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MG MGF Technical - Car Died, Burnt out rotor arm

A month ago I changed the distributor cap and rotor arm and after about a month the car died, called the RAC and the rotor arm was burnt out. Changed the arm car perfect. Then today, 2 weeks later same problem and the rotor arm has gone again. I bought the 2nd arm from a different place and was a different make (Unipart. Any ideas what is causing this??
B Smith

I forgot to say that the car is a 97. non VVC
Brian
B Smith

Brian,

What do you mean by burnt out? Where exactly? Due to arching at the tip, or somewhere else? Perhaps destroyed by heat, rather than electrical burn out, some sort of frictional heat? Describe what the burnt out rotor looks like...



Mark Clayton

Ok there is no sign of burning/arcing. The arm looks like new, but it will no longer carry an electrical current to the tip to fire the plugs, so I take it that there is a wire that runs from the centre of the arm to the tip. If I fit a new one the car is fine again until the arm fails.
Brian
B Smith

That section between the the centre contact & the tip is usually covered in an insulating gel-like substance. If this is all still intact, then there is no rational explanation, except that you've been extremely unlucky to have 2 rotor arms fail in this unusual way. (sounds unlikely doesn't it?)

Have you tried testing the rotor yourself? Is the continuity really broken, or could it be something else in the cap for example (or HT lead) which is disturbed and corrected when you go through the action of putting the new arm on? Then a few months down the line the problem comes back again. Tried cleaning the contacts on the coil?
Dave

Hi Dave,
yes I have tested the rotor arms by putting a 9v current through from the centre but nothing comes out of the tip, which it does on the new one. Also the strip is in tact on both the useless arms and there are no sign what so ever of any damage. Very strange I admit, but a pain when you are in the middle of nowhere.
Brian
B Smith

Brian,

It might be useful to disect one of the defective arms (are they both the same make? genuine Rover or not?). Looking at the one I've just taken off, I assume the metal tip to the rotor centre is one complete piece bonded to the arm, with insulation poured into the intersecting channel during manufacture. It would be interesting to see if it's mechanical failure or some sort of electrical burn out? Correct me if I'm wrong but I know the voltage that passes through to arm is really high but the ampage is low. Surely it would need significant ampage to burn through the link (in the same way a fuse burns out). You'd expect the evidence to be obvious in that situation. Perhaps it's a manufacturing defect? Can't imagine the arm is subject to any stresses which would cause it to fail through fatigue... See if you can get to the physical evidence of what has failed - I would be really interested after my recent experience.

One final thing - Is there some sort of contamination of the centre contact which is somehow insulating it (and preventing a good electrical contact perhaps?)
Mark Clayton

Mark,
since my last I replaced the rotor arm on Sat everything fine, Took it for a run sunday fine all day, tried fast exceleration, high speed everything fine. Today (Sun) drove 3 miles same thing has happened. All 3 arms are different manufacturer, but non were MG genuine. Ground out the center insulation on the arm, which revealed a ciol type wire from the centre to the tip. Checked with meter, from center to tip, nothing. Checked from center, along the coil to the tip end and there is a circuit all the way, until you touch the tip then nothing. ran some solder down the tip to the coil, now you get a current fron center to tip. Filled with epoxy resin, still all fine, but will it be safe on the car, or will it blow something else? Thought of running a good gauge piece of copper from the center to the tip, but thought that it must be a coil for a reason. Well car sat outside, I will ring MG tomorrow to see if they know. The nearest MG dealer is about 15 miles and this is one reason why I did not use genuine. But would it have made a difference
Brian
B Smith

Sorry it's beyond me - Hopefully someone with more knowedge will be along shortly with advice.

As you say, If it's designed thst way then maybe it is designed to protect somehow? A fault on the ignition coil perhaps? Then again, I don't really have a clue...
Mark Clayton

I have had this reply which makes sence and I will change the Dist cap and arm again with genuine to see if this cures it.
Brian

MG Mike, Lancs, UK
I’m not familiar with the rotor arm on the K series, but other similar rotor arms have a resistor between the centre and the tip as you suggest. It is there for electrical noise suppression and normally of a high value (>10K) due to the very small current involved in the spark. One reason for the resistor to fail is too high an ignition voltage. The ignition voltage can be higher than ‘normal’ if the plug gap is too great, or there are any other gaps in the ignition HT circuit. The High Voltage (HT) is generated by the coil, this voltage is not fixed and one influencing parameter is the plug gap + rotor arm to distributor gap + any gaps in the HT leads.

Interestingly, there are many devices on the market that exploit this. You can buy a ‘device’ that fits into the king lead (from coil to dizzy), that is just a known gap, this causes the coil voltage to rise further before the accumulative gaps start to conduct. Many years ago, you could even buy ‘special’ plugs with ‘gaps’ internal, that the manufactures claimed gave increased performance. Visibly demonstrated at the usual side shows at car shows etc.

Anyway, I suggest that you check the rest of the HT components, i.e. HT leads, dizzy cap and your plug gaps. And the obvious, make sure the new components are not made in china.

B Smith

Brian,

Thanks for reporting back (certainly something new and fascinating to me) but it does make sense, as you say - interesting info about the resistor in the rotor (wonder if the genuine Rover one has it?).

Interestingly, I've just had an ignition problem myself and initially suspected the ignition coil (which proved not to be at fault). While I was reading up on 'coil related ignition problems' I came across an article which said that the coil can be damaged by prolonged exposure to high voltage cuased by incorrect plug gaps (too big), causing the spark to jump too far - That kind of backs up what you've just said... I wonder how many other times your particular problem has occurred and gone undetected - Whenever ther's an ignition problem, everyone says 'change the plugs, leads, distributor cap & rotor arm - which obviously solves the problem

I changed my coil (unecessarily but I had alredy bought it so I put it on), dissy cap & rotor arm, plus cleaned all engine bay electrical connections which solved my problem - I had a blue dissy cap which was non Rover and the advice I was given (from a few different sources) was to change to the genuine article.

I guess you will check the plug gaps & leads too to be on the safe side, as advised by the MG man - I was going to say 'Let us know how you get on' but we already know that the new parts will work initially and, if all goes well, you won't have anything tø report back!!

Mark Clayton

Well this week end I replace the spark plugs, the rotor arm (MG Original)and Distributor Cap(MG Original). I was tempted to change the coil as parts co (Unipart) is only £30. Well will wait and see if this cures it, but it can be a couple of days, or weeks. I will carry a spare rotor arm from now on. Brian
B Smith

Well, fitted the new parts and the car has been running for almost a month now without a problem, so lets hope that, that has solved it. I will still keep a spare rotor arm in the boot. Thanks again for all who helped.
Brian
B Smith

This thread was discussed between 03/05/2008 and 02/06/2008

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