Fluctuating Speedometer

[Ian Vincent]

I have been beset by a flucuating speedo for a while now.  It's on a 1959 Tr3a which is RHD and has a Triumph 2000 O/D gearbox which has been converted to TR standard by ORS.

I have tried lubricating the drive cable (which as far as I know is the correct cable for the model and correctly routed) with graphite lubricant to no avail.

Earlier today, I removed the speedo and attached it to my battery drill running anti-clockwise and provided I keep the drill and speedo absolutely fixed in relation to each other the speedo will run quite happily at a constant 70mph.  It is however very sensitive to any slight change in angle between the drill and the speedo and will start to wobble a bit - but nothing like as much as when it is running in the car.

From which I conclude that it is not the speedo at fault but the cable/drive somewhere.

Does anyone have any idea on how I can address the problem?  Is it just a case of use a better lubricant or might there be an alignment problem that is causing the issue.

Note, when I say fluctuate, at what I reckon is about 60mph the needle is moving between just over 50 ish and just under 70 ish.

Rgds Ian

[james christie]

Ian, pluck up your courage and take the speedo out of its casing to have a look at the two little nylon drives that drive the mileometer and trip meters. These are driven by the worm gear on the shaft to the speedometer.

l’ll bet you a beer that one of them has a hairline crack that causes irregular interference - have a look at the thread on this subject.

 

james

Edited November 13, 2022 by james christie

[ijonsson]

In my case it was the bearing in the rotating disk that was worn. This caused the rotating disk from time to time touch the disk connected to the pointer, and make the pointer "jump"

[Lebro]

4 hours ago, james christie said:

Ian, pluck up your courage and take the speedo out of its casing to have a look at the two little nylon drives that drive the mileometer and trip meters. These are driven by the worm gear on the shaft to the speedometer.

l’ll bet you a beer that one of them has a hairline crack that causes irregular interference - have a look at the thread on this subject.

james

Sorry, but I beg to differ.  If the speedo in wobble free if the drill driving it is carefully set in line with the input (see above description from Ian) then the gears cannot be at fault. It is much more likely that the bush which the input shaft runs in is worn, allowing the shaft (& the magnet) to wobble around & catch on the aluminium disk, giving it a kick towards ta faster reading. 

Another problem I have found on 2 units now is the tiny bush which lives inside the input shaft - this is the bearing for the pin like rear end of the disk centre shaft.

(the bearing at the forward end is clearly visible at the hair spring end, & is just a hole in the brass plate.)

Bob

[james christie]

It was the fact that the drill has to be held straight that led me to my diagnosis. I await Ian opening it up to learn if I have lost my beer….

james

[Ian Vincent]

Thanks chaps.  I shall be opening it up later in the week and will keep you informed.

Rgds Ian

[ianc]

If James is wrong, Ian will be getting French beer from across the Channel.

The speedo in 4VC has acted this way ever since I bought the car in 1969, but doesn't worry me as it was moved to the left of the dash by the Comps Dept  in 1962.  The odometer works OK.  Only problem is that when grandson is sitting in the passenger seat, he keeps telling me when the pointer flicks past 60 on country roads.

Ian Cornish

[Charlie D]

11 hours ago, Lebro said:

...allowing the shaft (& the magnet) to wobble around & catch on the aluminium disk...

I had exactly that problem on my 3A (I actually have Jaguar instruments, but the principle is the same.)

In my case there was a screaming sound every so often and the needle would shoot upwards. I reckoned that the magnet was touching the disc due to bearing wear. All I did was put a drop of 3 in 1 on the bearing and it cured it. I know that 3 in 1 may be a bit too thick for such a delicate instrument but I think it filled the gap between the spindle and the bearing and stopped it moving. That was 2000 miles ago and so far all is OK.

 

Charlie

[james christie]

It won't be French beer - too much Brexit duty and hassle for the recipient

james

[Ian Vincent]

So,  I have now opened up the speedo and can find no evidence of a crack in either of the two gears that engage with the spindle but turning the spindle slowly by hand I can feel a point in the rotation when the resistance to turning increases slightly and I suspect that is the cause of the fluctuation.  Both gears have 20 teeth on them and they appear to be held onto the spindle by a small screw.  Would that be correct - see attached photos of the two pinions.  Apologies for the slight fuzziness, I will try and take some better photos this evening but I am waiting for the battery in my more sophisticated camera to charge.

The problem is that I cannot see what would be causing the slight resistance.

And one final question, is it OK to lubricate the speedo drive bush with graphite oil or is that a 'no,no'?

Rgds Ian

PS I have now updated the photos with some better ones.  The scratch marks are ones that I made to show which teeth are just about to engage with the driven screw gear when I feel the resistance on the input spindle

 

 

Edited November 16, 2022 by Ian Vincent
Updating photos

[Lebro]

I would remove the gears on their shafts, clean everything up, & re-lube with a little thin oil e.g. 3 in 1.

The input shaft, if you can get it out (some are riveted) again clean, & re-lube. Check for slop in the bush.

The slight resistance will probably be when the small claws push the ratchet wheel round for the odometer / trip meter.

Bob

[Ian Vincent]

37 minutes ago, Lebro said:

I would remove the gears on their shafts, clean everything up, & re-lube with a little thin oil e.g. 3 in 1.

The input shaft, if you can get it out (some are riveted) again clean, & re-lube. Check for slop in the bush.

The slight resistance will probably be when the small claws push the ratchet wheel round for the odometer / trip meter.

Bob

Hi Bob,

I am reluctant to dismantle too much of the instrument 'on spec' in case I damage something so I haven't removed either the pointer or the front face.  I assume the needle is just a push fit on the spindle?

When you suggest removing the gears on their shafts, I assume you are saying remove the small spring clip at the other end to the gear?

And to remove the input shaft, I assume I need to remove the front face of the instrument and then undo the two small screws that hold the main mechanism from the input area?

Rgds Ian

[Lebro]

Yes, that is correct. Only thing you are likely to damage is the hair spring, so keep that in mind. I use two flat blade screwdrivers, with a pad on the dial face to protect it, to lever off the pointer. When putting it back on it should point towards the white dot just below the end stop, then carefully lift it over the stop.

Good luck. Sending PM now

Bob

[Charlie D]

2 hours ago, Ian Vincent said:

I haven't removed either the pointer

Ian.

If you remove the pointer then the spindle that it pushes on to will spring back a bit.

If you do remove it then you need to make sure the pointer goes back in the same place it came off which will be before the "End stop" peg that stickes out of the face.

Just push the "end stop" till it is flush with the face so that the pointer can move past it to zero.

Charlie.

Just realised that Bob said something similar...

Edited November 16, 2022 by Charlie D

[Lebro]

Not all of the end stop pins can be pushed back, great if it is that sort, otherwise you need to lift the pointer over the top of it.

Bob

[james christie]

Ahem, looks like I owe Ian his beer! PM to you, Ian
Interesting to see how the little nylon pinions of the tripmeter and mileometer seem to be fixed to their shafts. Mine are just a push fit, I think.

They do look like they could do with a clean though!

james

 

[Charlie D]

11 hours ago, Lebro said:

Not all of the end stop pins can be pushed back

Sorry for any confusion caused. I was looking at later Smiths instruments.

 

[Ian Vincent]

Just to provide an update on this topic.

I partially stripped the speedo by removing the needle (it was very tight) with two flat bladed levers as suggested by Bob after liffting it over the small pin and noting where it came to rest.  This was slightly to the right of the small white dot referred to by Bob so that is where I put it back.  I then removed the face and separated the odometer from the worm gear mechanism, I didn't dismantle the odometer as that was all working properly.  The trickiest part as far as I was concerned, was removing the small springs that engage with the rachet mechanisms for the odometer.

I then removed all the gears including the worm gear and gave everything a thorough cleaning before reassembling it with a spot of 3 in 1 oil.  Reassembly was pretty much the reverse of disassembly and not too difficult.  I am not convinced that there was anything wrong with the speedo though as the gears (made of tufnol?) looked in perfect condition and there didn't seem to be any play in any of the bearings.  Once it was all back together I tested it again with the electric drill and everything seemed to work.

I have now thoroughly greased the cable in accordance with the "Smiths -Care of Instruments" manual mentioned in one of the posts cross referenced above and I can only hope that when I get a chance for a test drive it will work.

Oh, and I did need a bit of help from Bob with the front glass/rubber seals as my instrument has obviously been apart sometime in the past and instead of a seal either side of the glass there was just the one which meant that the glass rattled slightly.  It doesn't now.

Rgds Ian

[BlueTR3A-5EKT]

Can you not static test it with the tacho cable and the engine running?

[Ian Vincent]

2 hours ago, BlueTR3A-5EKT said:

Can you not static test it with the tacho cable and the engine running?

Pete, I’m not sure that would be any better than putting my battery drill on the back of it. I’ll wait till the rain stops and take it for a run.

Rgds Ian

[Ian Vincent]

So the weather finally relented and I was able to take a spin around the block this afternoon and check out my speedo - and it seems to be working ok!

At a steady 2400 rpm in top o/d the speedo was showing a pretty steady 60 mph which is about right for my car.  I have a 3.45 axle and a saloon 22% o/d with 165/80 tyres. 

I'm happy with that so whether it was the careful cleaning and lube of the instrument or greasing of the drive cable with the recommended LM grease, something seems to have worked whereas my lubriucating of the cable previously with graphite oil didn't.  Now to see how long it lasts.

Thks to all contributors.

Rgds Ian

[james christie]

Good result Ian, well done - and you’ve still got an operating tripmeter!

I’m interested in your speed calculation - for me, at 2400rpm and a 22% overdrive you should  be at 58,6 mph with a 3,7 diff.

Please tell me I’m wrong!

james

[Lebro]

Best way to check is with a GPS speedo (either from a satnav, or just a smart phone using a free app.

Bob

[BlueTR3A-5EKT]

1 hour ago, james christie said:

Good result Ian, well done - and you’ve still got an operating tripmeter!

I’m interested in your speed calculation - for me, at 2400rpm and a 22% overdrive you should  be at 58,6 mph with a 3,7 diff.

Please tell me I’m wrong!

james

…and the tyre size? 
 

this splendid calculator from Minty Lamb is very helpful

http://mintylamb.co.uk/gearspeed/

[Ian Vincent]

I used my own calculator based upon what one revolution of my rear wheels equals. I’ll compare it with the minty lamb version tomorrow.  As and when the weather clears again I’ll get the satnav out  

Rgds Ian

Edited November 23, 2022 by Ian Vincent