My son's 240Z just goes dead sometimes. The battery is good. It will start for a few days, then goes dead, no dash lights, no nothing. I've had four mechanics look at it and they all say they fixed it, but a day or so later it goes dead again. The battery is still charged, but even dash lights are out.

 

are you sure your ground strap is working. If nothing else try running the ground strap to a bolt in the body then to the starter (the best way to do it - ground thru starter is unreliable). Is the battery hold down frame secured and the battery isn't bouncing around the inside of the engine room. has the battery been load tested? my 83 had all sorts of gremlins (new battery). kept telling myself can't be - the battery is new. finally load tested and they gave me a new battery under warranty all problems solved.

 

@re you sure your floor band is working. If nothing else try running the floor band to a bolt in the body then to the starter (the best way to do it - floor thru starter is unreliable). Is power supply hold down frame secured and power supply isn't bouncing around the inside of the engine room. has power supply been load tested?

 

Use a voltmeter on the battery while trying to start it. The battery should be at 12 to 13 volts before trying to start it. It should only go down a couple of volts while cranking. If it's going down more while it's cranking, it could be a bad battery or a short. If it doesn't drop at all, then there is likely a flaky connection between the battery and the car (positive or ground).
If the voltage goes down a little while the car does not crank, it could be a bad ignition switch or solenoid. At that point, you can check for voltage from the solenoid to ground while trying to start the car.