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Ignition Coil Primary Voltage

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Old 07-13-2011, 02:10 PM
FDSJr FDSJr is offline
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Question Ignition Coil Primary Voltage

I am still trying to resolve an issue with continuously failing ignition coils on my 1993 Ram Van 5.2 L V8. My thought is that the PCM is causing it, since that was the only thing that was replaced shortly before the coils started failing. I have gone through 4 coils and a lot of towing since the PCM was replaced in mid-June.

Can anyone tell me what the correct primary voltage should be on the ignition coil. It ranges between 0.8 Volts and 1.2 Volts when the engine is idling. When I rev it up to 2000 rpm and higher, the primary voltage climbs to 5.6 volts.
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Old 07-13-2011, 02:33 PM
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Are you sure you're pinning the right wire? One is PCM ground and one is ASD feed (battery voltage) Never seen anyone measure voltage while the engine is running. There really isn't a spec for voltage drop across the coil but one would assume there would be a change with RPM. You aren't using race car wires with a stock coil are you?
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Last edited by TNtech; 07-13-2011 at 02:44 PM..
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Old 07-14-2011, 11:49 PM
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The wires are stock 1993 vintage. The current coil is an OEM Mopar replacement. All measurements were made wtih a Fluke 87 multimeter.

There is a 2 position connector that attaches to the coil primary winding. One wire is Green/Orange and the other is Grey. I am measuring directly across these 2 wires. I checked my method and determined that I was using a 100 mS sampling rate, which means that the max or min voltage would have to last for over 100 mS before the meter would record it. This gave me the readings ranging between 1.2 and 5.6 volts.

I changed my measurement procedure to a 1 mS sampling rate so that I would pick up spikes lasting more than 1 mS. This changed the results totally. Here is what I have now.

Measuring directly across the Green/Orange and Grey wires connected to the coil primary:
Idling: Min = 12 volts, Max = 147 volts
High RPM: Min = 12 volts. Max = 123 volts

Measuring between the Green/Orange wire and Ground:
Idling: Min = 12.8 volts, Max = 13.6 volts
High RPM: Min = 12.8 volts, Max = 14.0 volts

Measuring between the Grey wire and Ground:
Idling: Min = 1.2 volts, Max = 90.8 volts
High RPM: Min = 1.6 volts, Max = 69 volts

I am guessing that these high voltages, well above the battery / alternator voltage are generated by the decay of the magnetic field in the primary winding when the ASD shuts off the current to the coil. Since I can find no spec to compare them to, I don't know if they are normal or not.

So, what does this all mean with respect to my persistent ignition coil failure problem? At this point I have no idea, but I will continue to research it. There has to be something that is causing these failures and my opinion is that it is the new PCM that was installed immediately before the coil failures started to occur. The dealer simply keeps replacing the failed coils and claims that they must have been defective. I am skeptical that 3 defective coils have been installed in less than one month.

Any thoughts that anyone can offer will be appreciated.
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Old 07-15-2011, 12:44 AM
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One more piece of information. It is the ignition coil SECONDARY winding that is failing. The winding is apparently burning up and I end up with an open circuit in the secondary rather than the spec resistance. The resistance of the primary winding of a failed coil is within spec.
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Old 07-15-2011, 02:30 AM
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Quote:
Originally Posted by FDSJr View Post
One more piece of information. It is the ignition coil SECONDARY winding that is failing.
This can happen when the spark plug gaps are not set correctly or if spark is jumping to ground.
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Old 07-15-2011, 03:49 AM
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OK, on the Green/Orange wire. This is the ASD feed. It will reflect battery voltage. The Grey is the wire that PCM grounds to energize the coil. Measuring between them is not going to net you anything useful unless you have a running problem or no fire at all. There should be less than 5 ohms on that grey wire between PCM and coil plug. (Both ends unplugged)


Quote:
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This can happen when the spark plug gaps are not set correctly or if spark is jumping to ground.
Yes... If you have an inductive clamp to put on each plug wire individually, this may help you run it down IF one cylinder is spiking way high or way low. The way it works is the primary winding induces voltage into the secondary winding. The net voltage TO the plug ends up being in the THOUSANDS of Volts. The inductive clamp will reveal this.

Maybe the wirng harness needs to be opened up and inspected. If my memory serves me right, the coil primary wires are bundled up with the alternator cable? When they put the PCM in they possibly could have moved some old deteriorated wiring around. The 2 grounds to look for are the one on top of the alt bracket and the one just under the PS pump bracket. I wish I could help more, but I would need to have the car with me. Let me know what you find.
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Last edited by TNtech; 07-15-2011 at 04:09 AM..
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Old 07-15-2011, 02:02 PM
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Quote:
Originally Posted by FDSJr View Post
The wires are stock 1993 vintage.
Increased resistance as the wire ages in the older wires could lead up to coil failure. I highly recommend changing these out.
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ignition coil, pcm, primary voltage

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