martin62 Posted April 7, 2019 Report Share Posted April 7, 2019 Hi folks, I have an 1971 MkII Vitesse with a mechanical rev counter which seems to read roughly double the expected value. It returns to zero when the engine isn't running but reads just under 2000 on tick-over and right across the range is around double the value for the gear/speed. I am assuming the issue must be in the gauge itself (rather than the distributor/cable) as it seems very consistent, but has anyone any suggestions to check before I try and get a replacement? Link to comment Share on other sites More sharing options...
JohnD Posted April 7, 2019 Report Share Posted April 7, 2019 Has this happened out of the blue, or has something been done on the dizzie, or the engine, or drive cable or guage? Link to comment Share on other sites More sharing options...
martin62 Posted April 7, 2019 Author Report Share Posted April 7, 2019 Unfortunately it has been like this since I got the car, so I don,t have history of when it started to act up. Although the issues I have had since I got the car last year (everything from bushes missing in the remote gear selector, through a gearbox with no oil, and perished seals in the brake master cylinder so that the fluid that had been added to sell the car leaked out on the first trip), mean I would not discount anything 'stupid'. Link to comment Share on other sites More sharing options...
johny Posted April 7, 2019 Report Share Posted April 7, 2019 you could check the ratio number on the dial of the tacho, it might have 3.55 - 1, dont know if there are different ones but good to confirm what it is anyway.... Link to comment Share on other sites More sharing options...
martin62 Posted April 7, 2019 Author Report Share Posted April 7, 2019 That, at least, is correct on the face.....nothing to say the 'inner-eds' haven't been changed though! Link to comment Share on other sites More sharing options...
Clive Posted April 8, 2019 Report Share Posted April 8, 2019 I am wracking my poor little brain with this. The Dizzy end is simple and you can't change things. The cable is just that. So it must be the tacho itself. Having had a couple apart, one went badly, the other seems to have worked, there is not a lot that could happen as the cable drives a rotating magnet, and it drags the needle around. If there had been oil added or similar, or something got bent, it could alter the amount of drag, making it over-read. The other possibility is the needle has been moved on its "pin" so it still returns (maybe not as enthusiastically as it should) but that would give a constant overread of say 500 rpm. Best bet has to be trying another tacho. Or have a look inside yours? once out just undo the chrome ring, and 2 screws on the back. And the tachos are all the same ratio. So one off an early spit should match. Link to comment Share on other sites More sharing options...
Paul H Posted April 8, 2019 Report Share Posted April 8, 2019 Pretty sure I have a spare tacho from a Mk2 Vitesse should you need one Paul Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 8, 2019 Report Share Posted April 8, 2019 I would remove the innards and give the drive disc a good squirt of brake cleaner, these will read silly if oil grease or contact is made between the drive magnet and the needle drive disc. Any goo debris or contact will fly the needle up to silly reading , the insides are the same design as a speedo but no oddometer drive Needle support bearing are prone to fail then the discs make contact ., Is the needle return hair spring intact, never remove the needle without making the disc as the spring will unwind never be right ever again Link to comment Share on other sites More sharing options...
johny Posted April 8, 2019 Report Share Posted April 8, 2019 1 hour ago, Paul H said: Pretty sure I have a spare tacho from a Mk2 Vitesse should you need one Paul Paul can you confirm that the 3.55 - 1 that Martin has on his tacho is correct? Thanks Link to comment Share on other sites More sharing options...
johny Posted April 8, 2019 Report Share Posted April 8, 2019 oh alright no need as I see Clive says theyre all the same ratio anyway........ Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 8, 2019 Report Share Posted April 8, 2019 Certain 3,55 is the common ratio on all triumph nechanical tacho. Pete Link to comment Share on other sites More sharing options...
JohnD Posted April 8, 2019 Report Share Posted April 8, 2019 An electric tacho would have a simple expanation - its for a four, not a six cylinder, so it's getting (nearly) twice as many impulses. But a mechanical one? Pictures please, of the tacho and connections. Does anyone else share my vague memory of tacho conversions, from mech to elect. that preserved the cable for autheticity? That could mean your tacho IS electrical, which could use the above explanation. How many wires to the bak, and where from - one from the coil would be a giveaway. JOhn Link to comment Share on other sites More sharing options...
martin62 Posted April 8, 2019 Author Report Share Posted April 8, 2019 Sorry for the slow response folks, I have been working in Bristol today so had a near 400mile round trip commute. I will pull the rev counter in the next day or so and take a look. At the moment the cable looks fairly new, and is tightly screwed into the distributor but I haven't checked the dash end at all. Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 9, 2019 Report Share Posted April 9, 2019 at the head its the same sort of knurled nut if you have to lubricate the inner cable use graphite dust or a thick grease , the helical winding can transpose oil/grease up into the head there is a special grease for this i have some, it smells awful and once under your finger nails you have it for a week ...not recomended im sure you will find the discs are making light contact or oiled up , if the needle shaft brearing is shot its scrap. {Pete Link to comment Share on other sites More sharing options...
martin62 Posted April 10, 2019 Author Report Share Posted April 10, 2019 Well there is definitely a cable running from the distributor to the rev counter and it appears to be turning freely. The issue I now have is the rev counter appears to have been stuck into the dash using mastic (same with the speedo). I have removed the two stirrup brackets but the rev counter will not pull free. I guess the whole dash is coming out, it was on my list to refinish, just not yet! Link to comment Share on other sites More sharing options...
johny Posted April 10, 2019 Report Share Posted April 10, 2019 can you not get a blade in there to try to cut it free? As you say the dash needs a refurb so no worries on that front..... Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 10, 2019 Report Share Posted April 10, 2019 not goo its degraded rubber 0 rings they stick as good as gorilla glue needs some knifing, pulling and cursing followed by strong tea and out she comes Pete Link to comment Share on other sites More sharing options...
martin62 Posted April 10, 2019 Author Report Share Posted April 10, 2019 15 minutes ago, Pete Lewis said: not goo its degraded rubber 0 rings they stick as good as gorilla glue needs some knifing, pulling and cursing followed by strong tea and out she comes Pete I'm through the pulling and cursing and in the strong tea phase. Now have to fight to get it apart! Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 10, 2019 Report Share Posted April 10, 2019 Yes the rim and glass can be goo,d with the same nasty rubber, dont get it on your fingers Some degraded rubbers have nasty side effects on your skin. May need tompry the lugs to open the rim , it should rotate to align with the case cut outs, live in hope ,,,, maybe a dash of nectar in the tea Port and brandy work well as a corpse reviver The two screws is the easy bit Pete Link to comment Share on other sites More sharing options...
Badwolf Posted April 10, 2019 Report Share Posted April 10, 2019 I was experimenting with 'chroming' the bezel on an old rev counter and, as Pete says, pried the lugs but only a fraction to release the pressure. Then a twist and the bezel, glass and base parted company.. I was amazed. I never did try the chrome paint on the bezel. Wonder where I put it. I do remember stripping off the black paint to find lovely yellow brass underneath. Off to the garage to find the bits!! Link to comment Share on other sites More sharing options...
JohnD Posted April 10, 2019 Report Share Posted April 10, 2019 If that's the back of your tacho, no wires - my theory falls! J. Link to comment Share on other sites More sharing options...
martin62 Posted April 11, 2019 Author Report Share Posted April 11, 2019 14 hours ago, JohnD said: If that's the back of your tacho, no wires - my theory falls! J. There was the bulb holder and also a couple of (earth) wires on the stirrup bolts, nothing on the actual tacho though Link to comment Share on other sites More sharing options...
martin62 Posted April 11, 2019 Author Report Share Posted April 11, 2019 The rim and glass is off and the mechanism is out. It is surprising 'pristine' though so I am not sure what I should be trying to clean? Link to comment Share on other sites More sharing options...
johny Posted April 11, 2019 Report Share Posted April 11, 2019 There is a permanent magnet that spins around driven by the cable and is mounted just above that brass worm gear in the photo. It spins very close to the thin metal inverted 'cup' on top of it which if you gently move will move the pointer. The distance between these two is obviously critical for the calibration of the instrument so excessive wear in the bearings will cause misreadings as will any dirt that causes contact between the two. Cleaning/lubing is worth trying but overhaul and calibration is really a specialist job.... Link to comment Share on other sites More sharing options...
JohnD Posted April 11, 2019 Report Share Posted April 11, 2019 No argument with your remarks, johny, except that there is no provision in the mechanism for adjusting the distance between the driving disc with the magnet and the eddy current disc. There are two possibilities, neither of which are suitable for adjustment. A/ the gear ratio in the dizzy that drives the cable to the gauge. You will notice that the gauage face says , under "RPM x100", "3.55-1", which I think will be the ratio. I've just checked with a spare techo that I tink is froma Vitesse and it also syas "3.55-1" B/ the stiffness of the spring that opposes the action of the eddy current wheel This implies that Martin's DPO fitted a rev counter from a different car, or a dizzy from a different car. The second sounds much less likely! Dirt between the driving and driven discs would cause inetermittent jerking, not the smooth movement martin decsribes, so I tbink its the spring, whihc must have become weakened. You could try dismantling it, Martin, but where would you get another spring? I suggest sending it to an instrument repairer. Speedy Cables do this: https://speedycables.com/instruments/gauge-repairs-conversions/ John Link to comment Share on other sites More sharing options...
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