So, I've got my '79 F-100 with (I believe) Duraspark II set up with a Trisco tach from princess auto. I'm on my second one, and it stopped working. First one didn't work at all, except for the backlight. I believe this was due to me wiring it into the fuse panel. One of the empty fuse spots was grounded when the ignition was off, so I used it for ground, but it turns out that it turned to hot when the key was in the run position somehow. Anyway, I've got it grounded to the bolt holding the fuse panel to the body now, and that seems to work. Now, the first one's lights work great, but I got absolutely zero response from the tach when the engine was running. I've got the tach green wire hooked up to the terminal on the coil that says "tach test" and it didn't work at all. So, I exchanged it, and popped the new one in, fired it up, and it worked! Only ran it for about 3 or 4 minutes, turned it off, and went inside. Next morning, I come out, fire the truck up, and no response from the tach at all. LED backlight still works great though. is there some sort of fancy wiring that I need to do for the duraspark II with a tach?

 

Nothing fancy needed. If you want to use less wire and want to cleanup the engine bay a bit, you only need to run the wire to the duraspark box. I don't recall which wire it is but I looked at a diagram and figured out which one it was that comes from the coil and tapped into it. The other wires are just key on 12v, lights and ground.

 

Ya, that's what I thought. Yup, I've got all the wires hooked up to those spots. The coil wire is hooked up (at the coil) to the green wire from the ignition module, which goes to the coil terminal called "tach test". I'm just wondering if there's an issue with the coil wire, like a short. Don't see why the tach would dtop working overnight otherwise.

 

Sounds like it could be a ground issue? On all my tachs I ground to the screw on the bottom right of the fuse box. It's about an Inch long, works great. I run the power and accessory (light) wire into the fuse panel (power to radio fuse, accessory to any fuse that has Power with park lights on) as for the green wire, I just strip the wire a couple inches back from the coil (green wire) and run it into the cab behind the brake booster. Hope this helps. Also, you get what you pay for from princess auto... Auto value has a good one, its large therefore more acurate. I've bought 3, never any issues.

 

Just for reference. Plus there are usually dip switches that set for number of cylinders.

 

Ok. I think I got it figured out. Turns out I'm not an entire electrical dummie, but I am forgetful. And the tach is just poorly manufactured. My alternator hasn't been charging well, the battery tested OK, so I got to looking, and when I replaced the positive battery cable, I forgot to put one of the wires back onto the solenoid. Not sure what it's for, but the battery has started to charge now. It's not charging quick though, so I am trying to figure out whether that's a voltage regulator or the brushes and bearings and such. Anyway, tach was working sporadically before I fixed that wire, and same after I did that. I turned the truck off, and the tach stayed stuck at 600 RPM. I was confused, and started poking and prodding... When I pushed on the gauge face, the tach went back to 0. Curious, I turned the truck back on, kept my thumb pressed on the gauge face, and revved the engine. Tach works great. Took my thumb off, it was stuck at 2700 RPM. So, I think it's poor manufacturing on this one. Or, something did get damaged electronically when the other wire that I forget to connect grounded out against the chasis. Anyway, I'll return it again, and see if a new one works. My problem is, I cut a hole in the dash that is a perfect press fit for this exact tach... So, not super easy to fit any old tach in there.

 

Try charging up the battery with an outboard charger overnight so it's got a full charge.

 

Most gauges are one of a couple standardized sizes ... you could probably mount another one in the same size hole pretty easily.