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Shift light wiring question

490 views 2 replies 2 participants last post by  thunderjet302 
#1 ·
I'm going to be installing an Auto Meter shift light/digital tachometer: DPSS Tube, Black, Level 1 on my Thunderbird. The ignition coil wiring is the same as a 86-93 Mustang. The shift light has a three wire hookup: red to 12v switched ignition, green to coil negative, black to ground. I'm running the green wire to the coil negative wire and the black wire to a chassis ground. Could I just run the red wire to the positive side of the ignition coil? After all it is a 12v switched ignition source. I figure since I'm running the wire to the coil and already there that I might as well tap into the 12v positive side of the coil. That should work fine correct? I helped a buddy install a MSD shift light on his 89 Mustang several years ago and we followed the wiring http://static.summitracing.com/global/images/instructions/msd-8952_frm28827.pdf that had us connect the green wire to the coil negative side, the red wire to the coil positive side, and black to chassis ground. It worked fine.
 
#2 ·
If they have reasonable bypassing inside the shift light, and harmful pulses are not on the plus coil lead, it will work.

I've never looked at a T-Bird positive coil lead for noise, and I don't know what they did inside the shift light to protect it from transients on the 12V line, but it should probably work. A designer would have to be pretty slack to build something so sensitive to noise that you couldn't use it the way you describe.
 
#3 ·
The wiring for the 86-88 5.0 Thunderbird coil is identical to the wiring for the 86-93 5.0 Mustang. So if the positive lead for a 5.0 Mustang coil doesn't have any noise I would assume it should work fine.

If you think it would be any issue I can always tap the radio harness for a positive power switched voltage source.
 
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