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Sherlock Ohms

The Dodge Truck Was a Magnetized Mess

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Rob Spiegel
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Question about design
Rob Spiegel   11/2/2012 12:25:15 PM
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Here's a question from Mike Krysiak:

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?

 

Rigby5
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Gold
Car makers don't offer enough support
Rigby5   11/2/2012 12:58:52 PM
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.

bob from maine
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Platinum
Re: Question about design
bob from maine   11/2/2012 2:44:18 PM
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.

William K.
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Re: GREAT Detective work ...
William K.   11/2/2012 9:42:41 PM
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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. 

Where did the initial magnetization come from??

btlbcc
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Gold
Re: Wrong interpretation from hall Effect Sensor
btlbcc   11/2/2012 10:17:44 PM
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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.

 

Larry M
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Re: GREAT Detective work ...
Larry M   11/3/2012 11:45:04 AM
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William,

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.

Tool_maker
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Re: GREAT Detective work ...
Tool_maker   11/5/2012 12:49:33 PM
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  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.

Tool_maker
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Re: Question about design
Tool_maker   11/5/2012 1:02:12 PM
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  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.

Rob Spiegel
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Re: Question about design
Rob Spiegel   11/5/2012 1:55:34 PM
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Thanks for the explanation, Bob. But it makes me wonder why this problem is not more widespread. Do most cars have a different type of setup.

bob from maine
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Platinum
Re: Question about design
bob from maine   11/5/2012 3:17:25 PM
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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.

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