Yet another LED Problem thread
 

Hello, all

I searched and could not find this particular issue. Changed all my external bulbs (with the exception of the license plate lights) to LED's using the kit from Carmagic which includes the flasher relay and bulbs for the tach indicators. Works as it should as long as the headlight switch is off. Once pulled, the tach loses about 1000 rpm, and the turn signals do not work at all. Any ideas?

My guess is you goofed something with the relay wiring. Your tach should be mechanically driven, not sure how your doing rpms by turning the lights on, what happens at idle, does the car shut down? If your losing that many revs that sounds like your light switch is putting a huge strain on the alternator.

LED headlights as well?

My first guess is bad grounds.

I speculate the turn signals are actually grounding through the headlight filaments. and the tach light is actually grounding through the RPM signal or something weird like that.

Hard to mess up the relay wiring, as it is just plug and play, headlights are not LED's and the tach is for sure not mechanical. Any other thoughts?

I would be looking for an earth issue. You may have had a small current flowing backwards through an incandescent bulb allowing something to work but LEDs are diodes so the current can only flow one way.

Swap the indicator bulbs back as a simple test?