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MG MGF Technical - Any Ideas on what is causing this?

About 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. By that I mean that there is no current will pass from the cetre of the arm, to the tip. There is no sign of burning/arcing anywhere on the arm. 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 and that that has burnt away, but there is no sign of this. If I fit a new one the car is fine again until the arm fails. Anyway I changed the arm and the car is 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 it was a different make (Unipart0. Any ideas what is causing this?? (97 1.8i Non VVC)
Brian
B Smith

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.
MG Mike

Mike, that is very interesting as this problem seems to have started since I fited a new distributor cap which was not genuine. The main story to this is on the first post that I put on called 'Car died, burnt out rotor arm' but I added this as there was not much interest in that one, but now there is. Any way the first arm lasted about a month. The second which went on Saturday lasted about 10 days and the new one that I fitted Saturday burnt out last night Mon, so only lasted a day and a half. I will replace the distributor cap and arm with a genuine one and see if that cures it. Also checking the rest of the leads etc. I will let you know how this works but will not be able to get to a main dealer for a few days as there is not one local. Thanks again
Brian
B Smith

This thread was discussed between 04/05/2008 and 06/05/2008

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