Need help!! Spark problem
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Cargo Master
Joined: Nov 2009
Posts: 2,364
Likes: 117
From: starship enterprise
Need help!! Spark problem
I have an imtermitten problem with my 78 ford. On a damp day my truck will lose spark and only fire on 4 cylinders.Last time I replaced the cap, rotor, plugs, wires, and coil as well as greased all the connections. It fixed itself and now it is doing it again.
I ohmed out the coil and checked the voltage, Never found a problem. Swapped the distributor and the module, no change. Checked voltages and everything looked fine.
It only does it when it is raining and damp. Anyone have any ideas? Ignition switch, bad ground, anything? Thanks.
I ohmed out the coil and checked the voltage, Never found a problem. Swapped the distributor and the module, no change. Checked voltages and everything looked fine.
It only does it when it is raining and damp. Anyone have any ideas? Ignition switch, bad ground, anything? Thanks.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
How do you know it's only running on 4 cylinders? That's a rather specific conclusion. If you have no spark, how are you running at all? Are you saying that some cylinders have spark, while others do not?
What do you mean "checked the voltage" at the coil? I assume you're talking about the primary voltage? What were the conditions - engine running/off, position of the key, what readings did you see? All of this matters. The same for your coil measurements. Were these measurements done while the problem was happening?
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What do you mean "checked the voltage" at the coil? I assume you're talking about the primary voltage? What were the conditions - engine running/off, position of the key, what readings did you see? All of this matters. The same for your coil measurements. Were these measurements done while the problem was happening?
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Cargo Master
Joined: Nov 2009
Posts: 2,364
Likes: 117
From: starship enterprise
All tests were done per the ford factory service manual, with a voltage tester. I did not go into the specific results of the tests due to the fact that the results were not showing any kind of malfunction per the manual. Key position and voltage readings and test procedures were properly followed and in spec.
I know that 4 cylinders were firing because the header tubes are hot for those cylinders and cold for the rest.
The cylinders are getting spark, but when I watch it is erratic and inconsistent, sometimes strong sometimes weak, swap wires and/or plugs no change.
Thanks and keep em coming.
I know that 4 cylinders were firing because the header tubes are hot for those cylinders and cold for the rest.
The cylinders are getting spark, but when I watch it is erratic and inconsistent, sometimes strong sometimes weak, swap wires and/or plugs no change.
Thanks and keep em coming.
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Cargo Master
Joined: Nov 2009
Posts: 2,364
Likes: 117
From: starship enterprise
I guess i weeded it to the spark last time with a compression test and replacing the ignition components fixed it. It is worse this time though, and I would like some pinch hitters to come up and give me a direction to go. It is NOT a fuel issue, and other than that it could be anything. I am open to suggestions.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
If you're seeing weak and erratic spark, it seems to point to an ignition issue. What voltage did you measure at the positive terminal of the coil with the key in RUN? Not second guessing your work, but you're basically saying everything checks out fine yet there still is a problem. Something's not adding up, so it's good to lay everything out on the table.
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Cargo Master
Joined: Nov 2009
Posts: 2,364
Likes: 117
From: starship enterprise
I don't remember exactly, I think it was in the 5s as far as voltage, and I don't remember the ohms for the coil resistance, but I think it was in the middle of their specs for ohms. NOW, this was all in run, not in crank. I was cranking it, so I still have to check that, it could be the problem, but I am leaning towards no, so keep giving ideas please.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
5 volts in RUN is a little on the low side, not enough to cause cylinders to miss completely but it's curious why an almost completely new ignition system has low primary voltage. Unless your meter was not using a good ground.
Can you describe how you're checking for spark? What is your plug gap at?
Can you describe how you're checking for spark? What is your plug gap at?
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Cargo Master
Joined: Nov 2009
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From: starship enterprise
plug gap is 0.040" and my spark check is if it will run long enough I hook the wire to a plug and ground the plug at the engine and look at the spark. Sometimes it sparks to the side like a bad plug and sometimes it is a no spark and sometimes it is a spark barely making it over the gap. The spark is yellow and light blue. This is only on the dead cylinders, I have never bothered on the firing cylinders.
I have swapped plugs, wires, and moved the plug around to different locations, and the conditions will get worse, but not better.
I have swapped plugs, wires, and moved the plug around to different locations, and the conditions will get worse, but not better.
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Lead Driver
Joined: Dec 2012
Posts: 7,870
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From: Tennessee
Sounds like a voltage issue . 5 is a bit low .Ck for proper grounds and resistor wire . I have not seen many issues on Fords , but GM resistor wires are known to degrade and drop voltage . That will cause issues . I would expect 7 -9 volts running . Does it have an add on external resistor ? That could be the issue .
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Cargo Master
Joined: Nov 2009
Posts: 2,364
Likes: 117
From: starship enterprise
5 May not have been the voltages, as I said before, whatever the ford book says the voltages are supposed to be is what they are, if they are supposed to be 7-9 then it was 8 volts, if the volts are supposed to be 10-12, then they were 11. I didn't write it down and just tested it twice, and both times were what the book said for the coil voltage to be right. I need to do the little test light trick to check the pickup and stuff.
Save the old ones
Joined: Dec 2002
Posts: 666
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From: Boyne Mich
The positive side of the coil should have 12 volts with key on / engine off. And 7-9 volts with key on / engine on.
This positive coil wire (red/grn i think) comes from the ignition switch and has a 1.05-1.15 ohm resistor.
Per JWC comments, I never seen it but if the resistance goes up, the running volts go down.
This positive coil wire (red/grn i think) comes from the ignition switch and has a 1.05-1.15 ohm resistor.
Per JWC comments, I never seen it but if the resistance goes up, the running volts go down.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
With Duraspark II, the voltage at the coil is 7 to 9 volts with the key in RUN, regardless if the engine is stopped or not. This is because the module grounds the coil as soon as it receives power; it will continue to keep it grounded until the engine turns over and it gets a trigger from the pickup module. This is one of the reasons you can't leave the key in RUN with the engine off for extended periods of time; it dumps about 4 amps to ground, draining the battery and overheating the coil.
Save the old ones
Joined: Dec 2002
Posts: 666
Likes: 4
From: Boyne Mich
That makes sense and I stand corrected. I wasn't thinking about the negative going to the module. And what the module might be doing with it.
Just last week, I traced out the red/grn positive wire to the switch. At the time, the duraspark II was disconnected. So I was getting the full 12vdc.
But still to help the op, 7-9 volts key in run, not 5 vdc.
Just last week, I traced out the red/grn positive wire to the switch. At the time, the duraspark II was disconnected. So I was getting the full 12vdc.
But still to help the op, 7-9 volts key in run, not 5 vdc.
