The Dodge Truck Was a Magnetized Mess

I have a 1986 Dodge W250 318CID truck that I use to plow snow and do work around town. One day, the truck started running terribly. It had no power, an extremely high idle, and severe knocking.

A quick check revealed that the spark advance was going to 53 degrees before top dead center immediately after starting, and the only method of pulling it back was to apply a 18-inch to 20-inch piece of vacuum to the transducer connected to the electronic module. This was more vacuum than the engine generated at an idle, so using a hand pump was the only way to keep the truck running.

The dealer was unable to offer a solution. A call to the Dodge factory service hotline that dealers use didn't offer any help. The distributor was a standard Mopar component with a Hall effect sensor to trigger the computer and thus fire the ignition coil. There was no vacuum advance on the distributor itself, only mechanical. I wired the advance springs to prevent any centrifugal advance, and the condition remained. I replaced the distributor with one from a 1970s era Dodge Coronet, and everything worked fine, although the advance curve did not meet the factory specification due to different weights and springs.

The only difference was that the drive gear on the bottom of the distributor shaft was bronze on the early unit and steel on the truck unit. The early distributor was completely worn out and not usable without rebuilding, but the difference in the gears appeared relevant. I connected an oscilloscope to the output of the Hall effect sensor and found it was sending out multiple output pulses per cylinder on the truck distributor with both Hall effect sensors, but it was clean on the older distributor.

On a hunch, I dug out an old parts degausser and ran the shaft and gear through the coil, and the problem went away. It appeared the steel gear and shaft were becoming magnetized by the wiping action from the camshaft and caused the Hall effect sensor to get false readings. The local Dodge dealer had found this symptom on several other vehicles, and its fix was to replace all components of the ignition system (distributor, coil, computer) at significant cost. It didn't have a degausser, and I didn't offer to donate mine, so I guess it continued to replace parts. The Dodge factory people seemed politely interested, but I never heard whether they changed the design. I haven't heard about this problem on newer models.

This entry was submitted by Bob Humphreys and edited by Rob Spiegel.

Bob Humphreys received electronics training in the US Navy and then worked as an electronic technician and test engineer in the semiconductor industry for 15 years. He built houses during the occasional layoffs, and he built and repaired boats for more than 40 years until age diminished his ability to continue. He is working as a senior process technician for a medical instrument manufacturer in Maine.

Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.

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