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Old 08-04-2011, 02:07 AM
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Burning through ignition coils

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?

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Old 08-04-2011, 08:07 AM
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Timing is to slow, should be about 6 degrees BTDC with vac advance unhooked and plugged.
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Old 08-12-2011, 01:27 AM
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What is the advantage over the stock 4 BTDC and and vacuum advance hooked up?
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Old 08-12-2011, 09:44 AM
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Summer, AC, it only happens in the summer, could the coil be mounted in a spot that is not getting air circulation only thing I can think of. This a factory set up or modified configuration ?
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Old 08-12-2011, 10:39 AM
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This is a factory set up and the coil has not been moved. The A/C also has difficulty staying cool. I have been removing the thermostat to prevent the car from overheating. My understanding is that if the cam is advanced too far, the car can overheat.
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Old 08-12-2011, 11:24 AM
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Are you replacing the coil with factory parts or discount parts, aftermarket ? Removing the thermostat is a bad idea.

If an ac system has been over charged the engine will run rough because the system has been over charged. Too much gas equals to high of a pressure, then trying to start the car with the compressor energized will make the car seem hard to start, same reason pressure in the compressor is high. The absolute only way to charged an AC system is to put in a KNOWN quantity of freon, there is a reason that tag is on the radiator shroud .

Last edited by unix; 08-12-2011 at 11:38 AM.
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Old 08-12-2011, 01:48 PM
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Check the voltage on the coil with the key in "run", it should be 8 volts or less. If the voltage is too high (full circuit voltage) then likely the resistor circuit is faulty which is not an uncommon condition with that system. Also has the ignition wires from the module to the coil been modified in some way? If the resistor circuit has been defeated then the coil will overheat and burn out. A high voltage condition is almost always the cause of repeatedly burning out coils.
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Old 08-12-2011, 03:30 PM
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I am using a lifetime warranty coil from my local autopart store. I also tried an accel high performance, both are burning out. The wiring has not been altered. My coil does not have a resistor. What I thought was a resistor is some kind of noise supressor for the radio I am told. I am going to put a 160 degree thermostat in see how that goes.

I had my A/C professionaly vaccumed, tested and charged. It is still difficult to start with A/C on.

I don't think removing a thermostat can cause this, but my AC is pretty cold at idol, but as I drive it heats up. My entire A/C system is new, The clutch of the compressor is engaging even at high RPM's. The metal from the A/C hoses turn white from ice while it is on, but as I drive it heats up. It is getting plenty of air flow.

The only two things I have done is remove the thermostat and move the timing set to TDC from 4 bTDC. I am going to fix that this weekend. If there are any more ideas I would like to hear them. I will also check the voltage to the coil.
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Old 08-12-2011, 04:17 PM
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Quote:
Originally Posted by Steven Pisonero
My coil does not have a resistor. .

The resistor will be a resistor wire not a ceramic type but if you have a factory unmodified DuraSpark ignition then the resistor is definitely required. Check the voltage at the coil with the key in "Run" and you most likely will find it reads full circuit voltage, this is a fairly common cause of coil failure on these systems. Contrary to popular belief the resistor circuit was/is to protect the coil when the engine is running and was not used to protect points on a points type system, it is still required on some solid state systems like the Ford DuraSpark..


Here is what your wiring should look like to help you see where the resistor should be. Does your module have two plugs or three?

www.sierrabronco.com/DuraSpark.jpg
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Old 08-13-2011, 02:23 PM
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I disconnected the coil wires and cranked it over. Voltage would range between 9-12V, then I left it on run or accessory and it was at 11.7. I guess that means I have an issue
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Old 08-13-2011, 02:50 PM
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Apparently you do, that lower voltage during cranking is the real reason for the resistor circuit but it is a common mistake to think is was to protect points on a points system, as in your case however it was still used on many solid state ignition systems. Due to the starter load the voltage will normally drop to about 8 to 9 volts during cranking so the coil is designed for that 8 to 9 volts so it can produce a hot spark on that lower voltage during starting, if the coil was wound for a full 12 volts it would make a weak spark during start. If the resistor circuit is defeated or fails in some manner then the voltage will return to full system voltage after the starter is released allowing the full voltage to reach the coil causing an overvoltage condition and resulting overheating. That's why you should have only 8 to 9 volts at the coil with the key in "Run" but you say you not only have full voltage in "run" but also in "Accessory"? If you have ANY voltage to the coil with the key in Accessory then you have a wiring error since Accessory should not supply any voltage at all to the coil. Adding a resistor to the ignition circuit (the "run" circuit) should solve the coil burn-out problem but you would also need the resistor by-pass wire from the start circuit to provide full system voltage ONLY during cranking.
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Old 08-13-2011, 03:29 PM
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My mistake, in the run position I am at 5.6V, I retested several times. The only time I have problems burning out coils is when I run my AC. The AC will bring the car to the point of overheating as well. What is the liklihood that the increase in timing is causing the car to get hot, consequently killing coils?

The body of the coil was right over the intake manifold. I flipped it around so that the body is away from the intake and exhaust. Maybe this will help with airflow and reduce heat

Last edited by Steven Pisonero; 08-13-2011 at 03:55 PM.
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Old 08-13-2011, 04:36 PM
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5.6 volts is a bit below the low voltage called for but not at all unusual when tested with the engine off, with the engine running and the charging voltage over 13 volts normally then that 5.6 will increase to around 7 or 8 which is just fine. If then you do indeed have the lower voltage in run and no voltage in accessory then all is well in that respect and apparently the problem is somewhere else, is the bracket allowing the coil to shake when the engine is running? Except for an over-voltage condition which will make a coil smoking hot there is not much to go wrong that would cause the coil to fail repeatedly, engine temperature is not a likely cause with the factory mount since a coil would have to be quite a bit hotter than an engine to cause failure. Certainly touching or being in close proximity to an exhaust manifold or headers could roast a coil however.
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Old 08-13-2011, 05:46 PM
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The coil is sturdy. The clips on the coil were a little loose, creating a tiny spark when giggled. I crimped them down well. Also when the A/C is going, I only get about 3.75 volts to the coil. The A/c is getting about 12. With the car running for a few minutes,the volts did not change 5.75 without ac on and 3.75 with.
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Old 08-13-2011, 07:28 PM
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Are those readings with the engine running? Both sound low if the engine is running and charging correctly, are you sure your meter is accurate? At less than 4 volts you would be getting a very weak and possibly erratic spark, so weak in fact starting could be an issue and probably misfiring as well, could this low voltage be the problem and the coils are not actually burning out? How did you test them? The coil will need more voltage than that to fire properly and indeed it may not fire at all due to the low voltage when it reaches max normal temperatures. Did the "bad" coils fire at all when cold?
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