P5b Tacho

[pipetech]

Hi has anyone fitted a Tacho to a p5b?
If so any recommendations on make etc.. or problems encountered
Cheers

[ray]

Pipetech,

Your question:- has anyone fitted a Tacho to a p5b?


This seems a bit of a silly answer ...but : yes, Rover did it! The original fitment is a Tacho.

Ray

[Phil Nottingham]

Coupes only of course and used the now obsolete induction loop method which was far from accurate after a few years

[benedict_h]

Jun 27, 2016 18:55:05 GMT Phil Nottingham said:

Coupes only of course and used the now obsolete induction loop method which was far from accurate after a few years

Hi Phil,
Do these show the letters RVI on the scale (where I stands for induction, as opposed to RVC (current))? I have one of those in my car. Haven't tried to connect it since I changed to electronic ignition, but as I understand it, the RVI tacho won't work. I know you can have your tacho converted to RVC, to make it suitable for electronic ignition.

Interestingly, I came across this Ebay shop by Spiyda:
stores.ebay.co.uk/Spiyda/Tachometer-Kits-/_i.html?_fsub=13715305018&_sid=80045078&_trksid=p4634.c0.m322

They offer RVI to RVC conversion kits for a reasonable price. Has anyone any experience with such a kit?
Seems a great idea to be able to calibrate your rev counter using your iPhone...
Regards,
Benedict

[djm16]

Early coupes had a tachometer that is triggered by current pulses in the power supply to the coil. The cable carrying power from the ignition switch to the coil takes one turn around a soft iron core on the back of the tacho. The magnetic field generated by each current pulse induces another current in the secondary winding inside the tacho. That current is then fed to the input of a monostable pair. The monostable remains triggered while its holding potential is dissipated through a R/C network with a time constant that determines the deflection of the tachometer needle per pulse. A rapid series of pulses causes a larger deflection than a slower series of pulses, but in either case the needle deflection is independent of the size of the current pulses in coil power supply, but depends only on the R/C time constant and the number of pulses per second.

What can go wrong? Almost the only thing that goes wrong is the 55 year capacitor in the R/C network breaks down but is very easy to replace.

Does it work with electronic ignition? Of course it does. It even works if you put a damping capacitor to ground at the coil.

Mine was temperamental for a while but now works fine after I straightened out the shape of the cable winding around the soft iron core. It had got flattened. I didn't even need to put a second turn of cable around it to boost the signal.


Note that if you change the car to negative earth, you will also have to rewire inside the tacho.

[benedict_h]

Thanks for that explanation, dmj16.. Most of it is a bit above my hat still, but I'll try and grasp the meaning of it before taking further steps towards my tacho.
Regards,
Benedict

[Phil Nottingham]

Perhaps I may do my Coupe's tacho and will leave it as as Positive earth with Dynamo although I may swap to negative earth to try a NOS -ve earth  one for a Mk3 I have had for years   I do not do electronics but can solder - what does said capacitor(s) look like as it probably costa few pence to buy

It would be nice to have consistent readings in all weathers

[ray]

Interesting comment Phil
(... and used the now obsolete induction loop method which was far from accurate after a few years)

In all my years in the club I have never heard of any complaints about the Tacho, lots about the Speedometer, but none about poor Tacho. Mine is still fine.

[petervdvelde]

Jun 28, 2016 11:59:04 GMT benedict_h said:

Jun 27, 2016 18:55:05 GMT Phil Nottingham said:

Coupes only of course and used the now obsolete induction loop method which was far from accurate after a few years

Hi Phil,
Do these show the letters RVI on the scale (where I stands for induction, as opposed to RVC (current))? I have one of those in my car. Haven't tried to connect it since I changed to electronic ignition, but as I understand it, the RVI tacho won't work. I know you can have your tacho converted to RVC, to make it suitable for electronic ignition.

Interestingly, I came across this Ebay shop by Spiyda:
stores.ebay.co.uk/Spiyda/Tachometer-Kits-/_i.html?_fsub=13715305018&_sid=80045078&_trksid=p4634.c0.m322

They offer RVI to RVC conversion kits for a reasonable price. Has anyone any experience with such a kit?
Seems a great idea to be able to calibrate your rev counter using your iPhone...
Regards,
Benedict

Benedict,

I have a RVI tacho on my P5B Coupe and it works very well for more then 20.000 km in combination with a Powerspark electronic ignition

Peter

[Warwick]

My tacho* adds the pulses from the Facet fuel pump to the ignition pulses. Very handy for checking the performance of the pump, when the engine isn't running, but not much help at other times.



* The Australian English spell-checker suggests that I mean taco, nacho, macho, or tachograph.
The US English spell-checker suggests taco, nacho, macho or tachyon. Now that could be interesting; a rev-counter that responds faster than the speed of light.

However, if I accept this advice and choose tachyon, the Australian spell-checker then suggests that I mean tachycardia. Now this could be very appropriate in an old Rover when, for example, one was driving in heavy traffic or at speed on an icy road. It would help one stay calm and press on by aiming to keep the needle well below the red line.

[cstorey]

Jun 28, 2016 14:40:03 GMT djm16 said:

Does it work with electronic ignition? Of course it does. 
Note that if you change the car to negative earth, you will also have to rewire inside the tacho.

Hmm..well, with all due respect, it more often than not does not work with electronic ignition. I have more experience of this on Jaguars than on Rovers but the RVI tachos  used on Jaguars of the 1968-1972 era rarely work with electronic conversions, and since the circuitry of those Smiths tachos is virtually the same as that on the Smith/Jaeger used on the coupe, there is no guarantee that it will work. If that happens , then the best thing to do is to convert it to an RVC type circuit

[enigmas]

My OEM P5 coupe tacho works fine with the Piranha electronic ignition that I fitted over 20 years ago...and the car was a daily driver until several years ago.

The Jaguar tacho that is fitted to my Magnette coupe project (3.5 V8) doesn't like the electronic ignition module (a points fired system) and works well till it hits 2500rpm and then freezes! Very annoying.

[Phil Nottingham]

Jun 28, 2016 18:35:14 GMT ray said:

Interesting comment Phil
(... and used the now obsolete induction loop method which was far from accurate after a few years)

In all my years in the club I have never heard of any complaints about the Tacho, lots about the Speedometer, but none about poor Tacho. Mine is still fine.

You are fortunate - they were not accurate when new - they over-read when it warms up or in summer all the time not a linear over-read either. Plenty about this on here going back to the start of this Forum and even further back on the old one although I can no longer access that one

[djm16]

If the fuel pump is taking its power from the lead from the ignition switch to the coil, then yes, I can see this happening. The solution is simple, power the fuel pump from a different lead.

Vz-a-viz other electronic ignitions - mine are all Pertronix points replacement. I cannot vouch for capacitor discharge systems. There is also the option of putting a second turn of the coil power lead around the soft iron core of the tach.

[Warwick]

Actually, I only noticed the correlation not long before the car came off the road, and realized what was causing the erroneous reading. Still much to do under the bonnet. I'll be replacing the Facet with a point-less (as distinct from pointless) diaphragm pump from an early Range Rover anyway. I don't like hearing the Facet.