Hi everyone,

I'm hoping that someone out there with more electrical knowledge than me can throw some light on a strange problem. I am in the final throws of a MK2 (1967) restoration. I have rebuilt and fitted a 'Big Valve' engine
from a Series 3 XJ6 (1985 vintage). The motor is fed by two HD8 2" carburettors and I am presently using standard points, together with a DLB105 Gold Lucas coil.
Because the XJ6 engine doesn't have a tacho generator on the rear of the camshaft I have fitted a tacho from a 1960's Jaguar 420, which takes its input from the coil. I have spent a few days attempting to obtain a smooth
700 rpm tickover, to no avail - anything below 1000 rpm resulted in lumpy, uneven tickover. I've set the timing to 8 degree BTDC as per a Jaguar 420 workshop manual (The closest equivalent I can think of to my
engine spec). I suspected that the actual tickover sounded just about right (and smooth) at around an indicated 1400 rpm, so invested in a hand-held digital tacho (£11.99 through Amazon). LO and behold when I checked,
the instrument tacho rpm is exactly double throughout the range. I've also tried connecting a Series 1 XJ6 tacho and that also shows half its true value - so the instrument itself appears OK. My car has been converted to
negative earth and the 420 tacho is marked the same. I've now adjusted my carbs to 700 rpm (using hand-held tacho) and it sounds as sweet as a nut.

Regards Keith

I have the wiring diagram for a Jaguar 420 and I've wired my instrument the same, so why does it read double ?? Any thoughts would be greatly appreciated.

 

Q unfortunately that is a classic symptom of an impulse tach that needs to be rebuilt. It is 50 years old or so and there are parts inside that have perished or no longer hold value---that service is out there no matter where you are. Good luck!

 

Thank you for your comment George. I'm pleased to report that I have now managed to rectify the problem. Having removed and dismantled the instrument I first of all un-looped the feed wire (someone on an MG forum was
advised to try this and it worked for him) and made it a single wire feed (if that makes sense). Anyway, that didn't make any difference so I then drilled a hole in the rear of the instrument (again, following someone elses
experience), allowing me to access the adjustment knob (slotted screwdriver operation). Turning this knob with the engine running allowed me to reduce the indicated tickover to 700 rpm. Whether the out-of-adjustment was
down to worn components, or whether I accidently turned it some months ago when I removed the glass to clean it, I'll never know. If the former was the reason then I daresay that eventually I'll need to get the instrument
rebuilt (I was quoted a very reasonable £75 by a nice gent at 'Speedy Cables' in the UK). Thanks again.

Regards Keith

 

Keith was aware of those "fixes' but none of them have ever been satisfactory for me for long. I am curious if it reads in a linear fashion when higher revs are attempted. Remember even a broken clock (analog) is right two times a day!

 

Hi again George. I haven't tried the higher revs yet, so fingers crossed (the car is in the garage attached to the house and the family keep complaining about the fumes !!). I'm hoping to take it on the public road for the first time in 14 years next week, so I'll post the outcome (I daresay I'll come back home with a new list of 'Things To Do' - lol !!

 

Well best of luck!

 

CaptainQ
The 420 Tacho is transistorised as you probably know, the trigger loop is one side of a transformer. and the output of the transformer triggers a one shot pulse generator formed by 2 transistors and some capacitors and resistors, this produces an output voltage which stays constant for a set period of time and then turns off until the next trigger pulse (from the coil), the faster the engine revolves the closer these pulses are together, so the average voltage rises and moves the needle further around the face. The calibration resistor changes the length of the pulse and therefore the average voltage at any point in the rev range. You will have needed to reduce the pulse length by half in order to read half the revs.

It seems coincidental that you are reading exactly double, my first thoughts would be that the coil is producing a reverse voltage (back emf) every time it fired, doubling the pulses to the tacho, but you would need to rig up an oscilloscope to see that. You could try another coil just as a trial possibly.

However, if you have managed to get it to read correctly that's great, and see how it works across the rev range.

Just a note, taking out the loop and using a straight wire would reduce the output of the transformer, if the transformer output was still strong enough it would trigger the pulse generator and the revs would still read the same, if the output from the transformer was too low, the rev counter either would not work at all or would be erratic if the output sometimes triggered the pulse and sometimes didn't.

Hope this is of some help.

 

Hello John. Thank you for the explanation as to how the Tacho is fed. I did try another coil (the current one is a Lucas DLB105 Gold) but that didn't change things, neither did the un-looping of
the supply cable. As I explained in my initial post, I also tried an XJ6 instrument, which also over-read - perhaps that one is on the way out as well ?? Next week (fingers crossed) will be the
moment of truth, when I start to use the car. Thanks again & Kind regards.