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Re: MGA tach on crutches (both legs cut off) help!!!!

To: "Clayton Kirkwood" <>
Subject: Re: MGA tach on crutches (both legs cut off) help!!!!
From: Barney Gaylord <>
Date: Fri, 07 May 1999 14:30:12
At 07:36 PM 5/6/99 -0700, Clayton Kirkwood wrote:

>Have a 62 MGA II .... tach started making odd sounds .... squeeling and
humming,.... indicator flipped and flopped around ....
>.... The inner cable turns nicely. The end that goes into the back of the
tach has rounded corners (I suspect something with this). ....
>I then .... put the engine end of the inner cable into a cordless drill
and the other .... into the back of the tach .... but it couldn't make the
tach move ..... although if I move the cable around a little I get the
jumpy needle somewhat.

Your suspicions are well founded.  The square end of the cable engages a
square hole in the spindle in the back of the tack to drive it.  When the
cable end gets rounded off it can just spin free without driving the tach.
When you wiggle the cable a bit the cable end can misalign and bind a
little in the drive spindle and make it turn a bit causing the needle to
jump a little.  First thing is that you will need a new tach cable.

>Now what????? I am assuming that the tach innards themselves are screwed
up, but the only way to get the thing apart enough is to drill out a
punched tube end holding a face plate ....

Whoa there.  You probably don't have to remove the face plate.  I assume
you are saying here that the needle fell off the hub and that the hub is
still attached to the small shaft.  If you want to remove the face plate
you have to pull the hub off the shaft (press fit parts).  Otherwise just
glue the needle back on to the hub with a little 5-minute epoxy.

Then you want to know what caused the problem in the first place, because
the rounded end on the cable is likely just a symptom, and you need to fix
the tach so it doesn't happen again.  It sounds like the innards are a
little worn and ran dry and started binding and squeeling causing stress on
the drive cable end, resulting in the rounded end of the cable.  Most
likely all it needs is a little light oil to make it happy and quiet and
all will be well again.  For test purposes you can use the good square end
of the cable to drive the tach.

Since your old cable is shot anyway, clip a couple inches off the bottom
end of the cable core, chuck it in your reversible drill, and use that to
drive the tach with the drill in reverse at top speed (running
counter-clockwise to match the sweep direction of the tach needle).  The
tach cable runs at the same speed as the speedometer cable does in 4th
gear, with a 5:12 reduction from engine crankshaft speed.  So if your drill
runs at 900 rpm the tach should show about 900x12/5 = 2160 rpm, give or
take a little.  Once you get it working well enough you can worry about how
to calibrate it.  First thing is to be sure the input spindle of the tach
is actually turning, and put a drop of light in in the bearing area, as
well as a little oil on the needle spindle bearing.

The face cover glass and trim ring come off with a little twist, and the
innards are removed from the case by removing two small screws from the
back adjacent to the input spindle.  The mechanics of the tach are quite
simple, just two basic movong parts.  The input spindle has a bar magnet
that spins.  The needle shaft (with a small clock spring) carries a metal
disc that faces the spinning magnet.  The spinning magnet has a drag effect
on the metal disk and pulls it around in the direction of the spin.  The
drag is proportional to the speed of the spin and pulls the indicator
needle up accordingly against the increasing resistance of the clock spring.

So you should check for free spinning operation of the input spindle and
for free rotation of the needle spindle.  Then put a very small drop of
light oil on the shafts in the area of the bushings and give it another try
by spinning with the electric drill.

For the MGA 1500 and 1600 (with 4.300:1 ring and pinion final drive ratio),
in operation the needle on the tach should run exactly parallel to the
needle on the speedometer in 4th gear, as the input cables turn at the same
speed.  The speedometer/odometer does 1450 turns per mile, and that number
is printed on the face of the speedometer, and that equates to 3480 rpm at
60 mph (one mile per minute).  The Mk-II has a 4.100:1 ring and pinion
gear, so the speedo/odo should turn about 1383 turns per mile (+/- a few),
and that number should appear on the face of your speedometer (maybe 1380).
 In this case the tach needle will read proportionately lower, about
3480/1450x1380 = 3312 rpm at 60 mph.

Now before you can calibrate the tach you need to know the exact speed of
your electric drill.  To get this number you use the drill to drive the
speedometer and get the readout of the odometer for a given period of time.
 Have a clock handy, run the drill at top speed, and record the distance
acrued on the odometer, estimated to the nearest 100th of a mile, when run
for an extended period of time (I suggest 10 minutes).  Take the turns per
mile number printed on the face of the instrument times the number of miles
acrued on the odometer and divide by the number of minutes run, and you get
the exact speed of the electric drill.  Example:  6.65 miles acrued in 10
minutes with 1380 turns per mile for the odometer would be 1380x6.65/10 =
918 rpm for the drill.

Now the speed reduction in the engine via the timing chain from crank to
cam (2:1) and the tach drive gear at the back of the cam (6:5) is the same
as the speedo drive ration in the transmission (12:5), which is what makes
the tach cable turn at the same speed as the speedo cable in 4th gear.  So
you take the speed of the drill times that ratio to get the speed the tach
should be reading and position the needle on the tach such that it will
register that speed with the drill running at top speed.  In the example
above it would be 918x12/5 = 2203 rpm.

You can check and/or calibrate the speedometer needle reading in the same
fashion.  In this case 918 rev per min / 1380 rev per mile x 60 mi per hr =
 39.9 mph (as 60 mph = 1 mile per min).  For further information on the
operation of the speedometer and maintainance and calibration, check out
this web page:

Side note:  The speed of a cordless drill can slow down as the batteries
become depleted.  For the purpose of this exercise it would be better to
use a corded drill to maintain a constant speed.

Good luck and God speed,

Barney Gaylord
1958 MGA with an attitude

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