RPM gauge 12V on earth?

[Theo_NL]

I bought a nice set of Smith gauges to fit in my Morris. The fitting in the original frame of the glove box is almost finished (need some grinding and painting).
[frame][/frame]

I noticed that at the back of the RPM gauge three connectors are available: 6V / + / 12V
[frame][/frame]

Probably to fit in a positiv earth setup?
Should I connect 12V in my car (negative earth) tot the + and earth the 12V?

[gtt1951]

Theo, looks like the + terminal is the common one, so your vehicle "earth" should go here - in your case the -ve return.
Make sure that no other metal parts of the gauge are in contact with the chassis.
What is going to be supplying the "pulse" - take off from the Coil LT side?
Regards, George.

[bmcecosse]

The gauge will likely go bang if you do that.... Keep the polarity correct at the gauge and if that means isolating it from all 'earth' paths , so be it.

[Theo_NL]

The gauge will likely go bang if you do that....

Do what? Connect vehicle earth to + terminal?
Don't want to spoil it, after all that work.

The gauge is isolated completely. Fitted in polycarbonate front and thin plywood back.

[Theo_NL]

Found the fitting instruction:[frame][/frame][frame][/frame]

[Theo_NL]

The terminal for the 'signal' is missing. I assume it is made from isolation material and the fitting the wire to the coil close to the backside will transfer the signal to the electronics in the backplate of the gauge. Right?[frame][/frame][frame][/frame]

[gtt1951]

Theo, I've tried to lift the images off your post to enlarge to read - quality was not good enough.
Email me the instruction pictures to
george.tondryk@btinternet.com
The moving-coil meters I have for measuring all points type operated engine ignition timing systems can record RPM depending on where the probes are connected. Using this info, I may be able to help you with fitting this gauge.
BMC, what does your reply refer to? My suggestion or the OP's one? Input has not been very clear.
Regards,
George.

[bmcecosse]

Sorry - I read it that you intended to connect your negative earth to the centre terminal! You MUST retain the polarity at the terminals - as marked. So DO NOT for example connect earth on a neg earth car to that centre terminal !! And yes - a simple block to fit over the sensing stud with cable twisted through it just as shown.

[mike.perry]

I purchased a 1960s rev counter, probably a Midget gauge, slight snag, it is + earth and my car is neg earth, alternator so I can't change it. The earth terminal is connected to the body of the rev counter so I can't just swap the terminals around. I am going to have it professionally converted rather than risking bxxxxxxxg it up.[frame][/frame][frame][/frame][frame][/frame]

[gtt1951]

Having seen the better resolution instructions and read through them, the 6V and 12V connections are actually -6V and -12V connections with the vehicle system's +ve connection being made to the "+" terminal.
The whole case needs to the "earthed", which throws up another question and the whole unit will not work as the soft iron "slug" that connects to the 4th "post", and acts as an inductor (transformer) to supply the coil "pulse" is not available.
The drawing shows the LT feed from the Dizzy being looped around this 4th post and then on to the ignition coil.
Having a "loop" shows that it is being used as an "electromagnet" to induce a pulse feed into the instrument.
There is an article at http://www.uniquecarsandparts.com.au/ho ... ometer.htm that tells you about the way this tachs work.
George.

[gtt1951]

Looks like 2 other replies came in whilst I was taking my time in responding - started at about 20:00 and had to take time out to cook my supper!
George.

[bmcecosse]

Oh - so it's a 'slug' and not just a holder for the loop.. To be honest - all this enthusiasm for 'counting the revs' leaves me cold. Why bother? How do the revs matter ?

[Theo_NL]

Having seen the better resolution instructions and read through them, the 6V and 12V connections are actually -6V and -12V connections with the vehicle system's +ve connection being made to the "+" terminal.
The whole case needs to the "earthed", which throws up another question and the whole unit will not work as the soft iron "slug" that connects to the 4th "post", and acts as an inductor (transformer) to supply the coil "pulse" is not available.
The drawing shows the LT feed from the Dizzy being looped around this 4th post and then on to the ignition coil.
Having a "loop" shows that it is being used as an "electromagnet" to induce a pulse feed into the instrument.
There is an article at http://www.uniquecarsandparts.com.au/ho ... ometer.htm that tells you about the way this tachs work.
George.

George, Thanks for the explanation and the link!

[frame][/frame]

Mike, could you show some detailed pictures of the 'soft iron' part? Maybe unscrew it from the back? Would be very helpful to see if I can make it myself. EDIT: No need anymore, I found the missing piece! It was loose in the bag of the gauge

Oh - so it's a 'slug' and not just a holder for the loop.. To be honest - all this enthusiasm for 'counting the revs' leaves me cold. Why bother? How do the revs matter ?

I know Roy you are not a fan. The link of George might give some good reasons : One of the main uses of the tachometer is assisting the driver in correctly relating engine speed to road speed, so that gears can be changed at the optimum combination of these two factors, achieving the best power/speed ratio and the most economical use of fuel.

[bmcecosse]

Go on - now you're pulling the other one....... If you don't 'know' when to change gear....

[Theo_NL]

[gtt1951]

Go on - now you're pulling the other one....... If you don't 'know' when to change gear....

Roy, on my daily use "modern" Peugeot 406 diesel estate car, I know exactly what road speed (within about 5mph) I'm doing from reading the RPM and knowing which of the 5 gears I'm in (from past observations).
The reason behind this? Is because the way that Peugeot work the speedometer (not through any physical link to the gearbox) means that, the road muck (and gritting salt) covered transducer FAILS and my speedo shows 0 mph for miles on end!
This also means that the Trip Computer gets it all wrong and the car's MPG readings and "mileage left in tank" need to be taken with a big pinch of French Sea Salt.
The tank equates to about 15 gallons and the range is normally about 800 miles - not bad compared to my 3 Moggies!
Because of the very good fuel economy of "Heavy Oil" cars, successive Governments have penalised users and put far too much Duty on this fuel on the grounds that "they get far greater mileage per gallon".
I know of at least one Split Screen Moggie that has received a diesel Van engine transplant (rear wheel drive).
Regards, George.

[Theo_NL]

I always thought a rev counter is really necessary considering that you cannot hear the Morris engine running to change gears on time

[bmcecosse]

Change up when the valves bounce....

[Theo_NL]

The result after fitting the gauges:[frame][/frame]

Could not go out for a test ride, so just started (after repair of the starter ) and let it run for a couple of seconds.
Rev counter is not working
I have been warned, on the Mini forum following was posted:

The figure above is helpful if you are fitting a Smiths RVI type tachometer to a car with a points-type distributor. (RVI will be somewhere on the gauge face). If your car has electronic ignition, do not bother trying to fit an RVI tach to it. Chances are very strong that the tach will not work correctly.

A better choice is to find a Smiths RVC type tachometer as was used on later MGBs and Midgets. As above, RVC will appear on the gauge face. The RVC tachs are a more modern design AND they will work with either points or electronic ignitions.

Mine is RVI, and I have electronic ignition...
I do not understand, so I have to dig deeper to find out why. Maybe electronic engineers on this forum understand why this combination is not working?

[panky]

Very nice Looks a bit cleaner than mine, I managed to chip the paint a bit