Revs But Wont' Run......

[kenmataya]

I was driving my 70 240z on the freeway, when its suddenly seemed to be out of gas ( it wasn't ).  Towed it home, put some more gas in just to be sure, still won't run.  It turns over, and will actually rev up - as long as the starter is still engaged.  As soon as the starter is off the car dies. Any help appreciated here.

 

Thanks,

Ken

[SteveJ]

My first guess is there is something wrong between the ignition switch when it's in the ON position and the coil. The emphasis is on the ballast resistor. If you know how to use an ohmmeter, take the wires off of the resistor and measure.

 

When the key is in the START position, you bypass the ballast resistor.

[Persimmon240]

Your non-run symptoms except when the started is engaged sounds like your ballast resistor has failed.

 

The same thing happened to my wife after an abortive shopping trip in a 510 we had 25+ years ago. She walked home. Yes, we are still married... the "Dime" returned to the earth.

 

Hopefully I didn't get my wires crossed. Good luck in solving your problem. 

[rossiz]

that exact symptom happened to my buddy's 240z back in college... i used to tune up my friend's cars for fun - had never worked on a datsun, only american muscle - and after cleaning the plugs, filters and fiddling with the carbs it would start and rev but as soon as the key was released to the 'run' position it would die. turns out i had knocked off a wire by the coil - found it hanging there, spade connector i believe, and once plugged back in all was well. never figured out exactly what wire that was, as i never worked on a z car again until i bought one a year ago...

[240260280z]

As Rossiz said, the voltage to the ignition coil takes two paths:

 

1. Starting... with the key to start a direct path from the battery to the coil is taken to maximize spark. the voltage to teh coil is ~ 12V

2. Run...with the key in the on position, the path is now through the ballast resistor which drops the voltage applied to the coil to ~8V. the lower voltage keeps the coil cooler and increases it's life.

 

Anothe problem in this area is the ignition switch right behind the key. Sometimes it just wears out and will work intermittently when it starts to fail.  In my case it would turn the starter when I tried to start but it would not send voltage to the coil. I managed to make it work by turning the key ~ 80% of the way to full stop when starting and it seemed to get good contact on two conductors (starter and coil) when doing this.

 

 

For your hands-on checks you have a few methods:

 

1. side of road spark check: Pull a plug wire and stick a spare plug or something conductive in it then lay this on the valve cover. Have a friend watch for spark as you crank in start then release to on. If the spark dies as soon as you release, then it is the ignition.

 

2. garage check:  Have a friend with digital volt meter or 12V bulb measure voltage to coil positive when cranking and when the switch is ON. there should always be a voltage.

[240260280z]

[kenmataya]

what resistance should i see at the resistor?

[240260280z]

A ballast resistor is usually between 1 and 2 ohms.

[Zed Head]

If the resistor has failed you'll get infinite resistance.  No current flow allowed.

 

A quick check would be to run a jumper wire between the two On and Start posts, or just move the On wire over to the Start post.  It won't hurt anything for a short while, the coil will just get hotter if you leave it.  If it runs that way, then you'll know the resistor wire is broken.

 

Edit - not sure, since my ballast is gone, but you could probably bridge those posts with a screwdriver for a really quick check, if you have a helper.

[kenmataya]

OK, voltage when cranking, none when on....  3 ohms on the ballast resistor.  ignition switch??

[kenmataya]

Ok, its running... Switched the blk/wh to a different spade on the coil.... Bad connection? This car has pertrononix, so wiring is weird. Theres one blk/wht directly to coil, another to the bottom of ballast. Theres a red wire attached at bottom of ballast that leads to dizzy. Top of ballast connects to grn/white that goes to front of car?? Coil reads 12v when running.

[kenmataya]

coil is not even attached to the ballast....