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A = word of=20 warning before reading this section. The schematic diagram and = analysis of=20 how the Marchal BPR24 regulator works are based on general theory = and my=20 having disassembled one rather than from access to original = technical=20 data. It is only intended as an aid for use by a technically = competent=20 person with access to suitable equipment in trouble-shooting = problems with=20 the charging system. If you are not competent or equipped to carry = out=20 this sort of work then it is best left to somebody who is! It is = very easy=20 to cause further damage to the electrical system of your=20 M201!

DYNAMO

The 24volt=20 M201 was fitted with the following dynamos: Paris-Rhone G15R39, = G15R48,=20 G15R51 or Marchal BPG24 all rated at 28.5 volt, 22 amp. They are=20 physically large for this rated output due to the fact that they = are a=20 sealed design and there is no forced air cooling provided by a fan = that=20 would normally be found behind the pulley. This large dynamo would = overheat if it were allowed to operate at a the higher output that = it=20 could certainly produce.

REGULATOR

The 24volt=20 M201 was fitted with the following regulators: Paris-Rhone = ZT3115a,=20 ZT3118a, ZT3120a or Marchal BPR24. As far as I know all of these = types are=20 of the three coil 'current voltage regulator' design which = consists of a=20 cut-out, current regulator and voltage regulator coil. The photos, = diagram=20 and following detail is based on the Marchal type but the = principle of=20 operation will be broadly similar in all cases even if the = internal layout=20 is different.

CUT-OUT: this acts as a one way valve allowing = electricity to flow from the dynamo to the battery (charging) but = not in=20 the other direction. Without it current would flow back out of the = battery=20 when the engine is turning slowly or has stopped causing the = dynamo to=20 operate like an electric motor in which case it would try to turn = the=20 engine and quickly flatten the battery.

CURRENT = REGULATOR: Under certain conditions (e.g. when = the battery=20 is flat) the dynamo is capable of producing a very large charging = current.=20 The current regulator makes sure that the current is limited to a = maximum=20 of 22 amps to prevent the dynamo overheating or the batteries = being=20 damaged.

VOLTAGE = REGULATOR: This provides the fine control of the = charging=20 current. As a lead-acid battery becomes charged the voltage at its = terminals rises. This voltage is sensed by the voltage regulator = coil and=20 used to reduce the charge rate to a small trickle by the time the = battery=20 is fully charged. Without it the batteries would over-charge = causing the=20 electrolyte to 'boil' and the plates to = buckle.

(N.B. for simplicity = the ammeter,=20 battery isolator and connections
to other circuits have been = omitted=20 from the above digram)

CUT-OUT = OPERATION=20

As the dynamo = begins to=20 turn there is enough residual magnetism in the pole pieces for it = to=20 generate a small output. (Unlike an alternator, a dynamo needs no = help=20 from the batteries it can 'self-excite'.) This small output is fed = back to=20 the field coils in the dynamo making the magnetic field stronger = which in=20 turn will make the output stronger and so on.

When the=20 dynamo is turning fast enough its output becomes greater than = 24volts and=20 it is ready to charge the batteries. This is sensed by the main = shunt=20 cut-out coil (several hundred turns of fine wire) and at this = point the=20 magnetic field that it creates is strong enough to pull the = contacts=20 closed connecting the dynamo's output to the batteries. The = charging=20 current passes through a second series cut-out coil (a few turns = of thick=20 wire) on its way to the batteries. This adds to the magnetic pull = of the=20 main coil to make sure that the contacts snap shut cleanly without = chattering.

When the=20 dynamo is slowed down a point will be reached when its output = voltage=20 falls below that of the battery. Current will start to flow in the = wrong=20 direction. At this point the magnetic field produced by the shunt = coil=20 would still be just strong enough to hold the contacts closed if = it were=20 not for the series coil. Current flowing through this is now in = the wrong=20 direction and its magnetism works against that of the main coil = making=20 sure that the contacts snap open cleanly without=20 chattering.

CURRENT REGULATOR=20 OPERATION

Current from=20 the dynamo passes through a 'series' coil of thick wire on its way = to the=20 batteries. The contacts are normally closed but are pulled open by = the=20 magnetic field if the current in the coil exceeds 22 amps. These = contacts=20 form part of the dynamo's field circuit. When they are open the = field coil=20 is connected to the dynamo's output through a 200 ohm resistor = which=20 drastically reduces the field current and hence the dynamo's = output. As=20 the output falls the contacts will close again effectively = by-passing the=20 resistor and restoring full output only to open again. In practice = they=20 'buzz' to produce the desired limiting effect.

VOLTAGE REGULATOR=20 OPERATION

The = main=20 'shunt' coil (several hundred turns of thin wire) is connected to = the=20 dynamo's output and monitors the charging voltage. When this rises = above a=20 pre-determined level the magnetic field it creates together with = that of=20 the series coil becomes strong enough to open the contacts. Like = the=20 current regulator's contacts these form part of the dynamo field = circuit=20 and have the same effect when they open. The difference is that = they open=20 if the voltage is too high rather than the current, again these = 'buzzing'=20 to achieve the desired effect. The second series coil helps to = ensure that=20 the contacts open and close in a narrow voltage band to provide = accurate=20 control. As soon as the contacts open the magnetic field is = immediately=20 weakened by the loss of current through this coil. The voltage = only needs=20 to fall slightly before for the field produced by main coil is no = longer=20 strong enough to hold the contacts open and they will close=20 again.

DIODE

This is an unusual = feature in a=20 three coil regulator which certainly makes the regulator unit = suitable for=20 negative earth operation only. It's principal purpose isn't = exactly clear=20 as it could have been included for more than one reason and I = don't know=20 if it was fitted to all types of regulator. It does not protect = the=20 regulator against the batteries being connected the wrong way = round as it=20 would be destroyed the moment that the cut-out contacts closed. = However,=20 it will effectively kill off any back-emf produced in the field = coil and=20 therefore help to prevent damage to the contacts. The french refer = to a=20 diode used in this way as "diode de roue libre" (thanks for this=20 information go to Thierry Lefrancois who also believes that this = is the=20 most likely reason for its inclusion). Another possibility that I = can=20 think off is that the types of dynamos fitted on M201s may also = have been=20 used on other positive earth vehicles. If one of these were = inadvertently=20 fitted to an M201 the diode would prevent the residual magnetism = (with the=20 wrong polarity) exciting the dynamo. It is important to realise = that if=20 the dynamo is wrongly polarised or the batteries are connected the = wrong=20 way round then severe damage to the regulator cut-out contacts = will be=20 caused in a dynamo / regulator based charging=20 system.

SERVICING &=20 ADJUSTMENT

There is no regular serving required so leave well alone. = If it=20 ain't broke don't try fixing it!

The Most=20 common problem I have come across is that the contacts of th = voltage=20 regulator coil can become coated in a fine hard to spot corrosion. = There=20 is no effective electrical connection between them and no charge = at all as=20 a result. Except in the case of a real emergency never use emery = cloth or=20 paper to clean them! A fine points file should be used (and this = is a=20 quote from from the French maintenance manual!) =

The main=20 problem is that the abrasive particles shed by emery paper are = difficult=20 to remove. These can grind away at the surface of the points = during future=20 use or even prevent the contacts connecting at = all.

The contacts,=20 particularly of the cut out, can become 'welded' together, = usually=20 as a result of a fault elsewhere that will need investigating. It = is=20 sometimes possible to separate them and clean up the surfaces but = they are=20 likely to cause future problems requiring cleaning on a fairly = regular=20 basis. Where this has occurred it is important to check that = overheating=20 has not caused damage to any insulating features linked to the = contacts.=20 (e.g. Heat damage to the tiny plastic insulator on both the = current and=20 voltage regulator coils on the BPR24 could easily result in the = contacts=20 becoming effectively shorted out)

Precise=20 adjustment of the coil units is a pretty tricky business and as I = stressed=20 before you must know what you are doing and have a good reason for = doing=20 it. You will need a decent accurate current / voltage measuring = device=20 e.g. AVO meter. Before going too far down this route it is well = worth=20 checking that the fault does not lie elsewhere. The diagram below = shows=20 the connections made between the dynamo and regulator through the = link=20 cable. It is also well worth checking that the dynamo brushes have = not=20 worn down to the point that they no longer make good contact with = the=20 commutator. Replacement brushes can be ordered from Lucas in the = U.K.=20 (Thanks to Baz Davis for this information). The set needed is = Lucas 140410=20 (Replacement for Hesselmann 40/0201) I cannot verify this but I am = told by=20 Baz that they are a very close to the originals and work=20 well.