Pickup in question is a 78 F-150 with a 400. I was driving this pickup the other day, went inside to eat dinner, came outside and it wont start. It does not even attempt to fire so I narrowed it down to an ignition problem. It has fuel and is getting fuel. The battery is fresh and shows 12.6 volts. I tracked my problem down to the coil. I pulled the ign coil bracket am getting 12v to the + side wire on the bracket. I am only getting 6v coming out of the ign coil. I had an extra ign coil laying around and tried it, still only 6v. I then went and bought a new coil, still only 6v coming out of the coil. I checked the bracket again with it disconnected from the ign coil to make sure and am getting a solid 12.6v. I clamp the bracket onto the coil and check the same + side wire and am getting 6v again? Is it possible that I have 3 bad ign coils including the new one? I live about 50 miles to closest parts store so I cant just run to the parts store and grab another coil. This one kind of has me stumped. What do you guys think. Any help is greaty appreciated. Thanks.

 

I am assuming that you have electronic ignition. I doubt that you have 3 bad coils. They do not go out very often. Are you getting signal from the ignition module?

Run a temporary wire from the positive battery terminal to the + side of the ignition coil and disconnect the stock wire to the + side and see what happens. If you do not get spark the problem in most likely the Ignition module, wires or distributor.

 

If I am getting 12v to the coil I should have 12v coming out of the coil correct? I was just thinking that the coil was the problem because I was only getting 6v coming out of it but have 12v going into it? Thanks for any additional help.

 

I had the same problem. It was the voltage regulator.

 

???Voltage regulator in the alternator? My truck wont start, charging is not an issue.

 

Use the search function on this site and put"Ignition Module Troubleshoot"in the thread search.Several posts in the 1973-1979 forum.Several good tests from FMC400 in those threads.See if you can use those tests and let us know how you come out.

 

I've always carried a spare ignition module behind the seat in every duraspark equiped truck i've ever had. They'll go bad once in a while. Sometimes they'll pop and backfire for a while before they die, sometimes they'll just up and die and your truck won't start.

 

Are you getting spark from the coil wire? and if so are you getting spark from one of the plug wires?

 

The voltage req is separate than the alt. could also be the ignition control as stated above.

 

You're looking at the coil the wrong way. The "voltage in to the coil, voltage out of the coil" language is incorrect. It only makes sense to look at voltages with respect to ground in this case. Furthermore, there are characteristics of the primary coil circuit that make measurements tricky.

The coil primary winding is downstream of a ballast which in this case is a length of special resistive wire in the dash harness. The ballast, in series with the resistance of the coil primary winding, creates a voltage divider such that part of the full 12 volts applied upstream of the ballast drops across the ballast, and the remainder (most of it, anyway) drops across the coil primary winding. That's why there is less than 12 volts at the positive coil terminal.

The negative terminal is much different. It's switched to ground through a power transistor in the ignition module. Before the module fires the coil, the power transistor turns ON such that the negative terminal of the coil is effectively shorted to ground. When the distributor signals the module to fire the coil, the power transistor turns OFF and the negative terminal of the coil effectively becomes open circuit. This attempt to rapidly stop current is multiplied on the secondary winding of the coil, creating a spark which makes its way to the spark plugs. Because the coil primary winding's path to ground is switched, you'll either see ground (zero volts) when the transistor is ON, or the source (12 volts) when the transistor is OFF (because no current flows through the circuit, so no voltage drops across anything). Both of these measurements are with respsect to ground. There is no case where it makes sense to measure the voltage between the positive and negative coil terminals.

For goodness sake, stop replacing the ignition coil. If three didn't solve your problem, four certainly won't. Do this test. Hook your coil back up. Connect a test light from the TACH TEST terminal of the coil horseshoe connector (find some of the bare metal) to clean, unpainted metal on the engine (ground). Get in the cab and crank the engine over with the key, and watch the light. It should BLINK. Post your results, and we'll go from there.

And one last time, stop buying stuff you don't know you need.

 

Ok first off I would like to thank you for taking the time to explain all of this. Second, I did not just start throwing parts at it an hoping for the best, I had one extra coil laying around, it did not work so I bought a new one due to the fact that I only got 6v coming out of the coil which I thought was faulty, that didnt work so I posted my problem..... I will try the test that you suggested tomorrow and post what I came up with. Thank you again for the help.

 

Have you tried to jump start it, then when you shut it off it wont start again? That's what my problem was.

This was also after changing the battery and alternator.

 

FMC400 I did the test that you suggested and the test light DID flicker when I was cranking the engine over....

 

So what do you think the problem is? Is there another test I should run?

 

Check the resistance between the orange and purple wires giong to the pickup module inside the distributor, it should read between 400-700 ohms, it's unlikely that's your problem since your coil is apparently firing but it doesn't hurt to check.