71' 240Z Key Running with Key Turned Off (Not Dieseling)

[HappyZ]

Any help is appreciated...

My 240Z starts just fine, but it keeps running after I turn off the key and remove the key.  It runs smooth as if I never turned it off.  The car does stop when I turn off the key and remove the lead from the alternator.  Another thing is I thought 240Z had to have the tachometer connected to run, but I could be wrong.  It does run without the tach.  I also swapped the ignition switch and still have the same problem.

Any help pointing me in a certain direction is appreciated.  Ideally, someone that experienced this before.

[SteveJ]

Let me guess...the car has an internally regulated alternator. It's backfeeding through the lamp wire to the ignition. 

MSA sells a plug that will take care of that.

Edited December 2, 2021 by SteveJ

[Zed Head]

I think you meant backfeed.

MSA has been backordered on the backfeed prevention conversion plugs.  Might need to show him the diode fix.

[HappyZ]

Thanks @SteveJ

I'm not sure, but I think it's an original type with a voltage regulator (see pics).

[HappyZ]

Sorry for the ugly pics and freaky wiring colors, but the PO sprayed the entire car with white paint.  The overspray still shows it's ugly self on occasion.

[HappyZ]

If it's not an internally regulated alternator, what could be the problem.  I really am stumped, so anything is helpful.

[SteveJ]

Let's break things down. You said the car will shut off when you unplug the alternator. That means the alternator is backfeeding the ignition. That would be on the white/black wire attached to that plug. You may want to use an ohmmeter to measure the resistance between the white/black wire at the plug on the alternator and the black/white wire on the wiring harness. You'll need to backprobe the connector to get to the black/white wire. If the resistance is low, there is evidence of your problem. You will probably still need a diode to prevent the backfeed.

Someone has definitely altered the wiring on your car. Nissan definitely NEVER used a wire nut on the Z wiring... (As an aside, I asked my wife if she could see what was wrong with the picture of the voltage regulator. She picked out the wire nut, too.)

How long have you owned the car? It sounds like the IPO left you some surprises. What ignition system do you have on the car? I'm guessing it's not points. Anyway, you can wire the coil so that the tach doesn't work, and the car will run. Post some photos of the area around the ballast resistor and coil. If it's as I suspect, I have a video to help you sort that out.

@Zed HeadThat was autocorrect on my phone.  I'm back at my computer now, so improper words are my own ducking fault.

 

[Zed Head]

With the external regulator doesn't the backfeeding happen at the regulator?  It's mechanical, I don't really know how the things works.  But, the "break" should be there, in the regulator, either a diode or a relay of some kind.  Maybe it's just a sign of a bad regulator.  Might be worthwhile to examine the plug and wiring very carefully, and the bottom of the regulator.  There is exposed circuitry underneath, if I recall right.  Might just need a cleanin'.

[SteveJ]

11 minutes ago, Zed Head said:

With the external regulator doesn't the backfeeding happen at the regulator?  It's mechanical, I don't really know how the things works.  But, the "break" should be there, in the regulator, either a diode or a relay of some kind.  Maybe it's just a sign of a bad regulator.  Might be worthwhile to examine the plug and wiring very carefully, and the bottom of the regulator.  There is exposed circuitry underneath, if I recall right.  Might just need a cleanin'.

 

Actually it's not too important to find the exact place for the cross connection. The diode could be incorporated at the plug at the alternator.

Of course, it might be easier just to replace the VR if it's bad, hence my suggestion of testing the resistance at the wires.

[HappyZ]

Thanks guys!

A bit of background.  I bought the car and only started it once, but couldn’t drive it…transmission and clutch were shot.  I put a clean L28 and 5speed in it.  The dash has been out since paint as I’m installing a vintage air in it…all of that being done by a local guy (Locked & Loaded in Daytona).  The wire harness is mainly on the drivers side floor, but everything is connected.  The A/C is almost done, but we didn’t want to put it all back together until we were confident the harness was good.

I’ll go thru your emails tomorrow and report back.  This the last leg of an 18 month project, so I’m super motivated.

thanks again.

Edited December 2, 2021 by HappyZ

[HappyZ]

it occurred to me that with the new AC installed I'll need a stronger alternator.  I have a Bosch alternator in my other 240Z, so I think I'll swap it into the Z having problems.  I also have the plug the connects to the voltage regulator plug.  I think that may fix the problem and I need the other alternator anyway.  More updates tomorrow.

[Zed Head]

It will remove the external regulator from the picture.  You'll know immediately.  Good luck.