I will try to calibrate the speedometer to indicate the proper mph. Just for good measure I will further test it against the family car.
Attempting to Repair an Erractic T140V Smith's Speedometer
- Thread starter ZILCH
- Start date
Help Support Triumph Motorcycle Forum:
Zilch, an even better way to calibrate your speedo is if you know the exact distance between two points,the further apart the better.
We used to have mile markers at various locations years ago and they were great for this purpose.
Failing that,a gps used on a straight level road should be right on the money.
We used to have mile markers at various locations years ago and they were great for this purpose.
Failing that,a gps used on a straight level road should be right on the money.
I ended up using the gps and it worked out just fine. The instrument itself showed the correct speed readout between between 30 and 70 after a few adjustments on the calibration screw and one needle reposition. It was almost to the end of the adjustment screw range though. I think the input drive magnet washers and/or seat may have some wear on them. I think long term I will plan to swap out the speedo with a cheap repop when the unit starts reading erratic again. Then send it to a shop that replaces the internal parts. At least I learned about the internal workings of a mechanical speedometer. I like learning new things.
Good for you +2 
You're braver than I am to go in there
You're braver than I am to go in there
I have never attempted to into the instruments. Nice job.
The hairiest thing about meters is properly re-sealing the glass bezel without distorting the chrome trim ring.
That is exactly what I was thinking.....how the heck did you get that thing apart???
I've looked at attempting to repair them before, but found no way to get inside without ruining it.
Getting the old trim ring off isn't that hard, getting a new one on and sealed is the tough part!
Exactly!
After reading GP's comment I remembered that I changed a glass in one about 40 years ago and the sealing and making it look good was the hard bit.
There a special tool that will work the ring into place. To get the ring off I used a screw driver and worked around the back of the ring until it was straightened enough to slid over the glass and lip of the cup. I followed the directions of some documents out there I found. It turned out ugly but it worked and the face side of the ring still looked good. To reinstall the same ring I slipped it back over the glass and cup lip and used an socket extension end to bend the ring back down in several spots. This hold the ring in place, to cover the ugly I used black electrical tape to cover the back side. Since the unit is being installed in the rubber cup on the bike all the ugly was hidden. For thee purpose of getting the bike instrument woking to be able to ride for the season it served its purpose. If I was restoring the instrument or repairing it professionally or long term, I would get replace the ring and get the special install tool for the ring.
I've tried bending the chrome ring enough to get it off like you say, but the glass cracked in the process.
Yes, glass cracks and/or chips, and the chrome flakes off the bezel.
If you are working with the older "bare" style, 'ugly' won't be able to be hidden by the rubber cup!
If you are working with the older "bare" style, 'ugly' won't be able to be hidden by the rubber cup!
