Knackered Rotor Arm

[JVY]

Afer spending weeks getting old girl ready for her MOT and being very happy she was running just fine, I was almost sure she would pass. Anyway, took her round to the garage yesterday and left her. I had only been home for a few minutes when they rang and asked how to open the bonnet. Had to tell them to open the glove box. Unfortunately, after about half an hour, they rang again to say the engine had stopped and she would not start. In the end I had to get her towed from the garage mninus MOT.

Today set about finding cause of the problem and eventually traced it to the rotor arm. Tests showed HT sparks going into dizzy but nothing was coming out to any of the leads going to the plugs. At first I suspected some sort of earth problem. However, everything checked out OK.

My car is fitted one of these strange Lucas rotor arms that incorporates a semicircular earth bar behind the HT contact and centrifugually driven rev. limiter arrangement. When I put a multimeter between the HT contact and the earth bar I got eadings of between 20-40 Megaohms - easly enough to take the HT straight to ground inside the distributor I assume. Despite repeatedly cleaning the rotor arm and even removing all the rev. limit gubbins, it still wouldn't send HT to the plugs. I can only assume some sort of internal insulation breakown that no amount of solvent and scrubbing was going to fix.

I went to my local Halfords and found a basic Halfords make rotor arm that looked like it would fit (but it was much smaller as it did not have the rev. limit gubbins). It cost the princely sum of £2.99 and was the only one left amongst a sad dwindiling supply of rotor arms they had left in stock. Halfords part number is HRA559.

Stuck it on (fitted OK) and car started the instant I turned the ignition key

I think I remember reading somewhere that these speed/rev. limiting arms can be problematic??? Also, would appreciate any advice on where to get a spare rotor arm please? Is it worth fitting a basic one without the rev. limit to save future bother? Somehow, I don't think I will be taking the old girl up to 5000-6000 rpm on a regular basis!!

Has anyone else came across this problem?

 

[Brian-Northampton]

I expect you bought one that looks like this:

http://cgi.ebay.co.uk/Lucas-Rotor-A...s=66:2|65:13|39:1|240:1318|301:1|293:1|294:50

I wouldn't bother with the Rev Limiting Arm. You want a simple Rotor arm for the 25D4 dizzy which is, I expect what you've bought. Once you're happy with it, I'd go and buy another one! Rotor Arms seems to have a bad reputation these days for build quality so it's worth having a spare.

Also might be worth giving the distributor doctor a ring http://www.distributordoctor.com/red-rotor-arms.html These Red Rotor arms seem to have a good reputation with the chaps in the TR-Register. A bit dearer, but manufactured to a better standard. Me, I've kept an old one from the 80's as my spare :wink:

Cheers,
Brian.

 

[JVY]

Brian,
thanks for the advice. I reckon that you are spot on - the Halfords rotor arm I bought looks like the Lucas item on the link. Also, think I will take you advice and order up a spare red rotor.

Looking at Dist. Doc website, their theory seems to be that modern items have extra carbon blacking in them. Perhaps there is a good reason for this. Although, it does seem strange that carbon is added to the plastic (presumably just to colour it) when traces of carbon through a realtively small insulator with approx. 30KV across it is the last thing you want.

The site also mentioned another issue that I had wondered about which is modern "sport" ignition coils that put out a higher voltage. Having read a few complaints in classic car mags in recent times about rotor/dizzy insulation problems and dud condensers, I had wondered if one culprit was maybe these higher energy coils that many people fit. Are they maybe damaging what was previously a good ignition system? Also, I wonder if the high energy coil draws more current on the low tension side (leading to more rapid contact breaker wear). Unless the new coils are much more efficient transformers than the originals, to output more energy then more must be going in.

Anyway, I digress. Armed with new arm, I shall re-book MOT and hope the old girl behaves herself this time. Her letting me down yesterday was just a rebuke for my self concieted smugness at daring to hope she would pass on 1st attempt.