Tach - Oh !

[nono17120]

Yesterday evening, I looked at my ignition coil.
The wire from distributor arrives on a "CB" marqued pod
A wire from electrique beam arrives on a "SW" marqued pod.

Currently, my car is "negative earth".

[nono17120]

I am not steel here
the tacho is steel on the dashboard...

[billwill]

This is the circuit of the Smiths RVI tachometer.

http://www.classictiger.com/techtips/motach.html

The diagram is laid out in an odd fashion because it was drawn up my someone with relatively little electronics knowledge, by tracing around an actual device. Many motor websites have since copied the diagram so there is litttle point in redoing the layout. I've not been able to work out what that 150 ohm resistor R7 is for or what effect it causes.

Each ragged pulse coming in through the signal transformer at top left is converted by the two transistors (which constitute a mono-stable) into a nice rectangular pulse of a fixed width. R2 and the sluggishness of the meter (A) then effectively measure the average voltage of all the pulses which is directly proportional to the rate of pulses e.g. the RPM. R4 varies the fixed width of the pulses to calibrate the meter.

For a 6-cylinder engine some of the other resistor values may be different.

Anyway it seems that there is no protection for the two transistors and the zener diode if the power supply is reversed, so I think it would destroy the transistor(s) or the zener diode.

It certainly won't work while the supply voltage is reversed.

[nomad]

nono: On your coil, SW stands for "switch" and CB stands for "contact breaker". If you reverse the polarity which, as was covered, you will want to reverse those wires. They are correct presently for a positive ground car.

Kurt

[nono17120]

On your coil, SW stands for "switch" and CB stands for "contact breaker".
Kurt

For french men, it is difficult to understand abbreviations.
Thanks a lot.

[nono17120]

nono: On your coil, SW stands for "switch" and CB stands for "contact breaker". If you reverse the polarity which, as was covered, you will want to reverse those wires. They are correct presently for a positive ground car.

Kurt

You say I am currently right for a "Positive ground" car. But mine has surely been modified and is currently "negative groud".
Do you think coil wires hasn't been reversed ?

[nono17120]

The ignition coil also needs to be reversed because although it will work both ways, it is more efficient one way. The are often marked + and --. The negative side will go to the distributor and then to earth on a negative earth car.
Eric in Burnley
1967 S3SE DHC

My car runs in "negative ground" and CB wire to distributor.
Can it run if I reverse coil wires ?
Perhaps better???

I will try, if there is no risk for coil, and maybe the tacho will start !

[ericbushby]

Nono,
I am not familiar with the CB and SW markings, but from what Nomad said above, then yours seems to be correct now with the return from the coil going to the contact breaker and then to earth ( my coil is marked negative to be the same).
If you reverse the coil it will still work, but I understand the spark will be slightly reduced and can cause reduced spark plug life due to the spark going the `wrong way`! Please don`t ask!
If you have electronic ignition then a common cause of tacho problems is that the power supply to the ignition system is fed via the white wire through the tachometer, as well as the ignition pulses to the coil. This can cause interference in the tacho but apparently it usually causes erratic readings not none at all.
I suspect that your tacho has died or has no input signal. The tacho also needs a 12 volt power supply to the tag on the back and a earth connection to the case.
You could remove the input loop and make sure it is clean. This is fixed on the back with one knurled nut and has the white wire passing through it twice. It is a current transformer.
Best of luck and remember -- they are all easy afterwards !.
Eric in burnley,

[nono17120]

back from the garage
I tried to reverse coil wires: the engine "seems" to run normally, but nothing at the tacho.
Tomorrow, I will have a look behind the dashboard.

[prezoom]

A good explanation of converting an RV1 tach to negative earth/ground, can be found in the Tech Articles at Moss Motors, mossmotors.com. If you have done the electrical work inside the tach, and are not getting a signal, it may need to have the direction the white wire goes through the inductive pickup on the back of the tach reversed.

[nomad]

As was mentioned the tach after being run with reversed polarity may have been damaged. Of course, it costs nothing but time to try to get it going. Besides the loop in the white ignition wire needing change some connections inside the tach need change as well.

BTW early Lucas coils were marked with the abbreviations rather than + or -. At least several that I have encountered.

Kurt.