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.
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?
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.
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.
Yes, I have magnetized things by rubbing them with a magnet. BUT WHERE did the original magnetism come from? Engine parts are NOT supposed to be magnetic especially gears. And I don't believe that rubbing non magnetic parts with non magnetic parts would cause them to be come megnetized. That was the point.
Hall effect sensors incorporate a magent in the design (that's how they work - the Hall sensor senses changes in the magnetic field due to the motion of a distributor cam, "gear" or interruptor wheel). Presumably it's possible for this small magnet to eventually magnetize the distributor shaft, etc.
Another possibility might be a short circuit (or partial short circuit) from the battery to the distributor shaft. Since automobiles use DC, such leakage could cause the shaft to become magnetized over time.
Demagnetization would solve the problem in either scenario, but only temporarily.
As I stated previously, no steel is PERFECTLY unmagnetized. By virtue of entropy, there must be a couple more molecules aligned than exactly 50% in each direction. With the two gears (cam and distributor shaft) rubbing egainst one another, each becomes more and more magnetized over time.
To back up your remark about no steel being completely without magnitism I remember compass reading when in the Army. You put the compass up to your eye so you could see both the compass needle and the sight wire attached. You lined the needle with the desired azmuth and picked your target with the sight wire. If you did it without removing your steel pot, or laying your weapon aside, the readings were very unreliable. In fact some times the steel pot could pull the needle to which ever direction you looked and if I remember correctly, you might be 10 or 15 degrees off.
I never really questioned from where the original magnetism came, but was well aware of it being there.
I think this is more an issue of cost rather than the desirable wear feature of steel. It has been my unfortunate experience to machine some different types of bronze that machined as hard as any steel I have come up against. But it is not a cheap substitute and in many cases is hard to justify the expense involved.
One such application involved the weapons industry and the final product had to be Non-sparking, as it was used in a cartridge loading application that involved gun powder. Sparks and gun powder do not mix, so any ferrous material on ferrous material was forbidden, but we still needed the wear characteristics of tool steel. I had to learn more about metallurgy than I ever thought I would need, but in the end the product worked well and the customer was satisfied. Always a good thing.
Newer vehicles have crankshaft position sensors and multiple coils. Not many even have distributors, instead having coils that fire multiple cylinders. Some still use distributors and Hall-effect sensors but not many. Considering the number of trucks, vans and SUV's that had V8 engines with a similar set-up from Chrysler, GM and Ford, there were relatively few that exhibited this problem. The only "unique" aspect of this truck was a 200A alternator, 2 large batteries and a 2000W inverter to power the occasional 115VAC power tool. There may have been eddy-currents drifting around but I was never able to measure anything unusual. This same set-up has run on thousands of marine applications with no ill-effects.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 3
Early in my career, I worked as a draftsman and remember the days of drawing on vellum with numbered pencils and Mylar with plastic lead. This was a fun experience in the sense that I ...
I've been using workstations for more than 10 years and love finding ways to get more performance from my system. With demanding professional applications that require more power each ...
A lasting memory from my first job as an engineer in an auto assembly plant is standing on hard concrete at six in the morning, vending-machine coffee clutched in hand, listening to ...
A quick look into the merger of two powerhouse 3D printing OEMs and the new leader in rapid prototyping solutions, Stratasys. The industrial revolution is now led by 3D printing and engineers are given the opportunity to fully maximize their design capabilities, reduce their time-to-market and functionally test prototypes cheaper, faster and easier. Bruce Bradshaw, Director of Marketing in North America, will explore the large product offering and variety of materials that will help CAD designers articulate their product design with actual, physical prototypes. This broadcast will dive deep into technical information including application specific stories from real world customers and their experiences with 3D printing. 3D Printing is
To save this item to your list of favorite Design News content so you can find it later in your Profile page, click the "Save It" button next to the item.
If you found this interesting or useful, please use the links to the services below to share it with other readers. You will need a free account with each service to share an item via that service.
Tweet This
1 
