Our '76 F250 with the 460 engine quits after a few minutes, often starts right up, then quits again. A "friend" says its the coil. The primary circuit shows 0.15 ohms resistance and the secondary shows 9000 ohms. The dealer can't tell me the correct values. Can anyone tell me what the coil readings should be? Thanks!

 

Well your situation might be a fun one! the 9,000 sounds pretty normal, the .15 seems a little low, but probabaly acceptable

Most of these systems have a resistor wire going to the coil Bat+ term. If you run straight 12V power to the coil it will heat up, and shut off, I've had this happen to somehting I built.
I've never tried messing w/ the stock Ford Ballast ressistor, or ressistor wire...whatever it is

I've set em up before by using a chyrsler resistor I think it was rated for 1.5 ohms or soemthing. Just bolts to the firewall, has 2 terminals, in and out! Can buy one at any Napa don't even have to tell em' what it's for, they'll know. they'll just ask 2 or 3 prong!
You must also have the starter solenoid that has 2 little posts. That second small terminal puts out 12V only when cranking, think of it as a little wire hooked on w/ the wire that goes down to the starter. This wire is to give your coil full 12V at cranking only, bypassing the resistor so it starts easier....I guess

Apparently if you had a mopar you had to always keep one of these in the glove box.....so on second thought maybe it's not a good idea????

Sounds like a resistor wire problem to me, and the only way I've fixed it is w/ this mopar ressistor....so don't take what i said as the only way, but something to think about....

 

Thanks LedheadELH: Haven't had a chance to play with the old truck yet. Last week I took the old coil down to the local NAPA store and compared the resistance values to a new unit. They were the same! Then I noticed something...

It pains me to admit my stupid mistakes but I failed to notice the scale on the multimeter said 10X - so, the 0.15 ohms is actually 1.5 ohms. If anything, a bit too high! But it seems that my coil is OK (unless it is shutting off as you explained due to overheating/getting 12 volts all the time)

Your info about the 'resistor' wire is interesting, I know nothing about that. I'll check that out once I've done to obvious stuff like cleaning and checking connections.

Appreciate your help,

Cheers

 

Coils are cheap, if in doubt, just replace it. However I don't think it is your coil as to fail they need to "get hot" and that does not happen in a "few minutes". At least I've never seen or heard of one failing that fast. I would suspect the sensor in the dist.

 

Bear 45/70 Yes, I know you are right but I'm supposed to be old and wise... which means not throwing parts at a problem until you blunder on the actual remedy! Besides, NAPA wants $33 for a $10 coil... (I only paid $100 for the truck)

There is also the chance it is the ignition switch - someone opened it long ago, so the contacts inside are probably buggered.

I'll check out the price of a new distributor sensor... and then there's a problem with the 4bbl carb emptying its float bowl overnight... heavens above!

Thanks for the input

 

The ignition switch is easy, just run a jumper wire from the batter 12 volts plus terminal to the plus terminal on the coil.

 

mrfixit. i hope this helps. i got this from the second chance garage website. you can either read what i added or go to the website. after reading the article it is obvious that gm and ford put the resistor wire in and chyrsler went with the resistor on the firewall.

Automotive Electrical Systems-Part 5-Ignition Systems

The next component is the ignition resistor. It is necessary because ignition coils are designed to step up battery voltage high enough - and fast enough - to keep the engine running at high rpm. That means that, as designed, the coil would produce too much high voltage at low rpm and heat up. Automakers long ago realized that there were two solutions to the problem: using two coils (one for low rpm and one for high) or an ignition resistor. Obviously, the resistor approach is the least expensive and most reliable, so that's what they did. The resistor used varies is resistance as a function of temperature, and limits the voltage to the coil accordingly. As the engine revs up the resistance lowers, allowing more voltage to the coil for fast running, and the reverse happens when the engine slows down. At idle, for instance, only about 7 volts is going through the coil primary windings.

The only time the resistor is out of the circuit is during startup, when the engine needs all the spark it can get. It's bypassed in the ignition switch's start position so that, during starting, the coil gets full battery voltage. Ignition resistors can take many forms, depending upon the manufacturer of the vehicle. Some builders mounted a big resistor on the firewall and some others utilized a special type of wire (resistance wire) running from the ignition switch to the coil. Still others used coils that were built with an internal resistor. None of these is any better an approach than the others, but it's important to know which type you have, and that you have one!

 

Most of your missive is incorrect. The resistor is to prevent the points from getting fried at low speeds, not the coil. The risistor is bypassed in start because the starter drops the battery voltage to 10.5 volts and that means the coil would see about 4 volts, not enought to fire the plugs. Full battery voltage (12 volts) is not available during starting.

 

Bear, your right about the start up voltage being less because of the starter drawing the battery to whatever.

However the resistor has nothing to do w/ points. Newer elec. ignition (duraspark) Have these resistor wires. I think(but not sure) even EEC IV systems have the resistor too. my guess is anything with a single coil has a resistor. Anything with coil packs does not

By the way, what's with the car in your thumbnail?!......reverse rotation marine engine or something??or just an optical illusion? Cool pic either way!

 

The only reason the newer ignition systems still use the resistor is that it became an industry standard sort of thing. I always found it interesting the when I still worked in the recreational marine industry that all the American engines used a resistor, but Volvo did not. With electonic ignitions it's about how you design it and has nothing to do with the coil getting hot. It also has to do with the lower voltage available during starting. But required, no way.

That was my 1969 Ford Cobra with 428CJ Ram Air.