From the manual:
12) Before you put the back of the tach back on, you should hook it up to your car and adjust the meter so that it reads 4000 RPM and 1000 RPM properly. This must be done because each tach meter movement is slightly different. See the end of the manual to read about how to make this adjustment. Note that the meter movement MUST be grounded to the frame ground of the car, or the tach board won’t work. You may have to run a jumper wire from the frame of the car to the metal of the meter movement frame to make the ground connection while adjusting the tach board. I usually will run three jumper wires from the tach as follows: a) The case of the tach to the case of the alternator to get a good ground b) The long stud on back of the tach to the large +12V terminal on the alternator (Be very careful not to allow this jumper to touch the car or the case of the tach, as it will spark and cause problems if you let it touch ground). c) The short stud on the back of the tach with the spade terminal should be jumped to the minus side of the coil (unless you have a MSD ignition, in which case it should be jumped to the MSD tach output). After hooking up these three jumpers, and starting the car, then the tach needle should move as you rev up the motor.
End of manual:
Adjustment:
The 4000-RPM ADJ pot adjusts the gain of the tach board. If your tach is reading too low or too high at upper RPM’s, but is correct when idling, you may need to change the 4000-RPM ADJ pot using a small screwdriver.
The 1000-RPM ADJ pot is used to adjust the lower reading of your tach. If your idle reading is always too high or too low, you can try changing the 1000-RPM ADJ pot using a small screwdriver. The 1000-RPM ADJ pot can only bring the lower reading of the tach up, it cannot take the lower reading down.
If you try to adjust the tach board while it is connected to the wiring harness and the engine is running, make sure that you do not allow any of the circuit board wires or components to touch anything metal as you could destroy the board. Make sure that the unit has a good ground when adjusting it as well, using a small wire connected from the brass case to a good ground (such as a screw on the dash).
The sequence that we follow is:
1) Turn the 1000RPM ADJUSTMENT all the way counterclockwise for 15 turns. After 15 turns, the pot will just stay at the no effect (0) setting.
2) Now run the engine at 4000 RPM and set the 4000-RPM ADJUSTMENT so that the tach reads 4000 RPM. If you are uncomfortable running your engine this high, you can choose to adjust the tach at 3000 or 2000 RPM.
3) Now run the engine at 1000 RPM. On most tachs, the meter will read 1000 RPM and you will not have to move the 1000 RPM adjustment at all. If the tach is reading too low at 1000 RPM (or idle), then you can use the 1000-RPM ADJUSTMENT to bring the reading up.
4) If you did move the 1000 RPM adjustment, then you need to go back to step 2 and repeat if necessary.
5) If the 1000 RPM reading of the meter is too high, then you may be able to fix this by very gently moving the two springs on the meter movement in the direction that will make the needle go towards zero. This will put more force on the needle, which will make the needle read lower at low RPM. After moving the springs, you will need to adjust the 4000 RPM adjustment pot again as in step 2.
We use a dwell/tach meter in order to know what the motor RPM is. I have also noticed that older tachs are a little sticky, and it is helpful to tap the tachometer case after each adjustment to make sure the meter is reading correctly. Another good tool to use is a digital timing light with RPM readout. If your tach is extremely sticky, you may need to loosen the jewel adjustment screw on the meter movement. This will allow the needle to freely return to zero and to not stick.
