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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?
 
#6 · (Edited)
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 .
 
#7 ·
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.
 
#8 ·
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.
 
#9 ·
Steven Pisonero said:
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
 
#11 ·
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.
 
#12 · (Edited)
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
 
#13 ·
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.
 
#14 ·
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.
 
#15 ·
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?
 
#16 ·
color code ign module ?

Do you have the right color of the rtv where the wires come out of the ignition module. Ford had a problem with the ignition modules on trucks in hot weather when the Executives at the assembly plant drove them home .as part of the quality controll test. (and a free comuter vehicle each day.) I knew one guy that used to stop at a convenience store about half way home and get a cup of ice on hot days so he could make it home.
 
#17 ·
Steven Pisonero said:
.....
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
Make sure that however the coil is mounted, the top/tower terminal
is NOT pointing straight up. The coil is oil-filled, and if it's mounted
upright then an air bubble will form at the high tension connection
and the coil will fail. (The oil cools the coil.)

The stock Duraspark coil is mounted at about a 45* angle, with the
tower pointed directly forward.
 
#18 ·
I ran the car quite a bit tonight, without the AC on. The temp never went over 210. When I got home, whether or not I had the AC on, we were at just under 4 volts with the car running.

The coils were bad. Once I would let them cool off, they may work for a short time, but would quickly stop working again I would replace it with a new one and it would work fine until I began using the AC again, then it would die. This would be especially be true in idol with the AC on. In fact every time I left the car in idle for several minutes with the AC on, I would burn up a coil.

I believe everything is charging fine. My battery is charging correctly and I do not have an issue with any of my accessories. When I chick the voltage going to my AC compressor, it is 12V. My AC has quit working. The lines will get white with frost but little cold air. Either that is a symptom or I am the most unlucky SOB ever. The clutch engages.
 
#19 ·
Steven at less than 4 volts those coils were probably ok and would have continued to fire had the voltage been higher and it would be normal for them to "die" at that low voltage as they got warmer and the internal resistance increased, the fact they would work when they cooled off just reinforces that opinion. You can check a coil with an ohm meter, the primary windings (terminal to terminal) should be between 0.8 1.6 ohms (a little on either side of that would still be ok) and the secondary windings (coil wire to either terminal) should be about 5,700 to 10,500 ohms. The fact is if you truly do have less than 4 volts in "Run" then your resistor circuit is bad and needs to be repaired. Do you still have any of the other coils so you can check them with an ohm meter? At that low voltage a new coil might work for a while but internal resistance would normally increase slightly and would vary with temperature most likely causing exactly the problem you describe. The service manual calls for a minimum of 6 volts on the primary wire to the coil with 8 to 9 volts max with the key in "Run", that voltage reading you describe is bound to cause problems so you need to repair your resistor circuit. Don't be tempted to just run a hot wire from the ignition to the coil as this will burn out coils and depending on routing could also damage the control modual. That has been a fairly common problem when using the DuraSpark conversion to replace a points system on older cars and some think it is ok because since the points are no longer used then the resistor is not needed anymore. This is not the case however and if the resistor is eliminated the system will work for a while but will fail sooner or later.

In any case you apparently have just the opposite problem now and you need to check/repair the resistor circuit so that you have at least 6 volts to the coil, can you take your modual off and have it tested? AutoZone and a couple of the other Auto parts stores will test them for free.



One other thing here, what procedure are you using to test the primary voltage? Are you checking the positive wire to the coil and grounding the meter to the block?
 
#20 ·
I checked the Ohms of both the good and last bad coil and both were at 1.6. According to my voltage tester, I am to test voltage with positive to positive and negative to negative. After warming the car I retested voltage. With the car in the run position, I was at 5.71 V, but with the car running, it drops to about 4.0. The the meter is showing what looks like negative Ex. -5.71 and -4.0. I touched the negative part of the meter to the block and got nothing, but when I touched the negative part of the meter to the negative battery terminal, it looked positive rather than negative and I got a reading of 6.5.

I replaced my module due to leaking about a year ago. I could have it tested again I suppose.

Is the line to the left of my numbers a negative sign? My instructions don't explain what the line means.
 
#22 ·
Test with the positive lead on the meter to the positive wire to the coil (the ignition wire) and ground the negative lead from the meter to the engine block, ignore the negative wire on the coil for now. If you are getting a (-) in front of the number showing in the meter display then you have the meter connected backward, are the leads plugged into the right sockets on the meter?


That 6.5 reading is about right and you should be getting the same, or very nearly so, when grounded to the block. It sounds as if maybe you are just not getting a good connection point when touching the block, make sure your ground test point is actually the block itself and that the meter lead is touching a good clean spot.


EDIT: I said that 6.5 sounds about right but actually it is on the low side and could be better, was that reading with the engine running?
 
#24 ·
You should have 8 to 9 volts with the engine running but 2 volts with the key on-engine off can't be right because that probably would not even fire the coil to start the engine. It looks as if the resistance circuit is the culprit and since there are a couple of variations of that system you need to determine which wire provides the low voltage in "Run" and test the resistance on that circuit, how many plugs does your module have?
 
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