An acquaintance of mine with a Mk2 has just had another rotor arm failure. He had one a few months ago and fitted a new one.
The new rotor arm was clearly marked as a Lucas part in a Lucas box.
However, it failed last weekend while he was on a Jaguar club run about 50 miles from home.
He left the car in a safe location and got a lift home.
He eventually fitted another new rotor arm and the car ran normally.
Does anyone have any idea what the problem is as I have heard of similar failures.
E.g. are we dealing with bogus parts or are modern electronic spark systems as fitted to this car just too much for the old spark distributor systems?

 

Bill,

Any idea reason for failure? My friend's Mk 2 spat the dummy recently with a destroyed rotor and we believe arcing between the carbon brush and rotor button caused the problem.
However, the part was many years old, not a late replacement.
It seems that these days it's pot luck sourcing replacement parts. Lucas boxes (or any other) are no guarantee of "ring-ins" and shoddy Chinese quality goods.

 

Further to my comments, I was contemplating fitting a multi-spark CDI ignition system to both my Jags.

Has anyone had experiences with these modern set-ups and any recommendations as to makes etc?
Prices also seem to vary wildly, from about A$70 to circa $A700

Thanks for any tips.

 

If the rotor arm looks OK mechanically, it's failing by providing a lower resistance route to ground than via the spark plugs. I don't see why a slightly higher voltage supply should find the alternative route more quickly. Obviously, an enormous voltage could break down a range of routes to ground. For me, it's a combination of possibly marginal electrical design and inappropriate plastic material that, under the right (or wrong) circumstances, gives an easier route to ground when the plastic part of the rotor heats up. It's been suggested that there's too much carbon in the plastic especially in traditional looking black rotor arms. It's an argument that seems to make sense.

​​​The rotor arm in my Daimler started to fail in what is a totally standard (for a HE V12) system. It has happened only once. It was a few hundred miles after installation. The replacement has been fine.

My advice is to keep a spare in the car and be very sensitive to the slightest hesitation or miss in the first few hundred miles after replacing a rotor arm. Once they start to short, they progressively get worse.

 

fit one of the Red rotors. The rivett fails in the black ones.
https://www.distributordoctor.com/red-rotor-arms.html

 

Wasn't there talk of Rotor arms failing because the rivet holding the copper arm to the spindle was failing. There is another version out there with a welded arm instead of a rivet which solves this problem. Not sure if my facts and description are correct but I think Glyn has more info.

I remember back in the 1980s my father bought a new Audi Quatro. (Oh how I wish I still had it). Anyway it had this new rev limiter system to stop the engine over revving which involved a rotor arm with a strong spring so when the engine over revved the spring had so much rotational force on it it contracted breaking contact and the engine would briefly cut out until the engine revs dropped. Good old engineering rather than computers and sensors. My father told me to try it out once and hold my foot down in second until the rotor cut the engine on the rev limiter. Unfortunately the rotor gave out the spring collapsed, the rotor shattered, the engine stopped and we were stuck at the road side 10 miles from home and the nearest garage. I did laugh.

 

See here: A Lucas box means nothing today unless it's NOS. The brand has been sold out to all and sundry. The green box means nothing.

Distributor issue ??? - THE INTERNATIONAL JAGUAR 'S'-TYPE REGISTER FORUM

 

Refresh page for edit.

 

In the 1980s, there was a lot of effort to make cars more sophisticated with bad old electro-mechanical devices including rev limiting rotor arms and relays. Thankfully, the Japanese brought us into modern electronics.

 

Units with a metal stud frequently suffer from the spark jumping to the distributor shaft & not the cap.

 

Barratts sells the 123 ignition rotor arm only because of all the trouble with rotors. They will still sell you the unit I used in my post above but only as a special order. The rest are distributor specific. e.g. V12. The studded horror is still around however.

 

Considering Bill's original question, until the adoption of EDIS and CoP arrangements, distributors with rotor arms were successfully directing sparks from otherwise modern, high energy, digitally timed ignition systems. There's no reason why they shouldn't work. But that goes for a lot of repro parts for classic cars: they should fit/work/not be a problem ... .

 

I have twice had a rotor arm failure on my Mk2 and in each case the failure was caused by a shearing of the mounting peg on the bottom of the rotor, thus allowing the rotor to spin loosely on the distributor shaft. Both of these rotors were older units. I now carry 2 extra rotors in the glove box which means, of course, that I have had no further failures.

 

No surprise there Prof. Carry a spare and no failure occurs.

 

The trick of carrying a spare in the glove box is one I have used with equal success in the V12 Jaguars. The most frequent failure point on the 5.3 litre HE engine is the GM module in the ignition amplifier. Please note that this failing part is not a Lucas item, despite the Lucas label on the amplifier casing. It is a genuine GM/Delco part. The cure? Well, replacement of the part, of course (a 10 minute job if you are slow), but the real cure seems to be the spare Delco1906 which you put in the glove box. Seriously.

 

Carrying a spare rotor arm in the glovebox is precisely my acquaintance's answer to the possibility/probability of future failure.
Personally I have only ever experienced one rotor arm failure in my 50 years+ of motoring and that was in a Series 3 XJ6 about 20 years ago.
As Glyn hinted, I suspect there are a lot of parts out there that masquerade as original but are "knock off" copies produced by who knows where.