Hot Rod Forum banner

Burning through ignition coils

27K views 59 replies 9 participants last post by  2old2fast 
#1 ·
1977 Ford LTD
351M

I have been burning through ignition coils since I bought this car a couple years ago. They overheat and it only happens during the summer. If I leave the AC on with the car in idle the coil is sure to overheat,

It is probably note worthy to say that the car purrs with the A/C off, but when it is on the car runs horribly. In fact it is difficult to even start the car if the AC is on, but if the A/C is off the car fires right up. I converted the A/C to R134 and replaced everything but the hoses. That had no effect on the running or starting what so ever

Last thing is I increased the timing from four degrees behind TDC to TDC. I replaced the carb also, no difference.

Any ideas?

Thanks!
Any ideas?
 
#28 ·
I think you have solved your problem, yes technically 10 volts is too high and is well above the recommended max but actually it is not all that uncommon to see it go that high and I doubt it will hurt anything. If however you had just by-passed the resistor circuit then that voltage would have been well over 13 volts with the alternator charging at normal rate and that would burn out the coil. Run it like this a half hour or so then check to see how hot your coil gets and if it is overheating from an over-voltage condition it will get really hot to the touch, if the voltage is really too high the coil will get smoking hot!
 
#29 ·
Steven Pisonero said:
Lastly, I cannot find the resistor we were talking about. The following schematic is an exact replica of my setup.


http://www.google.com/imgres?imgurl...T1ITqikFo2nsQLcs8WSCA&ved=0CEAQ9QEwAw&dur=579

You will not have a separate resistor in that system but instead the resistor will be a special resistance wire and it is that pink wire in the diagram, the zig-zag patten denotes resistance in the diagram. If you do locate the wire it will be pink in color and the insulation will have a soft feel to it plus most of them (but not all) will actually have "resistor wire do not cut or splice" written along their length.
 
#34 ·
You can wire in an aftermarket resistor and it'll work fine, if the
resistor wire is also bypassed. The resistor wire is located behind
the ignition switch, and is about 4-5 inches long. OldRed described
it exactly, although some of the resistor wires are a gray color.

The resistor value should be about 1.2 - 1.4 ohms. It's wired in
series with the positive coil wire, and placement isn't important;
the firewall is a good spot for mounting.
 
#36 ·
Steven Pisonero said:
I am having difficulty finding it, but I will give it a whirl tomorrow. Can you help me understand how a resistor wire can fail? Since it has failed, and it seems like the coil is getting full voltage, why not put the new resistor on without bypassing the wire?
Thanks
A resistor wire creates an open circuit when it fails, passing no current.
I don't believe a resistor wire can suddenly lose it's resistance and then
work as an ordinary non-resisted wire. Because of this, I think your
coil circuit has been modified somehow. (Maybe the wire has already
failed and a previous owner has bypassed it?)

If you're getting battery voltage at the coil positive terminal when the
switch is on, then install the ballast resistor and it will work fine. These
back-and-forth voltage readings are what's confusing. I think you might
have a faulty multimeter...?
 
#37 ·
You said you found an error and corrected it can you describe what you found?

Yes you can just wire in an after market resistor and I was going to suggest that but it appeared you had found your problem, apparently not so let's consider a few things first. One is that you are apparently getting erratic voltage readings on the wire you are using now so it might not be a good idea to use that one. The module controls the ground side of the coil so you can just run a new wire from the switch to the positive side of the coil (leave the negative side wired as is!) and use the ballast resistor on that, just splice it in someplace safe between the coil and the switch. I say place it in a safe position because it's normal for that thing to get hot enough to start a fire, that's why the body is ceramic, so put it someplace where the heat won't be a problem like an open spot somewhere on the firewall. Next you will need to run a by-pass circuit from the starter solenoid (I am assuming you only had one wire connected to the positive side of the coil?) and this will require a solenoid with two small terminals in addition to the battery and starter cables but on that model I think you will already have that. If yours only has one wire then a new solenoid will be required but they cost little and are easy to install. One small terminal on the solenoid will be marked "S" or similar for "Start" and is the (Red/Blue) starter wire, this wire will still be used as is, the other will be "I" or maybe "Ign" and is the by-pass wire to the coil. If you already have the two terminals and two wires connected (some models will and some won't but likely yours will have both) then use the starter wire as is but discard the "Ign" wire as it may very well be part of your problem. Now run a new wire from the "Ign" terminal on the solenoid to the SAME terminal as the new wire you have from the added ballast resistor. This new wire from the solenoid will have full circuit voltage on it but ONLY while the starter is engaged, it is necessary to temporarily (only while cranking) by-pass the ignition resistor circuit to provide a hot spark for starting while the system is under starter load.

When you make these changes, and they are quite simple actually, you will have by-passed the entire ignition voltage supply section of your system and your problem will be eliminated for good. The DuraSpark system is a good system but sometimes gets a bad rap because Ford could have done a better job of wiring the darn thing! :mad: I have done several points type to DuraSpark conversions on Mustangs and a few other old Fords and they do work well, they are really simple and easy to install or, as in your case, repair if needed but sometimes they need a little "improved engineering" such as that wiring mod.

A few more points, the only wires you actually disconnect will be at the coil and the solenoid, DO NOT disconnect anything from the Red/Green wire from the ignition (it has a couple of wires spliced into it). Just connect your new ignition wire to this Red/Green wire and leave everything else because it also supplies power to the solid Red wire on the two-wire plug that powers the control module. After disconnecting the wire at the coil just clip the end of it and tape it safely out of the way in the harness, do the same for the wire that is disconnected from the solenoid. When you finish this wiring mod you will have two wires connected to the positive side of the coil (one from the ignition/resistor and one from the solenoid) and any factory wiring failure will be eliminated. This is not a "shadetree" fix and is actually the way Ford had the system wired on some models.


Good luck with this thing and let us know what you decide and how it's working out for you.
 
#38 ·
I have not yet put on the new solenoid, but I did put in the resistor. It helped substantially. The idling volts dropped from 10+ to 8+. Having said that, when the engine is revved, voltage goes to just over 10(better than 12.5 before). What do you guys think about a stronger resistor?
 
#39 ·
The resistor should be fine, those things are just a standard rating anyway, and if you are sure that wire is ok then you should have the problem solved now. Where did you mount the resistor? The reason I ask is that the factory by-pass wire will be tied into the circuit and needs to be at a point between the coil and the resistor, if it ties in before the resistor it won't do anything since it's voltage would have to pass through the resistor also. So you are saying that you do have a solenoid with only one small terminal (no ignition terminal)? If so then the factory by-pass may or may not still be in the circuit as you now have it, once the engine starts this wire does nothing and is only needed to provide a hotter spark during start. If it is missing from the circuit you may not even notice if the engine starts easily anyway but in cold weather and/or with a weak battery starting might be hard. If you replace the solenoid with the two terminal type and simply run a wire from the "ign" terminal to the positive on the coil, the SAME terminal the resistor circuit is connected to, then all should be well.
 
#40 ·
While driving, if I hit the gas, the car sputters and will even die out. I placed the resistor in three places: right before coil, right after the plug that connects to the firewall wires and right before the plug. I got the same results. At slow speeds it runs perfect. Once the gas pedal goes over about 1/2 way it will sputter and die. I removed the resistor and hooked it up the way it was originally and I no longer had the sputter and dying.
 
#41 ·
Since I removed the resistor, the voltage to the coil is at 10-11 volts again. When the resistor was in place, the voltage was around 8, but it died. Some clarification. If the car is in park and the car is revved, it does not sputter, but when driving, if the gas is hit, the car will sputter and die
 
#43 ·
Steven Pisonero said:
While driving, if I hit the gas, the car sputters and will even die out. I placed the resistor in three places: right before coil, right after the plug that connects to the firewall wires and right before the plug. I got the same results. At slow speeds it runs perfect. Once the gas pedal goes over about 1/2 way it will sputter and die. I removed the resistor and hooked it up the way it was originally and I no longer had the sputter and dying.


Obviously you still have the factory resistor wire in that circuit and by adding the new one you are adding too much resistance. The voltage readings you have given us do not make sense, they are all over the place, and I suspect maybe your meter is not accurate or something is amiss with the wiring. Place the resistor near the coil and check the voltage both at the coil connection and the supply side of the resistor, that is check the voltage going into and coming out of the resistor. With the engine running you should have as much as 13 volts or even slightly higher going into the resistor and not much over 8 or 9 coming out on the coil side.

Are you checking the Red/Green wire going to the coil? That's the wire that's supposed to go the positive side of the coil and have the resistor voltage on it during "Run" so just ignore the negative side connector of the coil.
 
#44 ·
oldred said:
With the engine running you should have as much as 13 volts or even slightly higher going into the resistor and not much over 8 or 9 coming out on the coil side. .

I just noticed I made a serious mistake in that last post, to clarify I meant to say that the above would be true IF the original resistor wire had been by-passed or otherwise modified but I mistakenly left that out.
 
#45 ·
I put it in gear and reinstalled the resistor right before the coil. I had somebody power brake it until the car started to sputter. There was not a drop in voltage at all, but it did not rise either. I do not understand. Maybe the coil needs a higher voltage at higher RPM's. I tried two different coils and the car sputtered and died with both of them
 
#47 ·
I purchased a total of three resistors. One was supposed to be 1.3 ohm, but when I tested it, it came out to be 1.6 ohm. the others were 2.0 and 2.3 respectively. I thought I had the 1.6 on, but it was the 2.3. I put the 1.6 on and the sputtering only happened at very high RPM's. I just ordered a .8, 1.0 and 1.3 from summit. I believe one of them should work. From what I read, if the 1.3 works, then we can assume that the resistance wire is allowing full current and we can consider the ballast resistor a fix. If I have to go lower, then I should have the wire removed. Is this the right manner of thinking or should i have the resistor wire removed regardless?

I attempted to pull the resistance wire myself, but once I got in there, it was not readily available and I am not comfortable enough with my skill level, definitely a job for a pro.
 
#48 ·
Steven Pisonero said:
I put it in gear and reinstalled the resistor right before the coil. I had somebody power brake it until the car started to sputter. There was not a drop in voltage at all, but it did not rise either. I do not understand. Maybe the coil needs a higher voltage at higher RPM's. I tried two different coils and the car sputtered and died with both of them
The voltage may not be dropping but depending on the resistor you are using you could be seriously limiting the current available to the coil which will hurt you at higher rpm.

The coil needs a solid 8-10 volts to work well. Any less that that and the current will try to go up and will burn out the coil. If the voltage is to high you will overheat the coil. If you are going to add a ballast resistor, which is not a bad idea it needs to be fed from full battery voltage to the resistor, not through the original resistor wire. My feeling is that the resistor wire or one of its connections have failed and this is why it seems you cant get a consistent voltage reading at the coil. The old Chrysler style ballast resistors work well and you can look at the wiring diagram to put it in line.
 
#49 ·
Steven we are trying to help here but you still have not answered a couple of questions, first what correction did you make when you said said earlier you looked at the diagram and found an error? Next what is the voltage on the wire you are using before the resistor is added? Is it the Red/Green wire normally used for the ignition? Your first posts were indicative of too much resistance and a very low voltage, which can happen with an old resistor wire that has corroded or has broken wires inside it, but then you said you found and corrected a wiring error and now the voltage is too high, what did you correct?
 
#50 ·
Steven Pisonero said:
I purchased a total of three resistors. One was supposed to be 1.3 ohm, but when I tested it, it came out to be 1.6 ohm. the others were 2.0 and 2.3 respectively. I thought I had the 1.6 on, but it was the 2.3. I put the 1.6 on and the sputtering only happened at very high RPM's. I just ordered a .8, 1.0 and 1.3 from summit. I believe one of them should work. From what I read, if the 1.3 works, then we can assume that the resistance wire is allowing full current and we can consider the ballast resistor a fix. If I have to go lower, then I should have the wire removed. Is this the right manner of thinking or should i have the resistor wire removed regardless?

I attempted to pull the resistance wire myself, but once I got in there, it was not readily available and I am not comfortable enough with my skill level, definitely a job for a pro.
The resistor values are probably correct, most cheap meters (less than $300) will not give accurate readings under 10 OHMs.

Remove the resistor wire, it is most likely bad.
 
#51 · (Edited)
There is no need to remove the resistor wire since this requires opening the harness and can be intimidating to someone not familiar with wiring. Simply splice into the Red/Green ignition wire at the switch and run the new wire to the resistor then the coil, clip the end of the old wire at the coil and tape it safely into the harness then any factory wiring failures will be effectively by-passed, I have repaired a couple of resistor failures this way.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top