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## Gauge restoration

 To: mgs@autox.team.net Gauge restoration ValveCurtain@aol.com 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 pictures. 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 Forza! rick /// /// mgs@autox.team.net mailing list /// (If they are dupes, this trailer may also catch them.) ```
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