93 3.0L Coil Output Test Question

[Trfsrfr]

I'm trying to diagnose a 93 with a 3.0 that just started running very poorly while cruising.
No Check Engine Light.

I'm fairly certain it has recently new plugs, wires, cap, rotor, but am unsure of the coil.

While cranking it over today with the distributor wire off, I watched it spark to its mounting bolt.

Is this normal and a sign of a good coil?
Or should it not have sparked at all and this is suspect?

The engine rev's and accelerates today, but very poorly and misses a lot.
Yesterday when it happened while driving home, it barely made it.
Tried a new fuel pump, but that isn't it, although it does rev now.
Thanks-

 

[RonD]

An engines coil has a primary coil of wire and secondary coil of wire, the primary has the 9v-12v power from the battery, the secondary has a longer winding of coiled wire and connects to the spark plug(ultimately).
A coiled wire with power added is an electro-magnet, and also a transformer, it can "transform" low voltage high amps to high voltage low amps based the the length of wire in each coil.

When power to the primary coil is cut the electro-magnetic field collapses, this causes the secondary coil to discharge its low amp high voltage to the easiest ground OR lower voltage point, lower voltage is the same as a ground to higher voltage electricity, mount bolt would be a ground.

In a distributor setup the points would cut the power to the primary coil, in the TFI and other electronic distributors they use a transistor or similar component to cut power momentarily to the primary coil, this causes the spark to fire up the engine.

To answer your question yes, the secondary output of the coil(spark plug wire) could spark to the primary connections on the coil, they are lower voltage or 0 voltage, at the time the secondary coil discharges.

If the plastic on a coil is cracked it can do this when it is not suppose to, so would limit the amount of spark(voltage) at the distributor and spark plug.


Good read here on testing TFI ignitions
http://www.therangerstation.com/tech_library/4_0_Page.html

 

[Trfsrfr]

Fixed.

It was a broken metal piece inside the ignition pick-up. I couldn't see it until I pulled the distributor. It looks like it was rubbing on the rotary vane.
I was going to try and just just replace the pick-up, but I broke the drive gear when trying to remove it from the shaft. At that point it I just bit the bullet and gave them another 60 for a reman distributor.
Limited Lifetime Warranty of course.
All good now, runs great, with a new tune-up.

 

[RonD]

Thanks for the update

Any idea where that floating piece of metal was from?

 

[Trfsrfr]

It was part of the pickup. The shiny piece next to where the rotary vane rides.
And it wasn't floating as in being able to be tossed around inside the distributor. It had come loose I think and was floating inside its area in the pick-up, similar to what a soda can inside a pringles can would float, up and down but thats it.
It was all crudded up with metal shavings too, so I assume that at some point (when I gassed it to get around that car 2-3 months ago) that the two parts made contact and that was the start of all my issues.

Now that it runs it's time to finish the flatbed, or at least continue on it.