When Logic Does Not Make Sense

Posted 3/2/2009
By Arlen Mauland

We have received several Chrysler hotline calls recently concerning the 4.7L engine with coil-on-plug (COP) ignition. Webster's Dictionary defines logic as "valid reasoning." Therefore, logic would dictate that the related wiring or ignition coils are shorting out, but this has proved not to be the case. Our hotline specialists have discovered not a single cause but instead multiple causes that seem anything but logical.

"We have received several Chrysler hotline calls recently concerning the 4.7L engine with coil-on-plug (COP) ignition. Webster's Dictionary defines logic as "valid reasoning." Therefore, logic would dictate that the related wiring or ignition coils are shorting out, but this has proved not to be the case. Our hotline specialists have discovered not a single cause but instead multiple causes that seem anything but logical."

Call 1 Complaint: Fuse was blowing intermittently for the automatic shutdown (ASD) relay, resulting in a stall and subsequent no-start condition. One would immediately think of a shorted wire between the ASD relay and the ignition coils and injectors, but this has not been the case. After many calls on this vehicle, it was determined that the problem was caused by a loose timing chain.

Call 2 Complaint: The problem vehicle came into the shop with the engine misfiring badly and the ignition coils heating up and smoking only at a certain rpm. Testing with a lab scope revealed a suspect pattern from the cam sensor. The technician removed the cam sensor and discovered that the end of the sensor was loose. It is noteworthy to say that the cam sensor had been recently replaced for a previous problem. You probably have heard our hotline specialists say, "New does not always mean good."

Call 3 Complaint: This one was somewhat unique. The vehicle was misfiring. The coil drivers in the powertrain control module (PCM) were faulty, causing the codes to set for coil primary circuits. Since the root cause of the problem was not found, the coil drivers in the next two replacement PCMs failed also, and the same codes for the ignition coil primary current problems were set. A faulty cam sensor was found to be the root cause.
Apparently, the 30-amp fuse for the ASD relay had previously been replaced with a 40-amp fuse to "fix" a problem of the fuse blowing intermittently. The fuse quit blowing, but that caused the coil drivers in the PCM to become the weakest link.

How does any of this cause the ignition coils to overheat or fuses to blow? One theory is that the loose timing chain was allowing the cam and crank sensor signals to get out of sync with each other, or the signal from the camshaft sensor was not being recognized by the PCM. The PCM then does not control the saturation time of individual coils properly, causing an increase of current flowing through some of the ignition coils. The rise in current flowing through the ASD relay would eventually blow the fuse and cause the engine to stall. If the current flow does not get high enough to blow the fuse, the ignition coils start overheating. Apparently on this engine package, the PCM cannot tolerate much variation in the signals from the cam and crank sensors. The PCM needs to see precise signals coming in to control the outputs.

Computer logic may not always make logical sense.

Arlen Mauland is an Identifix Chrysler master and Mitsubishi specialist. He is ASE master and L1 certified.

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