Page 1 of 1

Lucas 2MC Capacitor Warning

Posted: Mon Aug 10, 2009 6:19 am
by b50root
Lucas 2MC Capacitor Warning

1967 was the last year for the Lucas Energy Transfer (or “ET”) ignition system for use on British
competition motorcycles. In an effort to streamline production and to meet US import laws, starting
in 1968 Triumph and BSA sold only “road” bikes with the battery/coil ignition. If you wanted to
ride off-road or race you were forced to buy a road legal motorcycle, remove the lights and
otherwise prepare it for competition.

Part of that preparation included removal of the battery and battery tray. The battery, which was
required for proper functioning of the battery/coil ignition, was replaced with the Lucas version of
the ”battery eliminator”, otherwise known as the 2MC capacitor. The 2MC was a large electrolytic
capacitor that connected to the harness in the same manner as the battery, but weighed about
1/100th of the battery.
Starting in 1968, Norton P11’s also came from the factory with a battery and a 2MC capacitor
fitted. Again, the P11 was a competition machine and most P11’s ended up with the battery
removed. Then, starting about 1970, all Commandos came fitted with a battery and the 2MC. By
installing both, Norton was able to hold the system voltage at high levels even though the battery
may have started to age and slip below 12V.

Later on, in 1971 BSA and Triumph introduced a series of “oil in the frame” 250cc and 500cc
single cylinder motorcycles with an aluminum electrical box just behind the head post and below
the fuel tank. The idea was that if you wanted to ride off-road all you had to do was remove the
lights, battery and electrical harness. All electrical functions required for dirt riding were contained
in this compact electrical box. Inside the box was the battery eliminator, the blue 2MC capacitor.

That was fine in late 60’s and early 70’s. The capacitors worked well and allowed many riders to be
free of the weight and maintenance of a battery. However times change, bikes values increase, and
their owners get better looking. Unfortunately, the 2MC capacitor is not aging as well. It is well
known that some types of capacitors (aka “condensers”), especially the electrolytic type, have a
limited life span. Apparently for the Lucas 2MC, that life is limited to about 30 years.

Internally, capacitors are nothing but 2 large pieces of conductive foil separated by a very thin
insulating film. As this insulator breaks down over time, a very small trickle of electrons begins to
make a path between the 2 charged foils. Since the capacitor is not supposed to pass any electricity
the path is technically a short circuit, but the starting amount is so very, very small. However, just
like a small leak in a earthen dam, soon the trickle turns into a small flow, then a torrent. In the
same way 2MC capacitors start life as a good insulator, then become a small resistor, and finally
after many years end up as a short circuit.

No less than 3 times in the last month I have had to diagnose and remove 2MC capacitors that were
faulty. All 3 had developed into “near short circuits” that were draining the battery, but offered
enough resistance that the fuse wasn’t being blown. All 3 owners complained of batteries that
constantly had to be charged, even though the bike was ridden regularly. The bike would run good,
but due to the internal failure of the 2MC, the electrical system acted as if the tail lamp had been left
ON when the motorcycle was parked. The owner always came back to find the battery was dead.

When a 2MC works properly it can disguise the fact that the battery is dying until the owner is left
walking. And when the 2MC starts to fail the owner may buy several batteries before discovering
the actual problem. When you add to this electrical game of hide-and-seek the fact of its inevitable
failure, you may decide that having a 2MC in your electrical system might simply be more pain
than it’s worth. Therefore, if your bike is equipped with the bright blue Lucas 2MC capacitor that
was fitted as original equipment, in my opinion the best thing you can do is to disconnect the RED
wire from the positive terminal. After disconnection, the RED wire should be insulated with heat
shrink tubing to prevent accidental re-connection.

If you decide to replace your 2MC and keep your motorcycle completely original, then it’s very
important that you get a freshly manufactured unit. The ageing process that causes the problem
happens regardless of whether the capacitor is sitting on a stock room shelf or installed on a
motorcycle. In other words, a “new old stock” capacitor could be “bad” even though it was still
untouched and in the box. If you are unsure of the quality of the capacitor your local bike shop has,
then you can look for a 4500-microfarad electrolytic capacitor rated at 30V from an electronics
supply house.

Hope this helps!
RF Whatley
Rodi British Bikes

Re: Lucas 2MC Capacitor Warning

Posted: Mon Aug 10, 2009 10:23 am
by minetymenace
RF Whatley wrote:By
installing both, Norton was able to hold the system voltage at high levels even though the battery
may have started to age and slip below 12V.
Not strictly true. When a battery ages (normaly through the build up of sulphate on the plates), it ability to hold a charge diminishes. You may well find that the voltage during charge is normal and the current reduces to a trickle (equivilent to the self discharge of the battery). When there is no charging going on (ie the bike not running), the battery terminal volts will fall quicker than normal. A 2 uF capacitor will not supply enough charge to hold the battery voltage at normal levels for very long. However, the capacitor may be able to store charge long enough for the bike to be started if the battery is either flat or old. Once the engine is running and the electrical generating system is up to speed, the bulk capacity of the battery is no longer needed, and a small capacity (or knackered) battery or condensor is sufficient to soomth DC and enable an ignition system to run without issue.

Re: Lucas 2MC Capacitor Warning

Posted: Sun Aug 23, 2009 12:27 pm
by minetymenace
Something I have been meaning to add for a while:
Electrolytic capacitors (the type that replaces the battery, not the type across the points etc) have a fluid electrolyte between the plates (rather than say a layer of solid insulation). As long as this electrolyte has not dried out, the capacitor should be good, they can dry out over a few years or may be good for many more. They can also leak, so if they are physically damaged then best confine them to the bin.

They can also go short circuit, but unlike most electronic components (but like some batteries) it may be possible to recover the capacity. The electrolyte reacts with the plates and builds an insulating layer where the short occurs, and are sometimes referred to as "self healing".

This may be worth trying: To recover a short (or low impedance) DC capacitor, apply a DC voltage to the terminals (considering they are polarised) and monitor the current. If current flows, reduce the voltage until the current is negligible. Every 10 minutes increase the voltage a little, making sure no current flows. Eventually you will reach the DC voltage rating of the capacitor, and will have repaired the short (or it will have gone bang in a spectacular fashion). It is probably only worth doing this if you are insistent on keeping your pride and joy 100% original, as a modern replacement is so cheap.

Have fun.

Re: Lucas 2MC Capacitor Warning

Posted: Fri Oct 22, 2010 12:35 am
by britgaskets
I have always run the 2mc with a gel cell sealed battery and this turn around on tuning and fixing the charging system seems to be better and the bike will run on low beam halogen with tail light without the battery. It will even idle with the light on but at low turn over it needs a blip or two every so often. Without light draw the bike has a steady low idle. I run wires from rectifier (modern diode) to one big tab and the other to the ignition on side to feed the battery. With the 2mc hooked direct to battery I think there is leakage but I'm not going to swap it to prove that point. If it ain't broke & works don't touch it rule.