Low voltage to coil...

[Turbo7MN]

So I've been fighting my ignition issue here for a while and I finally decided to stop being a retard and grab my multimeter on my way up to work on the Z. As I posted in my previous thread, the car has been sitting for 8 years, the ignition system worked fine for a while until we got the car started for a couple seconds... then everything died.

So at this point I've found that though the battery voltage is fine, it spins the starter plenty fast, I'm getting 4.5VDC to the positive terminal of the coil when the key is in the on position and about 7 when cranking. The tach no longer bounces when cranking. This is obviously not enough voltage to produce spark... at least not enough to run the car on anyway. We hooked a plug wire to the coil and grounded it to the valve cover and hit the positive terminal of the coil with 12VDC straight from the battery and we have concluded that it will produce spark when the sufficient voltage is applied.

Now it's just a matter of figuring out where I'm losing voltage... Another symptom seems to be that the external coil resistor gets incredibly hot... could this resistor be the problem? Thanks.

-Darren

[GILDIA]

I had the same problem with my 72Z, I by passed the BR and change the setup to pertronix (Ignitor + and a 3 ohms coil)

[Turbo7MN]

Thank you. I've looked into those and the Crane system as well... I'd like to at least get the factory system running because at this point I just want to get the motor in there now started. But if that's going to prove to be more trouble than it's worth, the pertronix setup wouldn't be a bad idea.

[240ZX]

Yah, there are a couple of avenues to take for an ignition upgrade but, any of them are better than POINTS....that's my opinion anyway! I choose the Mallory UniLite set-up but, the Pertronics set-up is just fine and I think is the least expensive way to go.

Tom

[Force Fed]

The older coils are made to run at low voltage, that's why the ballast resistor is there. W/o it, the stock coil will burn up.

[Walter Moore]

Darren,

I just went out and checked the voltage across the coil on my car. With the ignition on, but the engine not running I see 4.9V across the coil. The remainder of the voltage is being dropped either across the ballast resistor, the points, or the wiring in between.

I have been driving it all day, so I know that the voltage isn't too low to produce the necessary spark.

By the way, the voltage goes up when you crank the engine because Nissan bypasses the ballast resistor when the starter is engaged.

The resistor is more to protect the points than the coil. In any case I think that you need to look somewhere else for the problem. When I first tried to get my car running the point gap somehow "readjusted" itself at some point because I didn't have the locking screw tightened correctly. Look for something similar. Maybe the distributor has shifted and the timing is off. It doesn't take much to make a car quit running.

Oh, and I agree with you that it is better to get the stock system working before doing an upgrade. I always want to solve one problem at a time, and upgrading a non-working ignition system seems like a good way to run in circles.

[Turbo7MN]

Thanks walter.

We ran a plug straight off the coil before it ever got to the distributor and did not get spark from it until we ran a wire straight from the battery to the + side of the coil. I'm used to working on more modern setups and am not too familiar with the points-type system but it seems logical to me that if nothing's coming from the coil the distributor seems irrelevant. I don't know... I had a friend that actually knows about this kind of thing stop by and look at the distributor and points and he said it all looked fine. Unfortunately I probably wont be able to make it up to my friends house to work on the car for another couple weeks as well.

Is the resistor supposed to get extremely hot?

Edited October 1, 2008 by Turbo7MN

[Walter Moore]

When my car is running both the resistor and the coil get very hot. They get hot enough that you don't want to touch them, but not hot enough to instantly burn skin.

Could the plug gap be set too large? Running the coil straight off of the battery is going to double (at least) the voltage to the plug, which would support a spark across a larger gap at the plug.

The fact that the resistor gets hot tells us that there is current flowing in the circuit, which is a good thing. So the points are at least doing part of their job. The coil could have a short in a winding that partially reduces the effective output voltage. There are likely other potential causes as well.

You can by-pass the ballast resistor with a jumper wire for temporary testing purposes without destroying the coil. That would at least take the resistor out of the equation while you find the primary problem. But don't drive the car that way for long because it will burn the points, and I suppose could eventually damage the coil.

[Turbo7MN]

Right, the coil was the first thing I replaced actually. But yeah, I actually burned myself on the resistor when I touched it. I'll probably try the resistor bypass when I get a chance just to see what happens.

My issue is that it did fire previously. I know plug gap isnt an issue. I tested the spark with a spare old plug that we pulled out of the engine anyway, not one of the ones I was using at the time. And of course, the fact that the tach is not bouncing concerns me greatly and seems like it would be an obvious symptom for something if I knew enough about the system.

Thanks.

-Darren

[Force Fed]

The resistor is bypassed during cranking to supply better spark at start up. Chryslers used the same system for years, with both points and electronic ignition. The resistor is there for both the coil and the points, I'm sure. If you look at the coil they are usually marked "for use with external resistor only" if they are made to run low voltage. Some of the newer coils don't need the resistor and can handle full 12 volts.

[Turbo7MN]

I guess I should have pointed this out before...

I know what the resistor does, and I made sure that when I picked up a replacement coil I got one designed for use with the resistor. I also knew it was bypassed when cranking... I downloaded a 73 FSM which isnt exactly the same as my 72 but its close enough for most purposes.