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Sherlock Ohms

The Dodge Truck Was a Magnetized Mess

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Mydesign
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Platinum
Wrong interpretation from hall Effect Sensor
Mydesign   11/1/2012 7:25:31 AM
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1 saves
"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"

Bob, these problems occurs with Hall Effect sensors. When I had done some experiments with Hall Effect sensors, I found that any magnetic field near to the Sensor will be interpreted as signal. This will lead to the malfunction of system, where sensor o/p is served as the input to the system. I had faced similar problem in Brushless DC motor drive, where Hall Effect sensors are using to drive the circuits.

naperlou
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Blogger
thing we do
naperlou   11/1/2012 12:03:40 PM
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Bob, I am impressed with your ability to track down and resolve this problem.  I was especially amused at your early solution, though.  The idea of using a hand vacuum pump to keep the truck going at idle was great.  The things we will do to keep moving. 

tekochip
User Rank
Platinum
Excellent Job
tekochip   11/1/2012 1:26:20 PM
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I really don't think I would have been able to figure that out.

OLD_CURMUDGEON
User Rank
Platinum
GREAT Detective work ...
OLD_CURMUDGEON   11/1/2012 3:26:31 PM
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!

 

Hop
User Rank
Iron
Magnetized distibutor shaft faults Hall-effect sensor
Hop   11/1/2012 4:22:37 PM
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This was an excellent example of real troubleshooting versus the dealer's "shotgun maintenance" approach of changing everything in sight. Good work!

Larry M
User Rank
Platinum
Who'd a thunk it?
Larry M   11/1/2012 4:37:19 PM
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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.

William K.
User Rank
Platinum
GREAT Detective work ...
William K.   11/1/2012 7:31:20 PM
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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.

Larry M
User Rank
Platinum
Re: GREAT Detective work ...
Larry M   11/2/2012 9:38:26 AM
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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:

http://science.howstuffworks.com/transport/engines-equipment/gear8.htm

bob from maine
User Rank
Platinum
Re: GREAT Detective work ...
bob from maine   11/2/2012 9:44:04 AM
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".

bob from maine
User Rank
Platinum
Re: thing we do
bob from maine   11/2/2012 9:51:20 AM
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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'.

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