Wacky Tach, pegged all the way to the right.
Record Breaker
Joined: Nov 2011
Posts: 1,013
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From: Ontario, Canada
You might have a poor contact somewhere that makes the signal "flaky". On the early Jeep YJs, Chrysler made flex print circuit boards that the gauges connected to (I'm again assuming that Chrysler would use similar stuff on other models like the Dakota). With a bit of water on these (from leaky window or whatever), these boards would corrode, giving you a lot of connection problems - I would look in this area to make sure the instrument panel connections were really good before looking elsewhere.
Captain
Joined: Sep 2006
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mine started doing that then got worse then stopped working.
Record Breaker
Joined: Nov 2011
Posts: 1,013
Likes: 2
From: Ontario, Canada
If it completely stops, you're missing the +12 (could also be the ground) to the tach itself.
Record Breaker
Joined: Nov 2011
Posts: 1,013
Likes: 2
From: Ontario, Canada
The signal voltage will come from the speed sensor & the service manual usually has tests for various sensors. I don't know what technology is used in this generation Dakota speed sensor, but I'm assuming it's relatively simple, like what I have on my 92 YJ, so once you find the sensor, you'll see the wiring and you can trace that back to the source to make sure you have good contacts and good grounding. The service manual should be able to show you where the sensor is located, what the tests for the sensor are (if any), and the wiring diagram. You'll most likely need to make or buy a couple of test leads for your multimeter, that have alligator clips on each end. One end, you can clip to the standard test probe, and the other is useful in clipping to a connector pinout or even to a straight pin. I use a straight pin to stick through wiring insulation to make contact with a wire - when I pull it out, I put a little dob of RTV or the "liquid electrical tape" on the hole to keep out moisture.
The tach signal on vehicles with an ECU will be supplied by the ECU. It will be receiving input from sensors like the Crank Position Sensor and will then calculate the correct signal voltage to send to the tach - the service manual should be able to show you the ECU pinout & from that, you'll find one end of the wire (and one connector) and the other end should be at the tach, but follow the service manual schematic if it doesn't match my description - I'm just going by memory & this can get faulty with a bit of age.