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Gauge restoration

Subject: Gauge restoration
Date: Sat, 14 Apr 2001 20:17:56 EDT
Hello Gang,
   This weekend I decided to complete the rebuild of the gauges
in my '70 B-GT.  A few fellow motor-heads have said "Go for it!"
Others to have told me, "b&NEVER attempt to repair gauges" and
"b&that job should be left to the professional."  While the later may
be true, I tend to have problems with the word "never" -- unless the
patient about to undergo surgery is irreplaceable.
   Having rebuilt the gauges in my '63 e-type a few years ago, I
went after the grimy MGB gauges with little fear.  In a few days
(after I reinstall the digital camera driver on my computer) I will
post the "before, during and after" pictures.  After completing the
repairs, I made a few measurements on the instruments.  These
measurements are the reason for this post as they may be of some
interest to of you.
   The fuel and temperature gauges are generally thermal devices.
That is, a bimetal strip in the gauge flexes in response to heat,
caused by a coil of high resistance wire wrapped around the
bimetal strip. The bimetal strip is attached to the gauge indicator
hand that moves across a scale.  Below are the measurements
and observations I made on the restored gauges.

Temperature gauge:
Voltage Current     Indication
2.0v        30ma        B= between C and N
4.0v        60ma        N
5.0v        75ma        B= between N and H
6.0v        90ma        H
7.0v        100ma   high calibration dots ( .. )

Fuel gauge:
Voltage Current     Indication
4.0v         60ma       Just above E
6.0v         90ma       3/8
7.0v          105ma ~ 1/2
8.0v          120ma 3/4
8.5v        130ma   7/8
9.0v        135ma   F
10.0v       150ma   >F offscale

   Ohm's Law says E=IR or Voltage equals current times resistance.
Rearranging, R=E/I. Therefore, we can compute the resistance of
the (hot) coil in each gauge thus:

R(temp) = E / I = 4.0 / 0.060 = 67 ohms

R(fuel) = E / I = 9.0 / 0.135 = 67 ohms

   What this means is that the resistance of the gauge coil is the
same and the only differences are the point where the hands attach
to the bimetal strip and the scale printed on the gauge face!
   You will notice that the fuel gauge scale is nonlinear (3/4 is not half
way between 2/4 and 4/4).  This is because the fuel tank geometry
is nonlinear.  The gauge is!

   I also rebuilt the speedo and the tach but that is the subject of
another post and is best supported by a website choked full of
   Okay, so perhaps you didn't want to know how I spent my
weekend.  Yet, it could be worse.  I could have posted the starting
grid for tomorrow morning's Formula One race. :-P



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