MASCOT March - April 2003

Electrical Gremlins

Barry Paine

I was interested to read about Bill Tull's condenser problems in the Last Mascot. Electrical problems can seem quite mysterious, because you can't see the stuff or even feel it at 12V. But however mysterious any particular problem might seem, there is always a logical explanation.

The following problem occurred on a Morris Eight, although it's the sort of fault that could easily occur on a Singer. Anyway they're only another sort of Bantam. The car suddenly stopped whilst being used for a local journey. Although I have a fairly standard routine for dealing with such situations, I had reason to suspect the rotor arm. The rotor arm was duly changed and the car started first time. Just as a double check, I then replaced the new rotor arm with the original, expecting the car to fail to start, but to my surprise she ran beautifully. There is of course a logical explanation to all of this, and that was that by changing the rotor I had disturbed something that was the real fault. In this case it was the HT connection on the coil, which had corroded to such an extent that the HT spark could not always get through. The waggling resulting from the removal and replacement of the rotor arm had just disturbed the connection enough to allow the spark to break down the corrosion.

When misfires and ignition failures occur people often change a selection of components in trying to eliminate the fault. Very often perfectly healthy rotor arms and condensers are discarded. There is a good test for a faulty rotor arm:

Remove the distributor cap and disconnect the so called king lead from it. Hold the free end of the king lead about 1/8 inch above the conducting metal on the rotor arm, and get an assistant to spin the engine. If the rotor arm insulation has failed, you should observe a strong spark from the king lead to the rotor. If the rotor

is healthy you should not see a good spark, although you might see a very small, feeble spark, which would be due to capacity current, or in other words the transfer of a small electrical charge to the conducting part of the rotor. I must add a word of warning here. Your HT system produces many thousands of volts and you can get a very nasty jolt if you're not careful. If you have fitted electronic ignition, the voltage will be even greater. When I hold the king lead near the coil, I grip it lightly with a good pair of insulated pliers a few inches back from the end and stand in such a way that I'm not in contact with any part of the car. I haven't received a shock so far using this method. I wouldn't recommend that vulnerable people put themselves at risk by dabbling with live car HT systems.

In respect of condensers, I've changed a few of these in the past whilst fault finding, but most problems have been with the connectors, rather than the condensers themselves. On pre war types, where the distributor base plate is Bakerlite, the condenser may lose it's earth connection. Most post war Singers up to 1956, (with the exception of the twin carb types), were fitted with the Lucas DKY4A distributor. On this type the condenser is permanently fixed to the metal base plate and so is unlikely to lose its earth connection, but it can lose its connection to the lead from the coil. A particularly vulnerable point is at the low tension connection where the stud has a square flat on its inner end which tightens against a piece of insulating material on the metal base-plate. If the nut is overtightened, the square may rotate and damage the insulating material. This could result in your points and condenser being bypassed, with consequent loss of spark. The same thing applies in respect of the tubular insulating material over the stud, which also prevents a short circuit to earth. Again, over-enthusiastic tightening could damage this and cause a loss of sparks. Further details of this distributor can be found in Bill Haverly's excellent article in the ASCO magazine of March 1984. For those on the internet this is reproduced in full on Mark Furneaux's new web site www.singster.co.uk.