240z turbo swap tachometer problem

[JessZ]

I did the L28et swap into my 240z and after wiring the stock tach up it only works when I turn the ignition key to start the car. The needle will jump to the correct RPM then go to 0 after the car is started. Also, when the car is idling or driving I can turn the ignition key a little past the start position without engaging the starter and if I hold the key there the RPM gauge will read properly, but then when I let go of the key and just drive/idle normally the RPM gauge stops working. Any ideas? Thanks.

[DCZ]

Get ahold of a 240z wiring diagram and look at the wires that go from the ignition switch to the coil. Basically (from memory) there is a black/white wire and a green/white wire. They both provide power to the coil but one of them (G/W I think) bypasses the ballast resistor during starting. Depending upon how the wires were jumpered when you did the swap you can provide power to the coil but the current may not necessarily be running through the tach to get there. In your case, the "start" circuit is going through the tach and the "run" circuit isn't. Remember that the 240z tach works via current, so any current going to the coil has to go through the tach for it to work.

 

Hope that helps. If you need more info post back and I'll try to look through my notes because I went through this when I did my swap.

[JessZ]

Thank you for the tip. I'll give it a try this week and get back to ya.