Using a 79-82 Turbo 4 Tach

Introduction:

I always find it cool when you can use a factory component and some ingenuity for racing instead of an aftermarket part. My stock 4-cylinder tach only went to 6-grand. My combination needed to be shifted above that point, but I didn't want to alter the inside appearance of my gauges. I found an 8-grand tach that matched all my other instrumentation in a turbo 4 Mustang. This tach was not adjustable between 4, 6, and 8 cylinder cars like the 6-grand tach, so it was reading twice as high as it should when used in an 8-cylinder car. After some research I figured out how to modify it to work in my application. I even calibrated my tach to make it very accurate. So follow along and you can perform the same modification.

Tools Required:

  • Philips #2 screwdriver
  • 1/4" nutdriver
  • Soldering iron and solder
  • Ohm meter, preferably digital
  • 100K ohm resistor

Step-by-Step Procedure

1) Set engine RPM using the idle screw to 3,000 RPM using another tach if possible. Or using a 4-cylinder tach set to 6,000 RPM.

2) Remove instrument cluster from dash. Remove tach from instrument cluster.

3) Remove back of tach to reveal the circuit board.

4) Locate rpm calibration resistor on circuit board.

5) Measure resistance across resistor with an ohmmeter.

6) Solder 100K ohm resistor on to circuit board at the same terminals that the adjustable resistor is soldered.

7) Place the ohmmeter back on the terminals and adjust the adjustable resistor to 1/2 the previous measured value.

8) Hook tach back up to factory wiring and check it, the reading should be 3,000 rpm with the engine running.

9) Put tach back together, reinstall in instrument cluster, and put dash back together.

10) Reset idle to preferred setting.

Alternative Calibration Procedures:

1) Check with another reliable tach - I had an aftermarket tach that was on my Granada that I hooked to check the rpm readings. Check at idle through 4,000 rpm and assume rest of the scale is correct.

2) Non-Contact Optical Tachometer - A piece of equipment like this has an accuracy of +/- 5 rpm, therefore this is the most accurate calibration method that is necessary. This instrument uses a reflective strip and a light to measure the rpm. I had my wife hold the gas steady at different increments while I checked the rpm with the stroboscope on the crank pulley. Use rpms of 1000, 2000, 3000, and 4000 and assume the rest are right. My tach was right on when this method was used.



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