Correct voltage across coil ?

[Stanley]

Basic question I know but basic's where I'm at. 

It should be about 8 volts across the coil right ?  I'm getting about 4 volts across the ballast resistor which seems about right, but only 4 to 4.5 volts max across the coil. If it's supposed to be 8 there must be a resistance somewhere that's hogging about 4 volts.

Not the only electrical problem of course. One time, got zero volts across coil. Cranked the starter accidentally and turned the key back to "on" engine still off, then I got the 4 volts again.And with the engine running, still got the 4 volts or slightly less, but every 5 seconds or so it dropped to about 1 volt. Maybe there's a vibration-related intermittent resistance too.

Wish the engine harness drawings in the FSM were drawn better, and showed clearly what wire went to what component.

 

[SteveJ]

Don't measure across the coil. Measure from the positive terminal of the coil to ground. If you are measuring across the coil with the points closed, you will see zero volts. If you are measuring across the coil while the car is running, the voltage will seem low, especially if your meter does not respond quickly from the voltage going up and down 13.5 times a second (or more if your idle is higher).
Here's a basic breakdown of the wiring for the ignition:

1. Starting the car: The ignition switch connects the White/Red wire (12VDC+) to the Green/White wire. The Green/White wire goes to the tachometer and comes out Black/White. The Black/White wire goes to the coil.

2. Running: The ignition switch connects the White/Red wire to the Black/White wire. The Black/White wire goes to the ballast resistor and becomes a Green/White wire on the other side of the resistor (resulting in a drop in voltage). The Green/White wire goes back into the wiring harness where it joins the Green/White wire described above.

The negative wire of the coil goes to the distributor where it meets up with the points. The points will ground the circuit when they close to generate the spark.

[Stanley]

Will try that tomorrow.

[Stanley]

Yep, got 9.2 volts from coil positive to ground at the body with the engine running. About right I guess. Still got the 4.5 volt from coil + to ground with the key to "on"  and engine not running, though. That might mean something, but don't know what. Also, the 9.2 volts dropped down to 3 or 4 volts briefly a few times. My recent book learning needs some more experience to back it up. At least I understand it a little better after reading post #2.

[SteveJ]

Unless your car is running rough, I'm not sure if there is an issue here. Am I missing something? Are other electrical systems malfunctioning in your car?

[Stanley]

Sometimes it starts right up, sometimes have to crank it 10 seconds or more before it starts. Hot or cold doesn't make a difference. Today for example, ran some errands, 3 easy starts including the first one, and one where I had to crank it a while. Only other problem is intermittent right turn signal.

Shouldn't it have about 8 volts coil + to ground with engine off and key "on" ? Battery is strong I think, at least it doesn't slow down any when I have to sit there cranking it.

 

[SteveJ]

Do you have points? If the gap is closed on the points (or close to closed), you won't see full coil voltage from positive to ground. Having the key on with the engine not running is why points will burn up. They aren't designed to have a constant flow of current.

For the hard starting, are you sure it's not a fuel issue?

[Stanley]

Have points. Now I see why no full voltage with engine off. This stuff should be in the book.   Don't think it's fuel related, fuel pressure is good and carbs are maintained, running rich at low end (SM's) but don't think that causes the intermittent problem.

When I first started the car, the coil to ground voltage kept dropping off to 3 or 4 volts every 5 seconds or so, but after a running few minutes it was steady. No clue on that one.Gotta work today but will try to repeat that tomorrow.

Found a 0.5 volt drop across the positive battery cable connector to post, doubt if it matters for this but I'll clean it before it gets worse.

[SteveJ]

What I told you about points and voltage around the coil is not in the manual because it was Automotive 101 back when these cars were made. We won't even go into how, in a pinch, people would set the gap with the cardboard from a cigarette carton after they filed the points some because the points got fouled.

[JSM]

1 hour ago, SteveJ said:

What I told you about points and voltage around the coil is not in the manual because it was Automotive 101 back when these cars were made. We won't even go into how, in a pinch, people would set the gap with the cardboard from a cigarette carton after they filed the points some because the points got fouled.

Talk about a flash back. I remember my dad doing that as kid!

[SteveJ]

Come to think of it, @Stanley, have you checked the gap & dwell of your points lately?

[heyitsrama]

Hey Stanley, not to thread hijack, but what do you mean by intermittent cranking? My Z was doing something stilly yesterday that I'm still thinking about where if I take the coil to the dizzy and connect them, the starter struggles to turn over. If I break that connection it spins fine.