Birdseyeview
04-29-2019, 05:50 PM
I've got a twisted pair of 18 gauge wires coming from my engine rev-counter that I'm running over to a UMA brand 912 specific 3 1/8" diameter tach. The UMA literature talks about hooking up one of the twisted pair wires coming from the engine speed "output coil" (Rotax calls it the rev-counter) to an input pin on the tach connector and grounding the other line of this twisted pair to the same ground used by the tach. There are two grounding pins on the tach connector, both internally connected to each other and one of which should be connected to the engine ground. My engine ground is short coupled to a common ground bus behind the firewall and I'd probably use this as the grounding point. Does it make sense to run the second wire in the twisted pair to a ground, such as via the second ground pin in the tach? I had the impression that the rev-counter signal coming from the engine's tach sender is a once per rev DC voltage pulse (on one or both wires?). To ground one of these two wires, either directly to the engine ground, or common ground bus via the internal tach ground, just doesn't seem right somehow. I guess a DC signal has to be grounded somewhere but my Kitfox build manual schematic only shows one signal wire coming into the tach and the other is apparently not connected. I realize that the build manual schematic is somewhat generic and may not be applicable to specific instruments and UMA's wiring hookup description is cryptic at best. As a result this has got me a little confused. I may be off base in my limited understanding of what these two wires are actually doing. I plan to call UMA as well to confirm all this but I thought I might need to get some ideas and clarification from the forum experts so I can ask UMA more intelligent questions. Any thoughts?