I have 93 Dodge D250. I found the wiring harness under the dash to hook up a tach. I have some old gas tachs from my collection of muscle car stuff, but they do not work in the truck. (hey i had to try) They power up and the needle goes below zero and stays there. Some one here said to check the signal wire with a voltmeter. The voltmeter reads around .1 volts at idle and .12 at speed. Is this what it is supposed to read? the voltage does not pulsate at all, just steady at the afore mentioned voltages. I thought the tach picks up a pulsating signal to read rpms. What should voltage be and what will it look like on the voltmeter? Is the voltage steady and the currnet pulsate? Is my ecm maybe bad? I believe the crank sensor is fine, my ac and criuse still work. I would really like to put a tach in this beast but dont want to buy the wrong one. Im also going to install boost and pyro but i want the tach to be at least 3 inches so i can see it at a glance. Looking at stewart warner gauges, as they have a 0-3500 diesel tach but i want to know if it will work with my truck. Stewart warner has shown no interest in returning e-mails or phone calls so far. thanks in advance for any help!!