Would a bad ballast resistor cause an 86 F250 not to start?

If you have power when the key is in run, your ballast is fine.

I would be looking for power on the coil positive when it turns over next if you didn't already show 0 across the blown coil....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >

The ballast resistor only has one wire going to it and it is in parallel with the power cable. So in other words, I could cut off the ballast resistor and there would still be power at the coil....

Sam

Then what would be the pourpose of the ballast resistor? Jim

It drops the voltage down for something. I have never seen one tagged to ground though, normally I see them inline so when they fail, there is no voltage. One 'could' go to ground to steal some voltage too I guess, I just have never seen them that way.

For the inline ceramic ones that fail, you can just put the two wire plugs together to give you a full 12 volts.

I was under the impression that an 86 Ford has the same ignition as my

86 Jeep which has a ballast wire, not a ceramic resistor block. The power when the starter is turning comes direct to the coil via the starter relay bypassing the ballast.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >

The case of the resistor is grounded which I am guessing is the "second" connection which drops the whole circuit down.

Replaced my coil and the truck started. Crazy that it went when it did. It did appear to be the original coil from 1986 so I got my use out of it.

OHHHH I know this one:

How many volts does your coil need to run?

Not 12 as you would assume, but about 10v. When you turn the key to start the vehicle, the starter draws so much current that only about 10 volts is left. so the engineers built the coil to a 10v specs (put a voltmeter on the pos of battery and then the hot of the starter, and crank it with the dist disabled--- about 10-11 volts

Now the vehicle is running and the 12 volts is at the coil-- enter the ballast resistor, it uses up the 2 volts so the excess voltage doesn't burn up the coil

I learned this last summer.

most all cars today only use 10 volts at the coil still, the ecm can regulate some cars,

Good grief, from who?

Actually, the cited 86 Ford F-250 w/ Duraspark ignition is one of the rare examples in the last 20 years.

Shakes head in amazement....

Are you sure that's a ballast and not a RFI suppression capacitor?

nate

Don't remember the class sponsor; was basic auto electronics class. The instructor put a Volt meter on a 5.0 test eng and showed us. I'll have to look for the book. Was rather amazed when I quizzed my father-in-law about it and he knew the answer but not the why.

Was the class free or did you pay to attend?

I suppose it would depend on where the instructor connected the volt meter. If he connected to the negative side of the ignition coil, then yes, I'd expect to see some fraction of system voltage. At the positive side of the coil, odds are that you'd see full system voltage. GM went this way in 1975 Ford went this way with the intro of TFI, basically around 1984 with the exception of carry over DuraSpark ignition used on HD truck applications and police cars thru the late 80s early 90s. ChryCo went this way around 1988 or so. Earlier on some models. Can't think of a single application offered in the last ten years that doesn't feed full system voltage to the positive side of the coil.

Not sure what the question was...

The reason for a resistor in the ignition primary is to control current thru the coil primary. The anecdotal observation is that it also drops the voltage to the coil, but voltage is more or less meaningless until you have electrons flowing (current). Bear in mind, when the magnetic field in an ignition coil collapses, there is upwards of 300 volts induced into the coils primary windings. Knowing that, what difference could a few volts (10 vs. 12 vs. 14.5) make? None of this obviously should ignore the fact that voltage to the coil can fall too _low_ and affect spark output.

The class was free to me but the shop paid a couple hundred per tech attending. The test engine had sensors from all sorts of engines wired up to it, the instructor could hit a few switches and cause all sorts of problems to happen. Great learning experience.

Restriction on the fuel filter? fuel pressure maintained constant, amps increased and volume decreased. It was good to SEE the cause and effects of problems on the car.

We did a voltage drop test one test lead on positive of battery and one on the hot of starter. Great tool for diagnosing many problems on a car.

The instructor also did something to demonstrate the 10v system on the coil as it related to starting, but I don't remember exactly what at this time... Gust had another class on Volumetric efficiency in an engine and diagnosing the total fuel trim problems. (How much effect does a rear o2 sensor have on fuel trim...) That one flipped my learning around. Got to find that book

I'll now try to dig up the book with the wiring diagram; it showed 12 v through the starting circuit to the coil and a separate line going through a resistor to the coil hot for running.

If the coil stays with a constant 12 v while running than the chances of killing a coil (primary side) from heat are much greater.

Take care

Like the man said, that is an old way of doing it and is not necessarily dependent on the coil.

My 86 Jeep has the old Ford ignition and I bought an Accel SuperCoil for a hotter spark. The Accel coil can run on either a direct alternator power level or use a ballast system.

I asked Accel about using it hot (14+ Volts) for the hotter spark and they told me if I did I would cook the ignition module. It even warns about that on the Accel site.

You saw one old test engine. That does not translate to 'all' coils.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >

the engine wasn't that old, perhaps 7 years F.I. Modern cars still do reduce voltage to the coils, mostly in the ignition modal if I remember correctly. The critical point being that during a start, the car pulls the available battery voltage down to 9-10 volts, and a full 12v coil wouldn't have enough voltage at that time to saturate and fire correctly. It applied it the old days and still today. Every starter test we do shows 9-10 volts at start and the car passes with that number.

Still looking for better references, this was just a quick search

Steve

Not so much a question of age or fuel system, it's a question of what exercises that particular simulator was designed to emulate.

They most certainly do not. Don't take my word for it, pop some hoods and do some testing with your voltmeter. Better yet, use a lab scope, that way you can see all the dynamics over time.

Problem is, coil saturation is not a function of voltage, it is a function of current.

Yes, and modern e-core style coils will output enough secondary when fed 9-10 volts to start an engine, especially now that we have advanced fuel control and delivery.

Going back to the original post; the Ford Duraspark system in question will actually shut itself off when cranking voltage goes under 10 volts. You can demonstrate this for yourself by connecting your carbon pile to the battery terminals, installing a spark tester on the coil wire, cranking the engine (remotely) and adjusting the carbon pile during crank. At almost exactly 10 volts, the spark will go out.

Amazing how many get this stuff wrong.

Hell, I bet I can find ten times as many articles that state that oxygen sensors measure oxygen in the exhaust, (they don't) but you can bet your next paycheck that [that] is the way it is still taught.

About 20 years ago, I made similar statements in a GM specialized electronics training class. The entire room went silent. (not a good thing) Thing was, of the eight of us in that class, five were full time GM instructors**, the other three of us were dealership line mechanics. I got a very good lesson on how an ignition system works that day, a lesson I won't ever forget.

** the five were; Bob Bendixon Perry Days Don Fritz John Porter some black dude from Detroit who's name I forget cause he was the only one not from Milwaukee. the instructor was Roger Hassler.