Drum speedos

[toddao]

Yesterday my drum speedo stopped working by racing up to 200kmph then swinging down to zero.
No problem, I thought I'll just replace it with the new unit that I bought as the old one would only work with the trip counter pressed in and even then would have fits sometimes..

Well, I swapped them over and.. nothing. After lunch will check the cable - are these prone to breaking? Thought perhaps also some problem at the diff from when I changed the clutch but this would surely have manifested before not two weeks later?

Also, I could not, for all the head scratching work out how the drum actually turns! I've got a Mk.1 TRS tacho on the bench and that's totally different - it's obvious how the gearing turns the drum but on these ones there doesn't seem to be any contact between the movement of the cable and the flywheel of the drum??

The cable enters through the back, turns a shaft with a weight on it ( which I would have thought acts like a runner on the flywheel of the drum but doesn't) then follows through to the gearing of the trip counter and kilmetre counter - this is straight forward. Any ideas??

[jonathan_dyane]

Yesterday my drum speedo stopped working by racing up to 200kmph then swinging down to zero.
No problem, I thought I'll just replace it with the new unit that I bought as the old one would only work with the trip counter pressed in and even then would have fits sometimes..

Well, I swapped them over and.. nothing. After lunch will check the cable - are these prone to breaking? Thought perhaps also some problem at the diff from when I changed the clutch but this would surely have manifested before not two weeks later?

Perhaps as the original speedo failed it seized, and caused the cable to snap, or perhaps the square end which enters the instrument to become rounded?

IIRC the speedo cable is in two parts joined by a plastic adapter on the outside of the bulkhead, but I could be wrong...

[toddao]

Thanks Jonathan. Yes it's in two parts : one from the tacho to the other side of the bulk-head then another part that runs down to the diff. I'm going back now to hopfully sort this out. I'm going to apply a drill to the cable in on the old tacho so I can see how it works.
It's so weird that the TRS tacho ( 1985) was made in Japan and the two Veglia ones that I've got (1986) were made in France and they're not interchangeable - even with Mk.1s there's some problems with compatibility

[toddao]

Ok, I found out how the thing works. The flywheel which is turned by the speedo cable is magnetised and as it spins it turns the flywheel beneath the drum which is quite delicately suspended and sprung almost with something like a watch spring. I can't actually believe it works!

The bad news is - my drum doesn't turn. I tested the unit and the upper part of the cable and they're fine. I then drove around without the instrument panel a la Mad Max to see if the complete cable turns and it doesn't. Something awry down the diff end then! I can turn the lower part of the cable by hand as well which, if it was connected would surely not turn?

I'm thinking that this could be related to the clutch change but,as I said above, it's worked for two weeks and I was careful re-assembling it. Oh well.. never boring.

[ken newbold]

I seem to recall from many moons ago that the drum actually turn by magnetic attraction. That's why you can actually accellerate or brake faster than the drum can record.

[jonathan_dyane]

Could it not just be that the lower cable is broken? If you pull on the inner you might find it comes out of the outer if it is broken. If you havent already done it I would remove the cable from the diff end and put the drill on there.

[mnde]

My drum speedo has got an odd quirk: from rest it doesn't register below about 20 miles an hour, then suddenly jumps to life. But when slowing down from speed, it works fine at slow speeds, even 5 mph... It's always done it

Instrument panel differences also exist with the GSA: the Jaeger and Veglia panels have significant differences with non interchangeable parts...

Mark.

[toddao]

Jonathan wrote

If you havent already done it I would remove the cable from the diff end and put the drill on there.

Will do this next. Ran out of time today and the wife ( as delicate as a drum speedo) has ordered me to look after baby now while having fits about the BX floor insulation hanging over the fire to dry ( this still isn't dry from the heater matrix leak six months ago!) making a nice aroma

[Aerodynamica]

My drum speedo has got an odd quirk: from rest it doesn't register below about 20 miles an hour, then suddenly jumps to life. But when slowing down from speed, it works fine at slow speeds, even 5 mph... It's always done it Confused

This is what my CX did - I dismantled it and applied very light oil to the spindle ends and to the magnet's spindle and then a tiny bit of spray grease and it was cured. Similarly the CX drum tacho didn't read accurately below 1500rpm it just stuck there even when it was idling at 750rpm. Same thing: light oil and tiny bit of spray grease and it'll work.

I find that the BX drum speedo is pretty accurate on mine but the LED tachometer always seems to lag slightly. It's a Veglia: I can't help but think that Jaeger items are better made. The difference between the S1 CX Jaeger and slightly later Veglia was quite noticeable.

[Mike Phelan]

Probably a bit late with this, but drum speedos operate in exactly the same way as most of the pointer variety.

You may yawn!

The part that is rotated by the cable is indeed a magnet; the drum (or pointer) is an aluminium drum or disc, and has no actual mechanical connection with the magnet. It has an attached hairspring to allow it to return to zero.

The rotating magnet field produces an electrical current through the ally, which then tries to follow the magnet with N-S attraction. The faster the magnet (and therefore car speed) rotates, the more pull is given to the drum, opposing the spring.

HTH

[toddao]

That's a good explanation Mike! Now I understand a bit better..

It occurs to me that the lower part of the speedo cable is in fact one piece with the whole speedo drive - is this correct? If this has failed will I have to replace the whole thing including the casing? Or is there any other failure that anyone is familiar with - could the plastic gearing that drives the speedo and runs off the r/h drive shaft slip off its bearing?
I'm going to have to take the r/h drive shaft out again etc. and want to prepare myself for what awaits in there!

[Mike Phelan]

Hi Todd
Not completely sure what you mean by "the whole speedo drive" - if you disconnect the cable from the speedo head does the inner cable rotate when the car is driven? If it is, the fault is in the head, if not, it's the cable itself or the bit on the transmission that drives the cable (can't remember what that bit is like as not had a BX for a few years now) ISTR it is a plastic multi-thread worm gear on the RH end of the transmission housing.

[mat_fenwick]

I think you should find a small white plastic housing on the box (assuming yours is a BE1 box - what engine do you have and where is reverse in relation to the other gears?)

To remove the cable from this housing there should be a rubber 'pin' with a cylindrical head - pull this to stretch the 'pin' and remove from the housing. The pin locates in a groove on the speedo cable, and once it is removed the cable should pull out.

To remove the housing itself you undo an 11mm(?) bolt and pull it out. There should be a worm gear visible which drives the cable. It is possible for this to fail, although I haven't heard of this happening. I have previously had one problem with a 2nd hand box missing the drive...

NB All the above assumes you have the BE1 box (and that is similar to the ones I have seen) which I would have thought you have!

[toddao]

Mike, the cable doesn't turn when I drive. I tried this yesterday by driving without the instrument panel

Mat, don't know what the gearbox number is - it's a 1986 n/a diesel.

I didn't realise that one could disconnect the cable from the speedo drive - also access is almost impossible without removing several 'pieces' first. The drive itself is held there by three bolts and the r/h drive shaft goes through it..

[Mike Phelan]

Ugh! It's down below, then, as Mat sez.