I've got a '73 F250, with a stock 390 I'm wanting to get a new coil for. Right now I'm reading the following voltages while running.

10.6 volts to the coil (from bat. terminal on the primary side of coil to the neg bat. terminal on the battery)

and

4 volts across the primary side (bat. terminal to dist terminal)


I'm aware that, stock, there's the resistor wire involved to knock the primary feed down to a lower voltage, but it seems that it might have been removed-but the coil looks stock OEM style and it's obviously been on there awhile (I recently bought the truck)

So I'm confused as to why I'm only seeing 10.6 volts instead of 12.6+ while running? Other than possibly just a dirty connection under the dash. OR my main question - when those resistor wires go bad, to they ever lose resistance and allow more voltage to the coil? I'm guessing they don't...

Any thoughts?

I will be doing a Pertronix upgrade eventually, but for now I'm about to take a road trip and would just like a back-up coil handy...

 

10 volts is fine.

The resistance wire comes from the switch thru the firewall, directly behind the engine, then snakes back and then comes to the disconnect plug where it combines the ign/ start wires to the coil.

The pink wire is very supple and a lot thicker than the other wires in the bundle. It also says resistance wire: do not splice or cut

 

10 volts is fine for a stock coil that would normally have the resistance wire before it? (I was assuming they liked to see @ 7 volts)

Or 10 volts is fine for a 12v aftermarket coil?

Or are they essentially the same and Ford just uses the lower primary voltage to get longer life?

I'm going to go double check if I still have the res. wire or not...

 

10.6 volts is a little on the high side for a stock coil, but not by much. The stock setup has a resistor wire in the primary circuit because the stock coil is rated for 7 - 9 volts. Some aftermarket coils are rated for 12 volts in which case the resistor wire is bypassed. The voltage rating has to do with the resistance of the primary winding inside the coil and how much current the winding can handle before burning up. What you need to do is measure the primary resistance of the coil, which is done by probing the BATT and TACH terminals with the coil disconnected. You should see around 1 ohm. If it goes much above 1 ohm, you will see more voltage drop across the coil primary which would explain the higher reading you are seeing. If the reading is much more than 1.2 or 1.3 ohms, you should replace the coil.

As a side note, to answer your second question - resistors almost always fail open; rarely do they fail short. However, the resistor used in the stock ignition setup is not a discrete component, as you know it's a specially doped length of wire therefore it could potentially short to sheetmetal, but that would cause other problems. Corroded connections usually lead to increased resistance. The higher the resistance in the path, the more voltage drops across that resistance and the less voltage in the divider is available for the coil primary (BATT terminal).

Another side note, keep in mind that any voltage reading taken on the BATT terminal must be taken with a grain of salt because when the engine is running, the TACH side of the coil is contantly brought (close to) open-circuit as the Duraspark box fires the coil. The coil is fired by the module removing the coil from ground, and this happens at a very rapid rate. When the box is grounding the coil, the coil primary is grounded and the primary is a complete circuit. But when the box lets go of the coil, the voltage you are seeing is actually floating and shifts the reading that the meter sees. To make things even more complicated, when the box is grounding the coil, there is actually a small saturation voltage drop from the TACH terminal to true ground, which shifts the reading slightly again. It's not by much, but it's always a slight source of error. It's just something to keep in mind.

 

I assume your running points, not Duraspark

 

Good catch - in that case, replace Duraspark box with breaker points. Everything else is the same - the BATT voltage still floats when the coil fires, and instead of a saturation voltage while the coil is grounded, there is a small drop across the resistance due to the points switch contacts. The coil is similarly rated between the two systems. Those are just details though. The important thing you need to do is check the coil primary resistance.

 

Thanks guys, thats all great information. And it's points...Fresh reman distro and new points. Dwell's running at 29.5

So what you're saying fmc, is that that 10.6 volts I'm seeing might read closer to 12 with the engine off/key on perhaps? I'll check that too. Sounds like the resistor wire is probably just not there anymore then...too cold to bother looking tonight so I'll have to report back tomorrow.

It's been running great, I just want to be sure to have a good (and correct) extra coil on hand for the trip.

 

On a points setup, if you have the key 'on' but the engine is shut off, there are two possibilities. If the points are open, you'll see 12 volts on the coil as there is no current flowing through the resistor wire. If the points are shut, you'll see lower than that as current is flowing through the resistor wire and there is a voltage drop.

I'm not saying the resistor wire is not there; I'm saying it sounds like it might be, but the resistance of your coil's primary winding may be higher than normal because 10.6 volts is a little higher than normal for a stock coil. If there wasn't a resistor wire there, you'd probably be seeing higher than 10.6 volts. I think you should check the coil resistance as previously described to see what kind of shape is in. I think increased coil winding resistance is the reason you're seeing a slightly higher voltage. You might try poking around to try and find the resistor wire itself; it looks just as Bullitt390 described. It came this way from the factory and will be there if no one has touched the wiring. If that's the case then you'll want a stock replacement coil.

 

Gotcha. I'll check the coil resistance first thing then. Thanks again for adding some clarity to this. And sorry to offer the question up without knowing for sure about my rest. wire. I probably should've waited until tomorrow to ask once I had more info, but that would have been too easy...

 

It's cool! Let us know what you find...

 

Coil resistance across the primary side measured at 2.5 ohms (at 20 degrees F). (secondary was 7600 ohms)
The resistance wire was there in the engine compartment, and everything appears stock under the hood in that regard. (no new wire that's bypassing it straight to 12v)

I did not see the resistance wire coming out of the ignition switch, so I am presuming it plugs into one of those wires somewhere up under the dash, but I could not find where. Probably good that I couldn't - hopefully meaning it hasn't been futzed with and is wrapped up somewhere still.

All that said, it looks like it's definatley time for a new coil...I'll double check my incoming primary voltage again once I replace the coil (with a stock Napa IC10), and see how that 10 volt reading changes and report back, for information's sake.

Thanks again!