by Cutlass

If you want to hook up a tachometer to your DIS-equipped car, be aware that the DIS system has unique qualities that will dictate exactly which tachometers can be added to your vehicle. The DIS fires a spark twice in each cycle. Once at the top of the compression stroke when it's supposed to and another at the top of the exhaust stroke. The second spark is "wasted." The problem with hooking up a standard tachometer to it is that it will read twice the actual rpm. So at 1000 rpm, the tachometer would read 2000 rpm. In order to put a tachometer in your car, you will need to get the Tachometer and Tachometer Drive Module from your GM dealer. This module, basically, cuts the signal from the coils in half so it reads accurately. You can get an aftermarket tachometer much cheaper than the GM one, but make sure it is compatible with DIS ignition.

Credit: Vincent Ciulla, Auto Repair Technician,