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bob from maine
User Rank
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.

Rigby5
User Rank
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.

Rob Spiegel
User Rank
Blogger
Question about design
Rob Spiegel   11/2/2012 12:25:15 PM
NO RATINGS
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?

 

bob from maine
User Rank
Platinum
Re: thing we do
bob from maine   11/2/2012 9:51:20 AM
NO RATINGS
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'.

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".

Larry M
User Rank
Platinum
Re: GREAT Detective work ...
Larry M   11/2/2012 9:38:26 AM
NO RATINGS
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

William K.
User Rank
Platinum
GREAT Detective work ...
William K.   11/1/2012 7:31:20 PM
NO RATINGS
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
Who'd a thunk it?
Larry M   11/1/2012 4:37:19 PM
NO RATINGS
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.

Hop
User Rank
Iron
Magnetized distibutor shaft faults Hall-effect sensor
Hop   11/1/2012 4:22:37 PM
NO RATINGS
This was an excellent example of real troubleshooting versus the dealer's "shotgun maintenance" approach of changing everything in sight. Good work!

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!

 

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