[xj] OT I guess but not very: tacho conversion

Hi all,

I’m working on another car at the moment: a 1975 Mini Pick
Up. Since this will not be a completely standard car (some
may see a pattern here;) I thought of using Ser I XJ gauges
in the dash. I’ve got a nice set of chrome bezeled speedo,
tacho and the smaller ones. Problem is of course that the
Jag’s tacho is for a six cylinder whereas the Mini only has
4. Does anyone have experience in doing a conversion like this?
For those that have no clue what a Mini Pick Up is:

cheers,–
Arnoud
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In reply to a message from Arnoud sent Wed 13 Jan 2010:

So the tacho will read low then. Don’t know if you can adjust
them that much. I’ve never tried. There should be an
adjustment pot on the pcb, If it doesn’t give sufficient range
of adjustment you may need to change it for a higher or lower
value. I don’t know if it would affect the accuracy though.

Dave C.–
The original message included these comments:

I’m working on another car at the moment: a 1975 Mini Pick
Up. Since this will not be a completely standard car (some
may see a pattern here;) I thought of using Ser I XJ gauges
in the dash. I’ve got a nice set of chrome bezeled speedo,
tacho and the smaller ones. Problem is of course that the
Jag’s tacho is for a six cylinder whereas the Mini only has
4. Does anyone have experience in doing a conversion like this?


Dave Collishaw '79 Daimler Sovereign '96 X305
Cambridgeshire, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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In reply to a message from sparx sent Wed 13 Jan 2010:

I assume the chrome-rim gauges are from an early S1 XJ. Did they
continue with chrome dials beyond 69-70ish? Reason I ask is because
at the same time the Series 2 E-type was using an odd sort of tach
and I wonder if the XJ was the same and it might not work for you.

After cable tachs faded away in the 1950s the E-type and other
early Jags used a small AC generator on the back of the inlet cam
and the tach was effectively a form of voltmeter. By the time of
the Series 3 E-type in 1971 they finally switched to a typical
electronic tach which counted the coil pulses. Between those times,
from about 68-70, the tach worked on a completely different
principle by measuring the current in the ignition circuit, or
something like that. This kind of tach can’t just be transplanted
as easily as a self-contained pulse tach.

Have I made any sense Dave? Is Arnoud going to run into problems?–
The original message included these comments:

So the tacho will read low then. Don’t know if you can adjust
them that much. I’ve never tried. There should be an
adjustment pot on the pcb, If it doesn’t give sufficient range
of adjustment you may need to change it for a higher or lower


68 E-type OTS, 99 Daimler Super V8, 92 XJ40
Cambridge, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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In reply to a message from PeterCrespin sent Thu 14 Jan 2010:

It’s no big deal on wiring it in Peter. It will be an
induction loop tacho that has the coil feed running through
the head to induce a pulse in the electronics.
I had forgotten all about those tachs with the impulse
generator. A similar set-up was used right up until about
the eighties for the speedo on a lot of commercial vehicles.
The pulse generator was enormous compared to the modern
equivalent.
I think some Triumphs had the same style of speedo/tacho
(I’m thinking Dolomite here) maybe Arnoud would be better
off flogging his XJ tacho and buying one for a four cylinder
Triumph.

Dave C.

Dave C.–
The original message included these comments:

I assume the chrome-rim gauges are from an early S1 XJ. Did they
continue with chrome dials beyond 69-70ish? Reason I ask is because
at the same time the Series 2 E-type was using an odd sort of tach
and I wonder if the XJ was the same and it might not work for you.


Dave Collishaw '79 Daimler Sovereign '96 X305
Cambridgeshire, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

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In reply to a message from sparx sent Thu 14 Jan 2010:

Hi Dave,

I have two classic Smiths 4-banger tacho’s (one with a red
tell-take hand) sitting on the shelf already. The point of
using the XJ one was uniformity in looks, but I guess I’ll
just have to live with it then. It’s not like this is the
only challenge I’ll have with this car (which will be
supercharged using a New Mini S’charger:p

cheers,–
The original message included these comments:

(I’m thinking Dolomite here) maybe Arnoud would be better
off flogging his XJ tacho and buying one for a four cylinder


Arnoud
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In reply to a message from Arnoud sent Sat 16 Jan 2010:

Arnoud,

I don’t think that recalibrating the S1 tach should be a
problem. The variable resistor used for adjustment has a
wide range, perhaps wide enough. If not, the simple
frequency-to-voltage converter circuit includes a fixed
resistor, which could be reduced in value, plus a capacitor,
which could also be reduced in value. The latter is known
to ‘‘fail’’ in S1 cars and E-types. When it does (in part by
losing effective capacitance) the tach read much too
high–just what you’re looking for.

The inductive coupling of this tach has already been
mentioned. It’s no problem with a points style ignition,
but can be erratic with some electronic ignitions. That can
be fixed, too.

The chromed bezels had a very short life, Pete. They ended
around 69-70. Not as short as the chromed AC balls and
chromed scuttle vent, though.–
The original message included these comments:

I have two classic Smiths 4-banger tacho’s (one with a red
tell-take hand) sitting on the shelf already. The point of
using the XJ one was uniformity in looks, but I guess I’ll
just have to live with it then. It’s not like this is the


Bob Wilkinson, 72 XJ6
Saint Louis, MO, United States
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In reply to a message from Arnoud sent Sat 16 Jan 2010:

Looks like you’ve got all bases covered Arnoud.
Bob thinks the tach will adjust, he’s probably right. If it
won’t go far enough and you don’t fancy dabbling with the
resistors or cap you could maybe just swap the appropriate
parts to make a four pot tach look like the XJ tach, or put
the guts from a four pot tach into the XJ tach.
Be a shame to do all that work to the car then not take care
of the details.

Dave C.–
The original message included these comments:

I have two classic Smiths 4-banger tacho’s (one with a red
tell-take hand) sitting on the shelf already. The point of
using the XJ one was uniformity in looks, but I guess I’ll
just have to live with it then. It’s not like this is the
only challenge I’ll have with this car (which will be
supercharged using a New Mini S’charger:p


Dave Collishaw '79 Daimler Sovereign '96 X305
Cambridgeshire, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

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