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Tachometer needle bouncing - grinding sound - what to fix inside?

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tylerwerrin Tyler Werrin
Santa Monica, CA, USA   USA
1972 Triumph TR6 "Lil Vroom Vroom"
Hi guys,

My tach never worked properly; while I assumed it might be the cable, I decided to take a look inside the tach today just to rule the cable out; at the very least, I wanted to rule the tachometer being the problem in or out.

I took this video - I'm trying to figure out what exactly is going on. It looks like the magnetic cup is dragging/wobbling which is causing the spindle to not register. If I spin the magnetic cup with just my fingers, i can feel it dragging, what feels like on the magnet which rides inside. If i hold the tach a certain way, or gently spin the magnetic cup with a specific force levied on it (gentle finger push) i can get the needle to register.

However, while idling, all i hear is a grinding sound; it looks like the cable is interfacing properly, but something is amiss within the device. What should I do?

The only in-depth rebuild I found online was at bullfire, but it doesn't specifically detail this problem.

Here's the video:



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TR3driver Randall Y
Confusion, Los Angeles, USA   USA
You've got at least two different bearings worn out; and very likely the tiny shaft in the middle is broken off. Unless you're really into clockwork (and have the ability to machine your own parts), I'd suggest either sending it to a professional; or looking for another tach.



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

tr4head Avatar
tr4head Rob B
Atlanta, GA, USA   USA
I have same problem with my speedo cable. I got a new cable and it does engage and run smoothly, but is about 20-30MPH fast (which makes me feel like I'm driving one of those new fangled sports cars). It would not appear to be a similar problem, maybe gauge calibration or wrong cable issue?? Cable fits fine. Cheers and Merry Christmas!smileys with beer

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TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Only a slim chance it might be a cable problem. See https://drive.google.com/open?id=0B2H2NJt34Offc092OGdtT0s1UDQ

Mismatched components is another possibility : How accurate is your odometer and trip meter? If they are correct (or close to it), then the problem has to be in the speedometer itself.

Is the 20-30 mph constant across the board? Or is it a percentage of current speed? Another way to put it: what does it read when you are actually doing 20 mph; and when actually doing 60 mph?

I don't really understand what causes them to read way high, but I had the same thing happen with my TR3. Best I can tell, the clock spring lost some of it's resistance. It was an interesting exercise, weakening the flying magnet just the right amount, but i'm not sure it's practical for most. Here's a shot of my low tech degaussing tool (using an old Weller soldering gun as a source of high current, low voltage AC):


It actually worked a little too well the first time, so I wound up having to remagnetize the magnet and try again. Here's my low tech (and very ad-hoc) magnetizing tool:


Certainly not the only way, nor even the best way; just what I did.



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

tylerwerrin Tyler Werrin
Santa Monica, CA, USA   USA
1972 Triumph TR6 "Lil Vroom Vroom"
if its bushings, I could see what I could find at the local Ace hardware store - they have a surprisingly large selection of stuff; perhaps even a nylon one would work?

as for the bushings you are talking about... which ones would be in question? theres a larger bronze sleeve that the entire mechanism is fed through - is it a tiny bushing which holds the tachometer needle straight?

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Here's the diagram from Tony Rhodes' speedo repair article. (The tachometer mechanism is the same, it just doesn't have provisions for the trip and odometer). It might be just the input bearing, but from the way the cup is flopping around in your video, I suspect the needle bearing is broken as well. And the bearing where the pointer shaft rides in the front plate may be damaged too.



You can find the complete article at http://www.triumphtech.x10host.com/PDF/speedo.pdf



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild



Edited 2 time(s). Last edit at 2017-12-21 11:23 PM by TR3driver.

tylerwerrin Tyler Werrin
Santa Monica, CA, USA   USA
1972 Triumph TR6 "Lil Vroom Vroom"
Ah, this is useful, thanks! where is the input bearing though? don't see it on the diagram - haven't drilled out the rivets on my unit yet to get fully inside the mechanism...

the needle bearing is probably damaged, especially since the unit has been grinding away like that since I put everything back together...

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TR3driver Randall Y
Confusion, Los Angeles, USA   USA
In reply to # 1503821 by tylerwerrin Ah, this is useful, thanks! where is the input bearing though?
Tony's diagram didn't call it out, but it's under the arrow that says "Main frame". ISTR there is an Oilite insert there, at least on the earlier units (I've only worked on TR3), but I've never tried to take one out so don't know how easy it is to R&R or what the size is.



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

ng19delta Avatar
ng19delta Scott Roberts
Merchantville, NJ, USA   USA
1974 Triumph TR6 "Gypsy Rose T..."
In reply to # 1503746 by TR3driver

Here's a shot of my low tech degaussing tool (using an old Weller soldering gun as a source of high current, low voltage AC):



That Weller soldering gun is as old as mine! But the original bakelite case was so broke, from age & accidents(more that), that I bought a new plastic case from Weller about 20 years ago & replaced the bakelite! They are GREAT tools- ESPECIALLY the heavier ones! I call it my "wrecking bar", and use it for old radio disassembly...

Scott



Edited 1 time(s). Last edit at 2017-12-22 06:16 AM by ng19delta.

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TR3driver Randall Y
Confusion, Los Angeles, USA   USA
In reply to # 1503839 by ng19delta That Weller soldering gun is as old as mine! But the original bakelite case was so broke, from age & accidents(more that),
Me too! Doesn't show well in the photo, but the case on mine may be more JB Weld than Bakelite! The entire back end and much of the top has been built up; plus the handle reattached to the body.



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

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