Assuming a factory tach, the "resistor" on the firewall is actually a filter. You can bypass the filter temporarily to see if it makes any difference, but the problem is more likely in the tach head. Begin by checking the wire connections to the back of the tach head and verify that the black ground wire makes a clean, tight connection to shiny cab sheetmetal. The other tachometer wire colors are brown (the signal lead from the ignition coil) and pink (switched 12 volts that powers the tach head). You can quick check the tach signal using a test light clipped to ground. The test light will brighten slightly as engine RPM increases. A quantitative test requires a primary triggered tachometer or an oscilloscope or a frequency meter. Some DVOMs have an RPM tach function as well. If all of the external wiring is healthy, the problem is in the tach head. In some cases a visual inspection will show a burned component on the tach head circuit board that can be individually replaced if it can be positively identified. It is not uncommon for resistors and diodes to fail.
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