Not sure its all part of a loom i made I guess its tracking across to the amp feed I think i will try and fit a suppressor on the amp feed
The problem is that the ignition system correctly produces a 'ringing ' voltage as the points open: hundreds of volts on the connection between the distributor and the coil. When you extend the lead to a tacho in a front engined car the wire is often less than a metre long so there is not a problem, When you stick a 5 metre long wire on it this is a quite effective radio antenna, transmitting into the loom and any other wires going the same way as the coil feed. Separating the tacho wire from the rest of the loom may help reduce the coupling. Using screened wire might help a bit as well, as the induced current goes into the screen which can be connected to the body at both ends. Connecting a resistor in series with the tacho feed at the coil end may help as all you want is the 12 volt pulses from the breaker reaching the front, not the high voltage ringing from the coil. The combination of the resistor and the wiring will cut down the interference. I dont know for sure but I suspect that a resistor of 1000 ohms or a bit more might keep the rev counter working. It is important to keep the high voltage on the coil or your spark will get weaker, as in fact it is weakened slightly just by connecting the tacho. My solution which is to have a tacho setup with a remote sending computer in the engine bay is another method .. the pulses only go a couple of inches to a counter. But I am pretty sure its unique because I made it myself and the manufacturer of the multi-function dashboard display I use (PLX Devices) does not sell a tacho module.
