Classic British Bike Magazine

Lucas MCR2 regulator

Understanding, fixing, and knowing when to give up


The mysterious innards of the Lucas MCR2 regulator. We love 'em. Others hate them. But if you persevere, they are understandable and manageable. Well, sort of...




The Problem


Our BSA WM20 regulator was on the blink and wasn't putting out the right charge. Why? Because (a) the bike had been laid up over the winter and was sulking, and (b) because the regulator hadn't (in any case) been touched/serviced/looked at in around ten years.


Unquestionably, general mechanical and electrical wear and tear had taken its toll on the electrical contacts within. Unquestionably, corrosion had also played a part by furring up the aforementioned contacts. And unquestionably, we needed to do something about it because a laid-up and sulking motorcycle is no good to anyone.


The options were to either:


Repair the MCR2.

Replace the MCR2 with a reconditioned unit.

Tear out the innards and fit a solid state rectifier.


After much argument, we decided to have a go at fixing it ourselves. Why? Because (a) we're stubborn bastards and we like to understand our classics as much as possible. Because (b) we haven't enough spare cash lying around to keep paying the experts and specialists. And (c) because we're simply not fans of the "soulless" solid state units.


Yes, you can just fit and forget these solid state boxes. But for us, part of the "fun" of owning a 1930-designed motorcycle (albeit manufactured in this case in the 1940s) is having the TOTAL classic experience with all its trials and tribulations. That's also why we haven't ripped out the engine internals and replaced them with a cheap and nasty Chinese chainsaw motor.


But on a more practical note, the thing about solid state regulators is that when they fail, they just conk out. Like an electronic ignition that's about to die, they don't warn you. They just give up and leave you stranded wherever you happen to be. But contact breaker (points) ignitions, magnetos and MCR2 regulators usually give you some warning that things are about to go belly up. And that's why if your bike (fitted with standard ignition and charging components) was running first thing in the morning when you rolled it from the garage, it will usually get you home under its own steam at the end of the afternoon or evening ride.


How the MCR2 works


Firstly, relax. We ain't going to get too technical here. Life's too short. And we ain't smart enough. But in simple language, the dynamo on our WM20 BSA supplies current (juice) to the lights and the battery. The regulator's job is (a) to switch that current in and out, and (b) control how much of that current gets to your battery.


These are the two functions of the unit.



And control.


Beneath the regulator cover are four regulator coils. Two of them are solenoids. A solenoid is simply an electrical switch. In other words, electrical current—via a coil or coils of wire—magnetises a pin and throws it out to make a contact in a circuit. That's all.


The other two coils have supporting (shunting) roles that you don't need to worry about unless you DO want to get technical.


Okay. Shunting? Think of a shunt steam engine shifting carriages around a freight yard. Same principle. The shunt coils kick in and move the current around the MCR2 freight yard via a different route. You don't really need to remember the shunt business, but it might help.


Anyway, these four regulator coils are two big 'uns (solenoids), and two little 'shunt coils.


The first (big) coil is a cut-out switch. It's designed to break the link between the generator and the battery. Why? Because if that link was left open, when you stop the bike, the battery would discharge into the generator through the wiring. In fact, the battery would try to DRIVE the generator like an electric motor.


Generators and electric motors, note, are essentially the same devices operating in different modes. So Joe Lucas, in his infinite wisdom, put a cut-out in the circuit. Meaning, in this instance, a solenoid. The bike stops. The engine shuts down. The regulator switches out the battery. But if you want to be accurate/pedantic, the circuit is wired so that the cut-out switch actually cuts-in. Why? Because the switch is "normally closed" and needs to open to complete the charging circuit. But it's easier to think of it the other way round. If this confuses you, forget we said this.


On the other hand, if you want clarification, here it is:






This means that when the engine is running, you NEED that generator to kick in and supply some juice. To make this happen, Lucas put in another BIG switch/coil/thingy that kicks-in ONLY when the generator is throwing out more volts than the battery needs.  Therefore, at low or zero revs, the generator will give you zero volts. You have to rev the bike to wake up the regulator and get it to switch in.


How many volts are we talking about?


Okay. We've got a 6-volt battery on our bike. So we need 6.3 - 6.7 volts to wake up the regulator. That means, when the bike's generator kicks out 6.3 - 6.7 volts, the regulator switch ALLOWS that current to pass on to the battery. Simple.


However, when the dynamo voltage drops below 4.5 - 5.0 volts, the switch CLOSES thereby stopping the dynamo from sucking the juice from the battery (as we mentioned).


That's the first bit sorted, Now for the second...


As the generator speed increases, it pumps out more and more volts, and if left unchecked that current would fry the battery. To control this, the cut-out switch kicks in and out at up to 50-60 times (cycles) per second.


This second cut-out switch shuts off the incoming current and turns it back on, and off, and on. Think of it as accepting parcels or packets of energy. When the balance is right,  this switch gives the battery only enough "parcels" to meet its needs. For a 6-volt battery, you don't want to charge it at much above 7.2 volts.


How does the regulator switch shut off the current?


Remember that shunt coil business? When this second switch detects more current than it wants, it opens. But when it opens, it wakes up the adjacent shunt coil. This coil (made from thinner wire, but with plenty more turns) has a high resistance. In other words, this shunt coil doesn't want to let too much current through. It resists it.


And because it resists, it effectively sends a "signal" to the dynamo. This "signal" changes the way the dynamo behaves. Specifically, this "signal" reduces the magnetism in the dynamo, and reduced magnetism means less charge.


A millisecond later, the regulator switch realises that the current has been reduced, so it closes again and shuts out the shunt coil. The current then comes on full-tilt, and that's suddenly too much again, so the switch opens and the shunt coil takes over again. This on-off, or in-out mechanism, happens 50-60 times per second. That's the ticking you hear from your regulator. That's the regulator switch and its shunt coil fighting it out.


So to clarify:






To adjust all this, the regulator has a number of contacts. These contacts have air gaps. As you adjust the gap, you adjust the spark that can jump the gap. If you increase the gap, you reduce the voltage that can pass. If you reduce the gap, you increase the voltage. This is why you must be careful when mucking about with a voltage regulator. If you screw it wrongly, it's screwed until someone else screws it right.




So what does FADE on the regulator mean?


This is simple to explain.


"F" means that this terminal connects to the "F" terminal on the dynamo.

"A" means that this terminal connects to TERMINAL 3 on the headlight switch.

"D" means that this terminal connects to TERMINAL D on the dynamo.

"E" means that this terminal connects to EARTH.


You can remember all this stuff if you want. But we'd recommend that you write it down somewhere (maybe on a strip of masking tape stuck inside the regulator box) and clear your head of information that you don't need to carry around with you.





MCR2 temperature compensation mechanism


This is what differentiates the MCR2 from its predecessor, the MCR1. The temperature control mechanism. This allows fine adjustment of the unit's contact depending on the air temperature. The air gap between the various contacts needs to be set accurately. If not, it just won't work properly. And that's why you need to know exactly what you're trying to achieve.


Testing the battery


Firstly, we had to ensure that both the battery and the bike's wiring was in good condition. The battery was old (our batteries are always old), and it was changed and charged on a low trickle. Then it was fitted to the bike. Then we checked the earth connection. A duff battery alone can stop your MCR2 from working, take note.


Then we tried starting the bike to see what was going on, if anything. Immediately, we got some electrical activity, both from within the regulator and on the ammeter. The lights were suddenly working too. But it was clear that something wasn't quite right.


Testing the generator


We needed to check that the generator was actually putting out some current. The way to do this is to look closely at the dynamo and find the two terminals. They'll be marked with a "D" and an "F" respectively. We removed the wires (marking them to remind us which way they were attached) and pushed them out of the way.


Next, we connected a small length of bridging wire between the two dynamo terminals (D and F). We had already prepared such a length of wire with a crocodile clip on each end. We clipped on that wire and dragged out our favourite Avometer.


One of the Avometer leads (the red/positive lead) was connected to the bridging wire on the dynamo (either the D or F terminal will do; it doesn't matter as long as they are connected).


The other Avometer lead (black/negative) was connected to earth. Connecting this way is important if you're running a NEGATIVE earth system. If you're wired for POSITIVE earth, you need to reverse the leads (black/negative to the bridging wire, and red/positive to the earth).


What does EARTH mean? Earth means the current is going to ground. Being depleted. Neutered. Or, if you prefer, the water is going in one end of the pipe at full pressure, and it's coming out at the ground end with some of the force removed. The frame will do nicely.


Then we started the bike and gave it a few revs. Up to around 1000rpm is fine. Any higher risks pumping out too many volts. We got a reading of around 10 volts, which is perfect. A reading of around half a volt suggests that the dynamo field winding might be the problem (don't worry about what this means; just make a note). A reading of around 1.5 - 2.volts suggests that the armature winding might be the problem (make a note and tell your dynamo repair man).


But our dynamo was fine, so we left it alone and cleaned the dynamo terminals, replaced the wires (on the D terminal and the F terminal, respectively) and had a cup of coffee and a good scratch.


To clarify, the above test simply proves that the dynamo is driving volts along a circuit that goes from the dynamo to the frame and back to the dynamo. Nothing more. Nothing less.


The next thing was to clean the regulator contacts.


Temperature compensation mechanism


As an adjunct to this feature, you might want to remember that air temperature will affect the charging rate. Why? Because heat affects conductivity, it's as simple as that. Warm metal allows electrons to move faster than cold metal. So on a hot day, your dynamo will be pumping out much the same voltage. But the regulator will supply more juice when the thermometer goes high than when the thermometer drops.


To deal with this variance, Lucas employ a temperature compensation mechanism. This is simply a bi-metallic spring. Bi-metallic means a spring made from two metals. Each metal has a different rate of expansion. The spring that expands fastest overpowers the "weaker" spring and makes it curl. The hotter it gets, the more it bends out of shape. And because it bends, it varies the adjustment/contact gap getting larger or smaller.


Put simply, the MCR2 works by controlling a contact gap. That gap controls the current. Those contacts are regulated by springs, and it's that spring tension that needs to be adjusted.


Adjusting the MCR2


Firstly, we had to ensure that we had a moving-coil (old fashioned) voltmeter, which we did. We've got three Avometers, one nasty digital meter, and one even nastier digital. The moving-coil meters are (almost) always preferable. That's because you can see the voltage rise and fall as the needle sweeps across the face, which means you get a better feel for what's going on. Conversely, a digital meter usually just flashes up the information you need, and you don't get a sense of how the component is reacting or operating. Actually, a digital meter will react to the electrical pulses coming down the line, so it probably won't give you a steady reading anyway. But a moving-coil voltmeter has a natural damping action, so the readout will probably be much more steady.


Don't get us wrong. We're a long way from electrically minded. Usually, after we've mucked around, a bona fide electrician comes around to fix what we've screwed up. Nevertheless, we like to at least try and understand what's happening, and a moving-coil meter is the tool to have.


Next we cleaned the cut-out relay contact and the regulator contact with some very fine glasspaper (800 grit worked for us). We were advised not to use emery cloth or a carborundum stone, and we're taking this advice. But the truth is, although we've heard various reasons why a carborundum stone and emery cloth are inadvisable, we don't have a definitive answer. BSA seemed happy with either. So if you know, please do pass the word. The important thing, however, is to lightly dress the contact faces, and then ensure that there's no grit or dust or particles remaining.


Next, we removed the terminal on the regulator marked "A", and we connected the Avometer's positive terminal to the "D" terminal on the regulator.  Then we connected the negative Avometer lead to the frame (earth).


To reiterate:


Avometer positive to D terminal on the regulator (with the regulator "A" terminal disconnected.

Avometer negative to the frame or other earthing point (engine/footrest, etc).


Then we started the engine and watched the needle rise and steady itself. The Avometer voltage is temperature dependent. Therefore it will read differently on hot and cold days. Why? Because of the earlier mentioned b-metallic springs.


Here's the voltage settings data

Cut-in voltage 6.3 - 6.7 volts
Drop-off voltage 4.5 - 5.0 volts
Reverse current 3.0 - 5.0 volts


Here's the temperature compensation data

Regulator - Setting on an open circuit
10° C (50°F) 7.7 - 8.1 volts
20° C (68°F) 7.6 - 8.0 volts
30° C (86°F) 7.5 - 7.9 volts
40° C (104°F) 7.4 - 7.8 volts


Next, if you're not absolutely sure of what you're doing, and if you don't have the correct tools, stop right now and talk to an expert. If you can find one. You need a collection of small screwdrivers, a 4BA spanner, some shim stock of varying thicknesses, and a set of feeler gauges.



What you're trying to is set the correct gap on the regulator points. There are two coils; one beneath each point. With the engine off, the regulator point is the one that's normally closed. The cut out is the opposite, meaning the point that's normally open (with the engine off, remember). Look at the diagram immediately above.


Diagram key


B is the bobbin

C is the armature carrying the moving contact, G

D are the armature fixing screws, and
E  are the fixing screws for the fixed contact, F


You can clean the points by removing the two fixing screws. A light dress-up with a carborundum stone will do the job. Just take it easy. You're cleaning, remember, and not gouging.


Then refit and slacken the armature fixing screws, D . That will allow you to set the gap, K. You're looking 0.018in to 0.020in.

You also want a gap between the armature and the core of the bobbin. That's marked on the above diagram with a letter I. That should be 0.012in to 0.020in.


Next, press the armature onto the bobbin core. The gap at the contacts G should be 0.006in to 0.017in.


You'll see that there are shims fitted to adjust the gap. Look at the fixed contact, F. Study. Scratch your head as required. Then shim as required, or bend where applicable. And remember that the settings are temperature compensated.


You'll need to make the adjustments off-load. So disconnect lead A and leave the other three in place (F, D & E). Then clip the voltmeter to the frame/chassis of the regulator. Connect the negative to earth (frame or battery).


Run the engine to a fast tickover; just enough to get the dynamo excited. Around 1,500rpm will do it.

Slacken the locknut of the regulator screw (see the diagram) and adjust the voltage accordingly (check the temperature chart, remember). Screw inward to increase the voltage. Screw outward to decrease. Make sure you nip up the locknut.


Finally, if all that fails, it will be time to talk to an expert—which we're not. But we have done this procedure before (long ago), so it is do-able. Keep in mind that we make many mistakes, so cross check this feature with other available information. And if you are an expert, and if you see where we've screwed up, please set us right asap. We'd rather be right than wrong.



If all else fails with your MCR2, a solid state unit will do the job. These devices are reliable, convenient, easy to fit, and cheap (around £30 at 2018 prices). And note that this is a 3 phase unit.


















£12.99 plus P&P

[for details, click here]


Copyright Sump Publishing 2014