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MG MGB Technical - 4 cylinder Smiths Tach to V-8

Anyone have a diagram of a converter to convert a Smiths tach to read a square wave signal from a V-8 direct ignition system (Electromotive)?

I am using a TR-7 5 speed instrument cluster w/ tach on a MGB-V8 conversion with a 215 Olds V-8 with an Electromotive direct ignition system.

HMF III
HMF III Fitzcharles

Hi,

For a RVI tacho, circuit below, reducing the total value of pot. R4 and resistor R5 will require more pulses for the same reading, ie 8 vs 4 cyclinders. With a bit of luck the pot has enough adjustment in it to bring it to 8 cylinder function.

Herb

Herb Adler

or, on second thought change out capacitor C5 for a 0.12uF one.

Herb
Herb Adler

I would have said the points signal is effectively square wave anyway. Electronic ignition can be different, which is probably why some cause problems with the earlier RVI tachs.

A TR7 would have had the later RVC voltage triggered tach, I would have said. It dates from 74 and the MGB tachs changed in 73.
PaulH Solihull

To get around this little problem, when I installed a '70s style GM HEI ignition system, I used a spare Smiths housing and installed the internals from an Autometer tach. It turned out better than I had expected. RAY
rjm RAY

Hi All,

The problem with electronic ignition and RVI tachs is that the power for the ignition is taken from the BAT terminal of the coil. Electronics invariably have a large capacitor on the power input, to smooth out voltage fluctuations. Unfortunately this also smooths out the current pulses, for the RVI tachs, thus giving poor or no triggering. The trick is to power the ignition from a circuit that doesn't go through the current sense transformer, in the tach. The clump of connectors on the RHS side of the engine bay, near the firewall, has a spare white connector, which is switched ignition and a good point to take the power for the ignition module.
I have done this with my RVI tach and a 123 dizzy and have had no problems.

Herb

Herb Adler

So how do you generate current pulses for the tach, Herb?
PaulH Solihull

Hi Paul, the coil is still energized via the tach, just not the electronic ignition.

The diagram shows what I mean. If the power to the electronics is taken from the coil, then that capacitor would supply power to the coil, after the tach, thus reducing the pulses through the tach.

Herb

Herb Adler

OK, that method of getting the 12v supply to the trigger from another white - say at the fuse box - is common (but not universal) for rubber bumper cars with ballasted ignition, but I've never heard it said before that doing that for RVI tachs solves the problems with that tach.
PaulH Solihull

Hi,

I can't talk about all electronic systems, but the 123 instructions basically say to connect the red (power) to the bat terminal of the coil. With RVI tachs this is after the current transformer and the input capacitor messes with the current pulse. I don't know exactly how, maybe rise time or magnitude. By connecting the electronics power before the current transformer this does not happen, and the tach works OK.

Herb
Herb Adler

Hi,

Back to the original question.

The type of Smiths tacho you have, RVI or RVC, would help in advising you.

I have just received a Jaeger (read re-badge Smiths) RVI tach, for 6 cylinder use. The only difference in components is R6, instead of 820 ohms it is 470 ohms.

Herb
Herb Adler

Any capacitor between pos and neg will tend to smooth out voltage pulses, converting a square wave to a saw-tooth. They a generally there to reduce spiking, like radio interference capacitors on the coil, voltage stabiliser, pump etc. Because the current pulses that trigger an RVI tach do go through a transformer, the smoother the saw-tooth input the smaller the output from the transformer.
PaulH Solihull

This thread was discussed between 29/06/2011 and 05/07/2011

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