'73 240z - No spark from coil

[StinkyCheese]

I've had my '73 240z for a couple of years and its always run pretty decent, but it overheats in city driving. Last fall I was driving and hit some rush hour and it got pretty hot (about 3/4 of the gauge). After a while I got out of traffic and it started to cool down some, but about a mile from my house it started running really rough and 1/4 mile from my house it died completely. I coasted downhill to my house and pushed it into the garage.

It wouldn't start, but I was so tired of the overheating I took the radiator out and got it rodded out. I also installed a fan shroud. After I got all that put back together, I've been trying to get it started, and it just won't start.

I took the coil wire off of the distributor and held it next to the valve cover while my daughter cranked the starter. I expected a spark to jump across to the valve cover, but there was no spark. Doesn't that automatically mean the coil is bad. When it WAS running, it would hesitate when it hit 3500-4000 rpm. Doesn't that indicate a bad coil, too?

I went and got a "IC70" coil from NAPA today. That's what their computer spit out as a replacement. Its a little skinnier than the old coil, but I tried it anyway. I don't get the coil-to-valve cover spark with this new coil either! What else can I try?

[FastWoman]

I'm assuming you have a points ignition???

When you turn your ignition on, do you get +12V to the (+) side of the coil? (You should.)

If you do, then what happens if you disconnect the wires from the (-) post, connect another loose wire, and then CAREFULLY tap the other end to ground? Do you get a spark then?

There's probably a ballast resistor involved somewhere in there too, but I don't know how it's wired. Nevertheless, you can determine if your coil is good by manually tapping a ground wire to the (-) post of the coil.

If your coil has +12 to the (+) post and tests good, then your next step is to look at your points. Are the points and condenser new/recent? Are they properly adjusted?

That's about all there is to the ignition, at least up to the point of the coil wire.

[olzed]

As Sarah says. Points . Condenser.

Brian.

Edited February 25, 2011 by olzed

[StinkyCheese]

Thank you SO MUCH for replying. I'm _this_ close to hauling it to a mechanic, but I'd so much rather work on it myself and learn...

Yes, I get 12 V on the + side of the cooil when I turn the ignition on. I took off the wires from the - post and put my test wire on. I touched it to ground and nothing happened. I did this with the ignition off and on, no reaction. Should I get a spark?

I attached a picture of my too-small-for-the-bracket coil...

[FastWoman]

Yes, a spark should jump between the coil wire and ground whenever you tap the wire from the (-) post to ground -- and also have +12 to the (+) post of the coil. Maybe your coil is bad out of the box.

There are at least two other possibilities, though. First, maybe you have a bad coil wire. What happens if you put one of your plug wires on the coil and try sparking it then? If you get a spark then, you should treat your Z to a new set of wires -- NGK, spiral-wound, of course!

The second possibility is that you have insufficient current to the (+) post of the coil. To test this, try jumping a piece of wire directly from your battery to the (+) post of your coil, and see if you can get a spark then. If so, then you'll want to trace down a corroded connection somewhere.

And again, I don't know how your ballast resistor wires in. Perhaps the (+) feeds to the resistor and then to the coil? So maybe you have a bad connection at the resistor? Dunno... I frankly don't know how those ballast resistors are wired and don't have a schematic at hand.

[SteveJ]

John,

You have a ballast resistor in your ignition circuit. You should have 12 VDC on the positive terminal of the coil when you try to start the car and 9 VDC with the key in the RUN/ON position.

Here is a quick rundown on how the stock circuit is wired.

Starting: The W/R wire carries voltage from the battery into the keyswitch. When starting the car, the switch connects the G/W to the W/R wire. The G/W wire goes into the tach, and the B/W wire goes from the tach to the positive side of the coil. The black wire comes out the negative side of the coil and runs to the positive side of the points and to the condensor and on to ground. The negative side of the points also runs to ground.

Running: The W/R wire carries voltage from the battery into the keyswitch. The switch connects the B/W to the W/R wire. The B/W wire goes into the ballast resistor, and the G/W wire comes out. The G/W wire goes into the tach, and the B/W wire goes from the tach to the positive side of the coil. The black wire comes out the negative side of the coil and runs to the positive side of the points and to the condensor and on to ground. The negative side of the points also runs to ground.

Having said that, failure points can include the condensor, the tachometer, and the points staying closed or open. (See page EE-35 in the factory service manual.)

Also, please let us know whether or not you have stock points or if someone installed a different ignition system (Pertronix, Crane, MSD, HEI, etc.). If you're not sure, take off the distributor cap and rotor, take a picture of the distributor, and post it.

Also, make sure the condensor is on the negative side of the coil. It looks like it's on the positive side in your picture. By the way, the condensor and ballast resistor look VERY old. Consider replacing them.

It would help, too, if you would tell us if the car is an automatic or manual. The automatic had a dual point distributor.

[StinkyCheese]

Wow, thanks for taking the time to help!

OK, I tried hooking the + side of the coil directly to the + side of the battery. Still no spark.

I hooked up a spark plug directly to the coil and it doesn't spark either... The spark plug wires I got from the Nissan dealer, but I don't know the brand. They're about 6 months old.

SteveJ, you say, "You should have 12 VDC on the positive terminal of the coil when you try to start the car and 9 VDC with the key in the RUN/ON position". My car isn't running, but the key is in the RUN/ON position, and I get 12V on the + of the coil. Does it have to be running for the ballast resistor to be used?

Thanks for the description of the circuits. I tried to draw it up, but I'm confused..

I have a standard transmission.

I took a picture of the dist. I made some notes on the pictures, I'd appreciate it if you could take a look.

[Zed Head]

Just wandering by. I notice that you've measured 12 volts on the + side of the coil, but didn't say what you measured on the - side.

[StinkyCheese]

I get 12V on both the + side and - side of the coil.

[Zforce]

If you are getting 12 volts at the -side of the coil your points are closed, coil fires when current stops flowing. Take a small screwdriver and open the points, I bet you get a spark, in that case try resetting your point gap. Also the rubbing block on the point set needs to be lubed with point grease, if you don't your point dwell will continue to deteriorate till the car stops again.

[5thhorsemann]

That second condensor under the coil, is that stock? mine doesnt have that. Of course I installed the pertronics electronic ignition, but there was only the distributor condensor. Try loosing that rascle and see if she fires.

[FastWoman]

Well, I admit I'm not really a points-ignition kinda gal, although I've had them before. However, it seems to me the coil is bad. I'm guessing it's got an internal short, as there seems to be conductivity between the (+) and (-) (evidenced by +12 at both terminals). I mean you've isolated the thing and tried firing it manually, energized directly off the battery, and it won't fire.

Another little test you can do is to unhook the coil, and hook the test leads of a mechanical multimeter (with a needle) to the wires that would otherwise be connected to the coil. Put the multimeter in the DC volts mode, at a scale that is appropriate to 12V. Now crank the engine. The needle should go up and down repeatedly as you crank the engine. If that happens, your points are good -- maybe not adjusted optimally, and maybe not in the best of condition, but still functioning.