I have a 2003 spyder eclipse gts 3.0l how do i install a after market tachometer?

It's true that aftermarket electronics can goof up engine management electronics if improperly connected. Never use the ECM (PCM) harness as a supply for 12 volts or ground. Conversely, never supply 12 volts or ground to the ECM except from the original wiring in the harness. Some wires have voltage or ground only intermittently to the ECM as data, not power.

If your tachometer has instructions for installation in a car with DIS (no distributor), you will have to obtain an electrical diagram for your car. Some ignition systems have a tachometer output, others don't. Cars that don't have a tach output may have a wire that goes from the ECM (PCM) to the ignition module that can work for a tach signal. This set-up requires adjusting the tach (per instructions) to operate with this type of signal.

dttech suggested referring to the instructions of the tachometer that you wish to use. You can find instructions online for the better brands of tachometers, such as Sun.

Good luck from Detroit, MI.

I really cannot tell you how to do that. You need to follow the instructions that came with the tachometer.

Please note: I would be very cautious about connecting aftermarket equipment into your computer input or output circuits. These things have been known to cause all kinds of drivability problems as well as make it impossible to pass an emissions test (if it is required where you live).

I have personally seen aftermarket add-on equipment cause everything from engine cylinder misfires to transmission shifting problems to engine stalling and no-starts. I do not recommend using any at all. However, there are usually no problems created by "stand alone" accessories that are powered directly from the battery and do not require splicing into any of the computer circuits.

My professional recommendation on this is that if you think you need a tachometer to operate your vehicle, or you just like the looks of a tachometer bouncing up and down, then you would be money and headaches ahead to buy a vehicle that is equipped with a tachometer from the manufacturer - you may be opening a can of worms that you do not want.

This is just my opinion based on 37 years of experience diagnosing and repairing automobiles (and having to clean-up aftermarket add-on messes) - do with it as you wish.