Jag dials

[cameronfurnival]

Well sorry to be a nuisance but I happened to pick up some Jag XJS S3 dials on ebay for £20. I only wanted the minor dials and was surprised to find the speedo was an electric one. Has anyone had experience of using these instruments before? If so how did you rig up a sensor?
The 6 cyl rev counter will need a shunt soldering in adn I am happy to try that out.

[cameronfurnival]

Re: Jag dials

just an update on the dials. Am fitting a rover V8 engine and box - the box already has an electronic sensor fitted. Just a matter of doing some sums and stuff I guess. :-)
Will dig out those old physics notes on FSD of meters!

[macdonald.dm]

Re: Jag dials

Hi Cameron.

I suspect the 'problem' you'll have with the speedo will be to do with rolling radius/rear axle ratios - ie: nothing to do with actual no. of cylinders. The gearbox sensor will almost certainly be reading the output shaft (propshaft) rotations. There's a reasonable chance that the speed reading won't be TOO far out! If it is, there are specialist companies who may be able to tweak it inside to match, if you give them your wheel circumferences and axle ratio.

Your rev counter, on t'other hand, WILL obviously be greatly affected by the different cylinder counts.

I'm guessing these clocks will have been bought-in items by Jaguar. Since there's therefore a reasonable chance that the actual electronics & mechs inside will also be supplied to other manufacturers (with a different fascia), I wonder if there's a way of 'switching' it inside to suit various cylinder no.s?

Do the actual clocks have a manufacturing name? If so, they'd be the one to contact. Or at least you'd be able to state the make when contacting recalibration specialists.

I can't think of any other easy way of adapting 8 cylinder pulses to suit a 6-cyl tachy.

[cameronfurnival]

Re: Jag dials

Thanks Donnie - I hadnt thought that they might be issued to other cars too - makes sense when youthink about it.
The rev counter will need a 330ohm shunt soldering in - i have worked that bit out but the speedo will be a problem. I agree that it is likely ot be measuring the rotation of the prop, I suppose there will be some sort of pulses coming out of the sensor and I need to match these up to the speedo. I have taken it apart and cant see any adjustment (potentiometers for example) so I may just have to bite the bullet and buy a more normal one - maybe a VDO? That will probably mean a matching tacho too. However I am convinced I can get teh minor gauges working satisfactorily. If anyone has advice on which speedo to go for (or has one spare!) please let me know.

[dave_kitson]

Re: Jag dials

I fitted VDO instruments to my V8 Cabrio, and spent many fruitless hours trying to get the speedo to work. There are several different types of sender, and the VDO speedo can be wired to suit whichever type you have, as well as having an in-car calibration facility. I found out the Rover sender type and wired accordingly. No joy. I wired for the other types. Still no joy. In total I tried three brand new senders (hard to get and expensive), without success. In the end I contacted VDO who were extremely helpful, but we still could not get a reading. In the end I tried a VDO sender that mounts next to the propshaft, and counts the bolts as they pass. This worked perfectly! The conclusions are that the Rover sender is a very odd beast, or gives a miniscule voltage, and only works with the Rover speedo; so wherever possible you should use the same make of sender as instrument (can you get/fit a Jag one?). I can dig out the technical info on the VDO speedo and Rover sender if it helps, or I can sell you a Rover sender if you want to try a new one (but I don't want it back! There is another just filling the hole in the gearbox!). Good luck! Oh and finally, jack up the back of the car, start it and put it in gear to check that the speedo works before you go to the SVA. I only found out mine didn't when I got there! (They will let you calibrate on site though).

[macdonald.dm]

Re: Jag dials

Hi Caneron.

Any manufacturer ID on the clocks? That would be my first point of contact - you might just get an enthusiast who'd be happy to spend some time replying to you with info on type of sender needed, etc.

I don't understand the rec counter shunt. I'd have thought it would be a bit tricky converting a 6 cylinder counter to cope with 8 cylinder pulses - but I dunno!

[cameronfurnival]

Re: Jag dials

my reasoning here is that the rev counter only counts pulses - it doesnt know how many cylinders there are - therefore if the full scale deflection of the needle equates to a certain number of pulses from a 6 cylinder all you need to do is change the electrics so that FSD occurs at a higher number of pulses. Sounds logical?

[macdonald.dm]

Re: Jag dials

Yes, that sounds logical, Cameron, but HOW do you change the electrics to suit?! If the clock doesn't have this facility built-in, then I think it'll be tricky (I remember I used to have an old rev counter from, I think, an aged Rover, and it had a 4/6 cylinder selector switch on the back - how convenient!).

It would be so much easier if the rev counter only took its pulse from, say, no1 cylinder; then it wouldn't matter how many cylinders you have! But, hey, that would be tooo easy.

[milliemarlin]

Re: Jag dials

I know the owner of a lesser kit car that modified a standard Sierra Speedo, to suit larger wheels, by adjusting the dials return spring. His was a mechanical drive. The Speedo had a spiral spring attached to an adjusting screw. He calculated the car’s speed at different RPM. With the car resting on axle stands and the rear wheels removed he engaged the various gears and adjusted the needle’s return spring at various rpm. The speedo was not very accurate. It diverged more at some speeds than others. It was good enough to pass his SVA.

This might be an easier alteration than changing resistor values.

Similarly if the rev counter is converting the engine pulses into an analogue signal to drive the needle you might be able to do a similar trick with that.

Of course if your needles are driven by stepper motors you would need another approach completely.

Paul