4g61t.org

'90 DSM:

- Tacho Interface monitors the ECU/Power Transistor's switching of both coil primaries, and adds the signals together so there are four electrical pulses output for every engine cycle; one for each cylinder fired. This signal is sent to both ECU, and tachometer via the noise filter.

- The ECU recieves the raw Tacho Interface signal on pin 109, and uses this to determine when to turn on the fuel pump. A failure in this signal reaching the ECU = no fuel pump.

- The Noise Filter is a simple RC circuit. It places a current limiting resistor in line between the Tacho Interface and the tachometer, and a noise filtering capacitor between the signal and ground. If the noise filter is not grounded (bolted to the IM) the tach will not work. Notorious failure part on the '90 model car.


'91-94 DSM:

The basic operation for the later DSM is practically (but NOT) identical, only the Tacho Interface and Noise Filter are housed inside of the (8 pin) Power Transistor. The catch in the circuit is that the later '91+ ECU provides a reference voltage on pin 109, and the Power Transistor grounds it out to produce the square wave signal for the tach (as well as fuel pump activation).

There's 2 parts need to get the tach to work. The 90 style tach interface on the coil packs and the noise filter.