Understanding MkIII electrics

Peter

We could be here all night on this one!! If we settled for a description of the original Lucas/Norton factory fitted system then I think the best might be a full article in Roadholder as it will need thinking through fully. In due course-can you wait?

Al Osborn.

Previously wrote:

Hi all, I have a '75 Mk III and I am keen to understand the electrics. I have a basic knowledge but could do witha laymans version of how the charging system (with and without Boyer hardware) works. I was always bamboozled by electrics - an RB340 regulator was like a magic box to me when I was serving my time on BMCs in 1971 at Storeton Engineering in Bebington, Wirral.

Now's the time with the help of my excellent friends in the NOC. Any guidance anybody?

Peter

Hi Peter,

While I agree with Al that a full description of the charging system may be a major job I would be interested in the background to your question. In the meantime here is a starting point that you can say is over the top or too simplistic ...

The 850MkIII charging system is very simple unless you are trying to understand the more esoteric functions.

You have a magnet whizzing round inside bunch of coils that generates analternatingcurrent, you only have two wires coming out and these go to a bridge rectifier that converts the alternating current to direct current.

Now the clever bit of the MkIII is that it does not have a single zener to clamp the voltage to something that will charge the battery without boiling it or blowing the bulbs. This is the way the earlier systems worked and the zener could only just about cope. It has two zeners, one on each footrest mount.

Now a little sidetrack to explain a zener diode, it is basically something that acts like a normal diode (a one way valve if you like) up to a certain voltage, but over that voltage it act like a pressure release valve and allows the current to flow in the opposite direction thus clamping the voltage at the maximum the zener is designed for. The excess voltage has to be lost somehow and this is converted to heat.

On the MkIII the bridge rectifier consists of two normal diodes (these sit up next to the starter solenoid and the blue thing), and the two zeners mounted on the big heat sinks doubling as footrest mounts.

The regulated voltage goes on to the battery and the rest of the system. Simple really.

If you have a Power Box then this is a simpler setup unless you want to understand how the internals of the box work

Let me know if this helps.

Regards

Tony

Tony,

Your description of the charging system will do as a basics. The only clever bit is the bridge rectifier. Normally these consist of 4 diodes in a 'square formation' In the case of the MKIII the original Lucas rectifier consists of just 2 such diodes, with the other 2 legs of the bridge made up by the Zener diodes. This causes the output voltage to be clamped by the zeners. Advantage-you get twice the power handling than using the one Zener diode. (which is needed with the higher output alternators)

Disadvantage-The final regulated voltage will be a 'diode drop' below the Zener voltage, which can mean in a lot of cases that the MKIII final charge voltage and hence battery stored capacity will be lower than it could/should be.

One small point is that should a Zener fail open circuit or become disconnected(same result)then your alternator output/charge current drops to half. One 'tricky' way around this is dispense with the original Lucas 'half' rectifier and fit a modern full wave bridge (all four wires connected) then with the Zeners in circuit as per the normal MKIII the resultant output/charge will be the same, but IF a zener 'disappears' then the actual output voltage/charge current actually rises a little. Of course it is always advisable to have some charge checking device like an LED voltmeter (BSM) to cover such situations.

A very brief description of the workings of the modern regulator/rectifier (Boyer Power box or Paul Goff-A O Services A Reg) is that they fit in the circuit in exactly the same place as the 'normal'(non MKII) rectifier and hold the alternator down-only letting the output voltage rise up to a predetermined battery charge voltage. By holding the alternator down then the alternator does not produce its full power unless it is needed and hence we are not having to lose the extra power in heat through the traditional Zener diode. Some people have worried about the power waste with the Zener, but the most you will 'waste' this way is some 70 watts ie about 0.1HP!

Al Osborn.

The Assimulator!! Ahhh...There were/are two types of these fitted to Norton Commando. The MKI MKII Commando (only I think) both had the 3AW assimulator, a '35mm film can' with three terminals under the tank. The MKIII had a black plastic 'widget' with 5 terminals sitting on the front top of the swinging arm (or somewhere very near) Both the 'widgets' did the same thing, they noticed when the alternator was giving some electricity they then switched a red lamp off -the one that came on with the ignition. Now the first one the 3AW was an electromechanical device on a similar principal to the flasher unit. BUT this unit has contributed remarkable to the poor name of Lucas electrical devices. ie the original 3AW is to be avoided as the most unreliable electrical device ever made. When it fails, which is very often, instead of warning of lack of charge it ensure there WILL be lack of electrical charge! Do not use this device! There is a an after market electronic version (this time actually in a 35mm film can!) which appears quite OK. The MKIII unit does the same job but this time it does it all electronically. To some extent this is a far more reliable unit but can fail.

The snag with all of these units is they turn a lamp off as you start your engine to show you might be charging, but later in your journey or if you didn't take notice when you started or if the lamp fails you could be embarrassed by not charging! I can supply two alternative units, a charge warning device that appears to operate the same way but in actual fact actually switches off the lamp when the battery voltage rises. Or another device the BSM, this is a 10mm multicoloured LED voltmeter that looks at the battery voltage. Ignition on-LED red- battery at rest. start engine-LED strawberry/orange-battery just charging. Engined revved up-LED green-battery on charge (all is well). Flashing red (fast) Over Voltage. Flashing red (slow) Under voltage, (battery flat).

So please be aware of the assimilator thingy.

Al Osborn.