Citroen XM Forum

White Exec wrote:
Love the completely flat dash panel on the Jensen, easily made up on a craftsman's bench. Rev counter (presumably) fed off the dynamo end-bearing is a nice touch.

Why would the Rev counter be fed mechanically, an electric rev counter is just a voltmeter fed from the coil -ve trip lead. Rev counters were commonplace on upmarket cars int he early 60's.
My Father had a 1964 Humber Sceptre that had an instrument set worthy a aircraft cockpit. Probably why he felt comfortable with a Gs next.

That Jeager dial set is the same as in the Humber, but this has aluminium centres, where as the Humbers were black only.

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I believe that jaeger instruments by that time were just badge engineered smiths units. Until the late 1950s they were all mechanical, using the same technology as speedometers. On engines where a camshaft drive was available, they were driven from the end of the camshaft (e.g. Jaguar XK), but BMC engines did not have provision for camshaft drive, so dynamo drive was used instead.

In about 1959, electric tachometers were introduced by smiths, but the first generation used a small AC dynamo driven by the end of the camshaft to measure the engine speed, so not applicable to BMC engines.

In about 1962, ignition driven tachometers were introduced, too late for the Jenson 541. A voltmeter across thee points BTW makes a dwell meter not a tachometer.

Thanks for the information relating to dates.

As you say, a Tacho is a voltmenter with a pulse count input.

The first (Smiths) tacho instrument I laid hands on (about 1965) was one of those IGN fed units. The feed to the coil from the CB was taken via the meter, in the shape of a couple of turns of the feed cable round an inductive clip on the back of the instrument.

I think it was a fairly basic unit, probably consisting of a simple "charge pump", where the incoming current pulses were short-term stored in a capacitor, which was in turn shunted by a discharge resistor. As the pulses grew in frequency (engine speed), so the voltage on the capacitor increased. The needle display, in glorious Smiths red, white, black and chrome, was a simple voltmeter.

Later on, this kind of charge pump design was superseded by F to V converters, the basis, as said, of both modern tachos and speedos.

Not sure about the parentage of Jaeger and Smiths. Several of the XM dash instruments and modules are Jaeger (France). I think the two companies' instruments were built to ape each other, enabling mix-and-match on the same dash.

The old classic round, white-on-black instrumentation, with white, red or orange pointers, takes some beating for appearance and readability, and ran right across vehicles of quality - Rover, Rolls, Jag, Daimler, Bristol, Van den Plas, MG... Still can't get excited about the cream and oval offerings of some recent cars.

I can't comment on the dates and solutions, of course, but in that specific article there are both text and pictures about the solution on that particular model. The tacho is driven mechanically by a cable from the dynamo, with a small gear unit (visible on the picture) to account for the different revolutions, not fixed to anything but just sitting in line between the two cables. It is specifically mentioned as an adorable example of wonderful British engineering craziness. [emoji3]

Besides, now that I read it again, this isn't only the first 541S model ever built but it's the exact same car that was unveiled in 1960 at the Earls Court Motor Show when Jensen introduced the model to the public.

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