Spridget Charging Systems
DYNAMOS/ALTERNATORS/CONTROL BOXES
Our cars have seen 3 main types of charging systems during their production cycle. These are as follows:
1. Dynamo with a 2 bobbin control box (voltage regulation only)
2. Dynamo with a 3 bobbin control box (voltage and current regulation)
3. Alternator with internal (voltage and current regulation)
If there is any doubt as to whether a charging system is working or not simply place a voltage meter across the battery and start the engine. No matter what system is in use as the engine speed increases then the voltage measured across the battery should increase to about 14.5 volts maximum. However with the engine running at say 2000 rpm, a reading of above 14.5 volts or below 13 volts would indicate a problem with the charging circuit/system.
Type 1 Dynamo with a 2 bobbin control box (voltage regulation
only) and Type 2 Dynamo with a 3 bobbin control box (voltage and current
regulation)
In these systems a dynamo is used to generate DC power and will generate voltages between a couple of volts and perhaps 20 volts depending on the speed of the engine. This generated voltage is then fed into the control box which limits the voltage to a maximum of about 14.5 volts. With the battery at just above 12 volts when this 14.5 volts of generated power is applied across the battery, charge (current) will be forced into the battery. If this is not the case and a fault is suspected then check the dynamo and control boxes as described below
Type 3 Alternator
In this system the alternator as the name suggests generates AC power that is rectified within the alternator itself. In the same internal unit is a voltage regulator that limits the voltage to a maximum 14.5 volts. The main advantage of these units is that they produce more power than the previous dynamos. Whilst the dynamos created 19amps (Frogeye) and 22 amps MK1 Midget and Mk2 Sprite onwards, the factory fitted alternator later Mk3 and Mk 4 (1500) Midget delivered a maximum 34 amps. Most replacement alternators that many fit today are even higher at 45 amps easily. With no need for an external control box and power in abundance there is no question which system is the best.
Workshop Manual
Here is the original workshop manual from Lucas on the Dynamo and Control Boxes. It is very involved and concise. I hope you will find it of interest if you need to go further than our suggestions Lucas Manual
Testing the Dynamo
Testing the dynamo is relatively easy and you will only require a cheap
multimeter or simple volt meter that can be set to read 30 volts DC. Accuracy
is not the name of the game as only a couple of values are possible, basically
they represent working or not!
As always with electrical systems it is of primary importance to check the
basic wiring for quality connections. Thus check the 2 connections D and
F on the rear of the dynamo for being sound and tight. The other end of
these 2 wires is at the voltage regulator, mounted on the bulkhead at the
rear of the engine bay on our cars and these connect to D and F on the 2
bobbin (RB106) regulator with either a screw terminal or spade connector
again check these for condition and security. Later cars with the 3 bobbin
(RB 340) control box have the same connections although they are contained
in a multi plug and this should be checked also.
WARNING: The following procedure involves working in the engine bay with
the engine running. Ensure there is no loose clothing and keep your hands
and the connections to the test meter clear of the fan blades. Have an assistant
available to switch the engine on and off.
Disconnect both cables from the connections at the back of the dynamo and
with a short piece of wire connect both terminals (D and F) together. Then
connect the negative lead of your meter (set to measure at least 30 Volts
DC) to the short piece of wire and the positive lead of the meter to a convenient
earth on the engine/ car body.
Start the engine, let it tick over and measure the voltage. It should read
about 15 to 20 volts depending on the speed at idle. Revving the engine
slightly should give a figure of about 30 volts of more.
If when doing this test you see next to no voltage (1 volt or less) then check the dynamo in the following manner suspecting that the field coils are at fault. The field coils are connected to F and earth and so set your meter to ohms and check between the F terminal and earth (dynamo case) The reading will be about 6 to 7 ohms. Less than this could mean the field coils are shorted to earth and above this reading suggests the field coils are open circuit. Both ought to have you buying a new dynamo.
However if when doing the voltage test you get a figure of less than 20 volts but more than 1 say about 4 or 5 volts then probably the armature will be faulty. I have seen a number where the commutator (the bit where the brushes run) falls apart mainly due to excess heat caused by overcharging. The only hope you may have is that the brushes may be worn out and new brushes are easy to fit. Other than this it is probably easier to simply buy a replacement dynamo, they should not be too expensive.
Setting up the Control Boxes
I have found in the past that both types of control boxes have been very reliable. About the only things I have ever done to either of them is clean the contacts or adjust the voltage bobbin. On other occasions I have changed the box for a new one. Both types are inexpensive when purchased from a local auto electrical retailer and are set up when purchased. However for those who want to do a little more here is info on the RB 106/2, the early control box with 2 bobbins.
RB 106/2
The RB 106 originally had screw terminals, later versions had lucar connections
(push ons) and were designated RB 106/2. 2 contacts are operated by the
2 bobbins. One of them opens and closes to regulate the voltage to a level
of about 14.5 volts when measured at the battery. Whilst the second one
opens and closes to disconnect the dynamo from the battery at low dynamo
outputs (engine revving very slowly) to avoid discharging the battery when
engine is idling or engine is stopped.
(Diagram 1 RB106/2)
Generally if when I measure the voltage at the battery when the engine is revving at more than 2500 RPM the voltage is less than 14.5 volts I will simply adjust screw 1 until the voltage reads between 14.5 and 15.0 volts. Turning clockwise increases the reading. If you suspect the control box requires more setting up then follow the next set up procedure.
Remove the wires from terminal (A1) and (A) and join them together. Then
connect the negative lead of an analogue voltmeter (set to read 20volts)
to the D terminal of the dynamo and the positive lead of the volt meter
to earth. Start the engine and increase the speed, watching the voltmeter
until the reading steadies regardless of increasing engine speed. The readings
should fall into the following chart depending on temperature
At 10 degrees C should get between 16.1 to 16.7
At 10 degrees C should get between 16.0 to 16.6
At 10 degrees C should get between 15.9 to 16.5
At 10 degrees C should get between 15.8 to 16.4
As you can see from the chart don’t worry too much about temperature
simply set the voltage to between 16 and 16.5 volts and you should be fine.
Also this test must be done very quickly (within 30 seconds of the engine
being started otherwise the system will overheat and cause poor readings.
For a more detailed set up please refer to the Lucas manual attachment above.
RB 340
As described above these control boxes have 3 bobbins inside. They are voltage
cut out (limits voltage to about a maximum of 14.5 volts), current cut out
(limits current produced by dynamo to battery to about 22 amps), and normal
cut out when the battery is more powerful than the dynamo (low engine speeds).
To adjust the RB 340 you will need to remove it's cover. This is held in
place by plastic rivets that have a plastic pin down their centre. This
pin can be pushed out from the front by a small diameter rod and when they
are pushed out the back you should be able to pull the cover forward and
thus expose the 3 bobbins. There is a special tool for adjusting the 3 cams
on top of the bobbins, to this day I have never had one but I normally adjust
these cams by simply turning them with snipe nosed pliers. Looking from
the front and from left to right the bobbins are, Voltage limit, current
limit and cut out.
When I want to adjust the voltage regulation I normally put a meter across the battery and rev the engine to about 3.5KRPM and simply adjust the left hand side bobbin until I see between 14.5 volts and 15. I do not have a simplified way of adjusting the current limit and I follow the Lucas manual when setting that. For the cut out I remove the brown and yellow wire connected to the WL terminal (in the centre). I then connect my volt meter between that terminal and earth. With a bit of load on the car system (main beam headlights switched on) let the engine idle slowly. Hopefully the meter will be reading less than 13 volts. Now as you increase the engine speed slowly the cut out should "cut in" at 13 volts. This will be seen as a slight "wobble" on your volt meter at what ever is the value that it operated. Adjust the right hand bobbin cam until this occurs at 13 volts. For a more detailed set up please refer to the Lucas manual attachment above.
Testing the Alternator
The alternator from a enthusiasts point of view is a simple device! why? because we can do very little to it should it go wrong. Like the dynamo to check the alternator we need to connect a voltmeter across the positive and negative of the battery. If when the engine is revved to say 3 KRPM and the voltage does not rise to at least 14.5 volts then either the connections to the alternator a faulty or the alternator itself is faulty. The original alternator fitted to the Midget could have it's brushes changed by removing the raer cover. Then remove the wire connectors fitted to the rectifier terminals. Now removing the screws holding the surge resistor and brush box into position remove the brushbox and replace the bushes. Later alternators were slightly different and many cars have had totally different alternators fitted that have easier access to the brush pack. These sometimes have a unit held onto the rear by a couple of screws and contain brushes and voltage regulator. It is possible to get new units sometimes normally from your local auto electrical specialists. If in doubt and you are happy that your testing has identified the alternator as faulty, then take it to your local auto electrical specialist who will sort it for you.