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1994 f150 4.9L standard trans ignition coil...

Resolved • Response time 1 hour

4 Jul 2013

1994 f150 4.9L standard trans ignition coil keeps failing couple months ago replaced coil distrib an wires an plugs today replaced 2 coils an rechecked all six spark plugs wires an dis cap no sign of shorts or wear gaps are at proper gaps also .044 two coil failures back to back.
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4 Jul 2013

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Ford Mechanic's response
4 Jul 2013
Dr. Hamman
Dr. Hamman
Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. Also keep in mind I don't know if you are a pro or a novice, so feel free to add any additional info at any time. I am also an electrical specialist and I will try to help you with this problem. On an ignition coil that takes a coil out is too much power draw, vibration, or excessive heat (normally from over powering) Vibration is something pretty straight forward that you can confirm by inspecting the mounting. If it is bolted on firmly in the factory location it is pretty safe to assume that vibration isn't a factor. That leaves too much power. To understand this part you need to have a good idea of what controls the power draw of a coil. The power draw is determined by the electrical resistance of the coil measured in ohms. There is even a formula that you can use to determine the power draw. It is the Amperage = Voltage (divided by) the resistance (in ohms). It works like this, if the circuit has 2 ohms of resistance, and it is a 12 volt circuit that means that amps = 12 divided by 2, so 12/2=6 the circuit will draw 6 amps. So unless it is the wrong coil, or the voltage is increased the coil cant draw additional power, causing an overheat failure. This basic formula will carry you through a lot of electrical problems. So what do we have left that could be the problem. It's is important to understand that no matter what happens because of the characteristics of electricity that the coil cant be overloaded unless it is over volted. Even if you directly short the output it still has the same internal resistance, so the power draw is set in stone so to speak. So now I am wondering what the voltage is to the coil. If the system voltage is too high (above about 14.5 volts) the coil might draw excessive amperage possibly burning out coils. But there should be other signs of over voltage, battery failure, and other devices burning out. I would measure the voltage at the positive terminal of the coil and the positive battery cable with the engine running to see what the voltage is. What I have seen on Fords is, sometimes the wire will break inside the plastic right before the connector to the coil. If you unplug a coil that isn't working and plug back in a new coil it might work for a while. I have also seen the 12 volt electrical connector loose good connection to the actual wire at the coil, and cause symptoms like coil failure. These problems are hard to spot. I would consider installing a new connector for the coil and trying it. On the vehicles I have worked on this has solved a lot of problems. Go over all of this and if you have any more questions at all, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer. I do not get paid for my work unless you rate at 3 to 5 Smiley Faces or Stars, a bonus is always greatly appreciated. If the answer is not clear, let me know what additional help you need, and I will be happy to assist you further.
4 Jul 2013
Customer reply
4 Jul 2013
After reading your reply I checked the plug and yes the wire in plug was loose in socket. I pushed into spade on coil an motor started. while motor was idling putting tools away to go on test drive while closing hood motor died. Replaced coil once again an removed factory coil plug an crimped on 2 female shielded fittings. Turned on ignition checked voltage at battery and post on coil power was matching 12.5. Tried starting motor did not start, while trying to start was getting no voltage over 12.5v at coil.
4 Jul 2013
Ford Mechanic's response
5 Jul 2013
Dr. Hamman
Dr. Hamman
I would stick a straight pin, like is used for sewing into the power wire to the coil about 4 inches away from the coil and check and see if there was power there. This allows you to get a power reading without stripping the wire. If the connector has problems, or if the wire is broken inside the plastic it should read voltage there when the coil isn’t getting any power. Remember that this type of problem can be intermittent, so you might plug it up, and it might work or not, and if it works it might go 1 minute or 1 week you can never tell. This is one of the challenges you run into on electrical systems with intermittent problems. It can make it look like parts are bad, that are really good, but have lost electrical connection. If the power wire to the coil isn’t getting power at the straight pin you push into the power wire, then you are going to have to get into the electrical system to find the problem. You could have an ignition switch that is failing, or a bad connection anywhere between the ignition switch and the coil. The ignition switch is a fairly common failure, but the electrical connector at the coil is the more likely problem. What I would do if it was mine is to install what is called a pigtail, I am not kidding. A pigtail is a replacement connector with a short piece of wire coming off of it, the short wire is the tail a guess. Any way you need a coil pigtail, and you will have to call around to parts stores, and see if it is offers for your vehicle. Go over all of this and if you have any more questions at all, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer. I do not get paid for my work unless you rate at 3 to 5 Smiley Faces or Stars, a bonus is always greatly appreciated. If the answer is not clear, let me know what additional help you need, and I will be happy to assist you further.
5 Jul 2013
Customer reply
5 Jul 2013
I put 2 new connectors (one on each wire) on the coil wires. Its getting 12.5 v when I turn the key switch on and also when I try to start the motor. So the coil should be hot. I am now wondering If the rebuilted distributor I got from Auto Zone a couple mouths ago is failing? Could be?
5 Jul 2013
Ford Mechanic's response
5 Jul 2013
Dr. Hamman
Dr. Hamman
I feel pretty helpless from here. When someone has a tough electrical problem I find myself wishing I was there so I could get to the bottom of it. I might be a glutton for punishment, LOL, I have been accused of it, but electrical repair is something I have done a lot. Now as far as the distributor goes, it is entirely possible the distributor is bad. But if this is the same symptom that you had before you installed the distributor, odds are the distributor isn't the problem. This type of problem can be real tough to track unless you can watch something quit, on a meter, like power to the coil, or if the distributor looses connection with the coil. I know you have been through the mill with this problem, hang in there and you will spot the problem. You should probably get an electrical schematic for the ignition and dig into it to figure out how to start testing the connection between the coil and the distributor. I would also keep monitoring the power to the coil. When tracking the problem and the engine wont start, never assume the coil isn't firing, always check it so you will know for sure. Using a spark tester on it, like one you but at a parts store, stresses the ignition more, and sometimes will make a problem easier to spot. Mainly because building a higher voltage that is required by the tester will often make the problem, like a bad connection show up. Go over all of this and if you have any more questions at all, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer.
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Dr. Hamman
Dr. Hamman
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I have 30 years of Ford Technician experience. I have built many custom Fords, I also collect them.
5 Jul 2013
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