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.
Rob, yes, replacing the steel gear with bronze would prevent the issue but create another; the government requires 100K mile emission warranty, the wear characteristics of the bronze gear would require replacement before 100K miles. Most old distributers (using ignition contact points, not Hall-effect sensors) taken from high-mileage engines of old had bronze gears with severe wear. This could be seen using Sun test stands or a good dwell tach which showed the dwell and timing jumping back and forth. Replacement gears were readily available through parts distributors. I don't think individual parts are stilll available for distributors or alternators or starters. Your choice is buy new from the dealer or remanufactured from the parts distributors. Considering a mechanic at a good garage bills between $50 and $85/hour, it just isn't worth it to pay them to rebuild anything.
It is clear car makers are at fault, not for trying a steel gear that will cause failure eventually, but for not giving the dealer mechanics the tools to diagnose it.
Clearly mechanics used to use oscilloscopes a lot in the past, to diagnose alternators, condensers, point bounce, advance curves, etc.
Oscilloscopes are still important for things like this Hall effects failure, and the diagnostic procedure should have been covered. In fact, the ODBII standard should already have caught this. All fuel injected cars should also come with a dash display for fuel pressure as well. Current cars are not being supported properly by the maker, and they are ignoring maintenance. It is getting worse. I stick to buying the older cars because they are far more reliable. New cars these days are junk because they are becoming impossible to maintain.
Would replacing the steel drive gear with a bronze gear(as in the older design) eliminate or reduce the chance of the distributor shaft from being magnetized?
naperlou I also had a seemingly endless supply of British motorcars and as a result, learned a great deal about bad design, poor materials, bad luck and how to 'make do'.
I always remember high-school shop class, our teacher wanted us to learn SOMETHING so he frequently repeated; "you magnetize steel by stroking and direct current; you demagnetize steel by heating and alternating current." The cam gear is helical so each turn of the distributor represents 10 or so strokes by one steel gear wiping against another steel gear. The old TV degausser was part of my ever growing pile of things I'm sure I'll never need, but in this case I was wrong. It still hangs on the wall of my garage right over the sign "Danger, HIGH Resistance".
William, you can magnetize any piece of steel by stroking it in one direction with a magnet. E.g., if you have a bar magnet and an unmagnetized steel rod or bar or horseshoe, you can magnetize the unmagnetized component by repeatedly stroking it in one direction with one end of the bar magnet.
Think about it. Magnetization is simply alignment of all the molecules in the same direction. If each stroke aligns a few more molecules due to the influence of the passing field, ultimately a significant proportion will be aligned.
No piece of steel is pefectly unmagnetized. There has got to be a bit of residual magnetism there someplace, even if it's only a couple of molecules. If the pieces are stroked in a repeated manner, the pieces will ultimately become magnetized.
When two gears are in contact, a wiping (stroking) action takes place as the teeth pairs com in contact. transfer motion, and leave contact. Check out this animation of the contact point motion:
Definitely some good reasoning as well as the detective work. Changing to cheaper materials is something that Chrysler purchasing has quite a reputation for. So even if the design started out good, purchasing may cut costs without any clue as to why it had been designed differently, and often without contacting the design engineer. Of course many of the managers were devoid of any clue, so the problem would not be discovered until it would damage the designers career.
I am not sure just how steel parts self magnetize, but we often had to demagnetize production fixture parts. So I guess that it does happen.
Bob, thanks for the excellent insight. I have been fiddling with Hall effect sensors on my 1992 Mitsubishi Expo (250 K miles) for some time. In hot weather I get misfires and diagnostic codes from the one in the distributor, and I periodically also get them from the sensor in the air-inlet servo--a valve controlled by a DC servo motor.
It never occurred to me that magnetization could be the source of either problem. I have a degaussing coil from an old color TV and I think it's going to get a workout.
This IS an excellent example of how INTERDISCIPLINARY knowledge is essential in any technical endeavor. Even though his main thrust was / is electronic troubleshooting, he had / has sufficient understanding of the principles of ignition in an internal combustion engine and the Kettering Ignition system, so was able to dig deeply into the problem.
What's really a total bummer is that his "discovery" went unacknowledged & unrewarded by the DODGE factory geniuses. It would be interesting to determine IF Dodge prepared a TSB (Technical Service Bulletin) for distribution throughout their dealer network, OR if it fell by the wayside, so the customers affected by this malady continue to pay through the nose at the dealers' incompetence!
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