REPEAT TACHO (RPM GAUGE) FAILURES

[ROBERT.JACKSON]

I have a Bosch dizzy with Aldon ignition and a MGB tacho (RVC type) and I have blown 2 tachos in the past week. It is wired corrrctly etc. the tacho works for a few hours then stopped working, then a second tacho fitted and the same occurred. the coil has been checked and also is the same as a spare I have tested. The only question I have is what is the voltage at the - earth conection on the coil with ignition off then on then engine running at tickover then at 3000 RPM ?. As I am getting 0 volts = ign off, 1.5volts = ign on, 6 volts with engine running at tickover 900rpm then up to 10 volts at 3000 rpm. The positive side of the tacho is conected on the fused side of the ignition switch. Could someone please test and advise if this is correct ? or throw some light on my tacho failures.

[Dane_Rescueman]

Re: REPEAT TACHO (RPM GAUGE) FAILURES

Bob,
The earthed connection at the coil is the "bit" that is switched on and off by the points, or in your case, the Aldon unit. Clearly, with points open (Aldon output power transistor at high resistance) the so called earth of the coil will be at system voltage. Your meter has lag etc and will not pick up the switching well at even low rpm and will thus register a changing voltage with changing rpm. An oscilloscope shows this up well!
The average coil is around 6 to 9 ohms so, depending on the internal resistance of your meter you will suffer a voltage drop across the coil with the engine still. This may be the reason for the 1.5v with the engine still, ign on.
Most tachos need a +ve feed from the ignition and an earth (-ve) to the chassis. A third feed is taken from the negative terminal of the coil. I always fit a capacitor in series with this feed to protect the tacho instrument from high current spikes.
I suggest that you check your tacho wiring and see if it has to have a feed from the ignition switch (+ve) a chassis earth(-ve) and the third feed from the coil negative terminal.

Good luck with it!

[Dane_Rescueman]

Re: REPEAT TACHO (RPM GAUGE) FAILURES

Hi Bob,

I have just checked a manual and the Smiths RVC should work Ok with electronic switch units.
On the Tach:
Terminal 1 goes to ign/+ve
Terminal 2 goes to coil CB/-ve
Terminal 3 goes to -ve chassis/earth.

Hope that this helps.

[ROBERT.JACKSON]

Re: REPEAT TACHO (RPM GAUGE) FAILURES

Could anyone confirm the Rev counter terminals as mine has no id. There ia a spade type and a round pole one only, which is - neg coil and which is + 12v live ?.I have had differnt answers on this question and would like a confirmed correct detail as this maybe the reason for my failures !!!!.

[Dane_Rescueman]

Re: REPEAT TACHO (RPM GAUGE) FAILURES

Bob,
Is it possible that you have a Smihs RVI and not RVC. There is a huge difference. Try this web site:
http://members.shaw.ca/tsmit/tachmod/tachmod3.html.
They may shed some light for you.

If you have the RVI then the instrument case is the -ve and should be wired somehow to earth.

Good luck.

[ROBERT.JACKSON]

Re: REPEAT TACHO (RPM GAUGE) FAILURES

F.Y.I.
I have it confirmed from MG owners club on my RVC Tacho.
The spade connector is the 12v feed from the voltage stabiliser while the pole is the coil connection. I have checked other coils - neg terminal voltages and my results above are correct.I have another tacho ordered and will test again when fitted.

[h_m_cumming]

Re: REPEAT TACHO (RPM GAUGE) FAILURES

Hi Bob, just to add my 2p worth. I don't think the 12V feed should come from the voltage stabiliser but from the 12V ignition feed. The V stab. doesn't give out 12V but about 7.5V which is designed to supply the bi-metal type temp. and fuel gauges.
Best of luck. Hugh.

[donsayers]

Re: REPEAT TACHO (RPM GAUGE) FAILURES

Ah! I thought the bimetal voltage stabilisers were designed to give an average of 10V. If not then this would explain a lot.