BACKGROUND

24 volt M201 type jeeps were fitted with charging systems by two different manufacturers. In building new M201s, Hotchkiss fitted a system manufactured by Paris-Rhone whereas, for rebuilding old 6 and 12 volt jeeps to the new 24 volt standard, ERGM at La Maltournée used a system manufactured by SBP and distributed by Marchal. Over the years of rebuilding jeeps the Marchal model appears to have become the more common of the two.

DYNAMOS

24 volt M201 jeeps can be found then with dynamos by two different manufacturers. MAT 2835 indicates that M201s built by Hotchkiss from 1960 were fitted with the Paris-Rhone G15R39, G15R48 or G15R51. The other dynamo, a Marchal dynamo BPG24, originates, I believe, from jeep rebuilds by ERGM. Both types were rated at 28.5 volt, 22 amp. They are physically large for this rated output due to the fact that they are a sealed design and there is no forced air cooling provided by a fan that 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 could certainly produce.

REGULATORS

Over the period of production (1960 - 1966) new Hotchkiss 24 volt M201s were fitted with a Paris-Rhone regulator (ZT3115a, ZT3118a or  ZT3120a) which bolts directly to the bulkhead with a degree of resilient mounting provided by rubber washers. According to MAT 2835 these different Paris Rhone types are interchangeable. The Marchal BPR24 regulator used by ERGM is carried by four resilient rubber mounts and two brackets which are bolted to the bulkhead. 

Paris-Rhone regulator	Marchal regulator

Both makes are of the three coil 'current voltage regulator' design which consists of a cut-out, current regulator and voltage regulator coil. An anti-parasitic filter is included in the base of each type of regulator to reduce radio interference.

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

MARCHAL REGULATOR

Although all Marchal regulators were designated BPR24, there were in fact two versions, an early type introduced in the early 1960s, and a later type from about 1970. The following detail is based on the later Marchal model but the principle of operation of all all types of regulator by Paris Rhone and Marchal is broadly similar and differences will be dealt with later.

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

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

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

(N.B. for simplicity the ammeter, battery isolator and connections to other circuits have been omitted from the above diagram. Note also that the diode and thermistor are specific features of a Marchal regulator. The diode is not included in a Paris Rhone regulator where a standard resistor is used in place of the thermistor)

CUT-OUT OPERATION

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

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

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

CURRENT REGULATOR OPERATION

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

VOLTAGE REGULATOR OPERATION

The main 'shunt' coil (several hundred turns of thin wire) is connected to the dynamo's output and monitors the charging voltage. When this rises above a pre-determined level the magnetic field it creates together with that of the series coil becomes strong enough to open the contacts. Like the current regulator's contacts these form part of the dynamo field circuit and have the same effect when they open. The difference is that they open if the voltage is too high rather than the current, again these start 'buzzing' generally at somewhere between 100 - 200 Hz to achieve the desired effect. The second series coil helps to ensure that the contacts open and close in a narrow voltage band to provide accurate control. As soon as the contacts open the magnetic field is immediately weakened by the loss of current through this coil. The voltage only needs to fall slightly before for the field produced by main coil is no longer strong enough to hold the contacts open and they will close again.

DIODE AND THERMISTOR

These are an unusual feature in a three coil regulator found only in the Marchal design. The thermistor is common to both Marchal types and the diode replaced a condensor fitted to the early type to reduce sparking at the voltage regulator contacts. The purpose of the diode in the later Marchal type is to effectively kill off any back-emf produced in the field coil and therefore help to prevent damage to the contacts by excessive sparking. The French refer to a diode used in this way as "diode de roue libre". Thanks for this information go to Thierry Lefrancois who also believes that this is the most likely reason for its inclusion (In English we call it a free-wheeling diode).  The thermistor provides a degree of temperature compensation to the regulated voltage. It is important to realise that if the dynamo is wrongly polarised or the batteries are connected the wrong way round then severe damage to the regulator cut-out contacts will be caused in a dynamo / regulator based charging system.

SERVICING & ADJUSTMENT

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

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

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

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

Precise adjustment of the coil units is a pretty tricky business and as I stressed before you must know what you are doing and have a good reason for doing it. You will need a decent accurate current / voltage measuring device e.g. AVO meter. Before going too far down this route it is well worth checking that the fault does not lie elsewhere. The diagram below shows the connections made between the dynamo and regulator through the link cable. It is also well worth checking that the dynamo brushes have not worn down to the point that they no longer make good contact with the commutator although this is fairly rare.

PARIS-RHONE REGULATOR

Despite its different external appearance the Paris-Rhone regulator performs the same function as the Marchal unit and uses the same external electrical connectors as shown above. The three types, ZT3115A, ZT3118A, and ZT3120A are interchangeable and represent an on-going technical development of the original model. They share the same casing although the ZT3120A has a slightly different shaped lid and is the easiest to adjust as adjusting nuts were finally incorporated into the design of the coil assemblies.

The principle of operation is the pretty much the same though the coils are arranged in a different physical order and orientation, there is no diode, and the field circuit is controlled by an arrangement of two resistors rather than a resistor and thermistor. One disadvantage of the Paris-Rhone design is that there are no adjustment screws on the coil assemblies and adjustment can only be achieved, as far as I can see, by physically bending the metal tensioners which is not an easy task.

INTERCHANGEABILITY

As already mentioned dynamos and regulators were supplied to the same specification by two different manufacturers and would certainly have been fitted to jeeps during production as matching pairs.  In an ideal world small differences in the electrical characteristics of individual dynamos and regulators, even from the same manufacturer, also mean that a regulator should ideally be set up while connected to the actual dynamo it is to work with.

In the real world a good condition Marchal regulator is certainly interchangeable with another Marchal regulator or Paris Rhone with Paris Rhone. Even so, there can be small differences in the regulated voltage and I always subsequently check the voltage regulation, and adjust if necessary, as soon as it is practical to do so.  Given that it is possible to connect a Paris-Rhone dynamo to a Marchal regulator and vice-versa as the connectors are the same, they can end up mixed together in which case checking the regulated voltage to see if an adjustment is necessary is again important. In the case of my first M201 it has a Paris Rhone dynamo with Marchal regulator both marked with an 'A' sticker and the photo below from Barry England Davis shows one of his jeeps with the opposite combination, a Paris Rhone regulator with Marchal dynamo.

I have not come across any official army documentation that explains the labelling system under which dynamos and regulators had stickers attached marked 'A', 'A B', 'B', 'B C' or 'C'. As both Marchal and Paris Rhone dynamos can be found marked 'A' it is certainly not to identify the manufacturer although many believe this to be the case. I have once seen a crate of three factory fresh sets and as I recall they were all marked 'A'. I suspect that the system may be based on climate zones but I have no proof of this. Certainly, extremes of temperature significantly affects a lead-acid battery system and the regulated voltage setting that is required.
 

The 'Hotchpotch' of dynamo and regulator labels

			Can you add any more photos to the hotchpotch?
Marchal reg marked 'A'	Marchal dynamo marked 'A'	Paris Rhone reg marked 'A B'
Can you add any more photos to the hotchpotch?
	Paris Rhone dynamo marked 'A'	Paris Rhone reg marked 'C'	Paris Rhone dynamo marked 'B C'

INTERCHANGEABLE OR NOT?

Physically the two different makes will connect together and electrically they will work together but invariable may need adjusting to the correct voltage (not easy with a Paris Rhone regulator). Experimenting with different combinations I have found that the regulated voltage can vary by up to a volt within the range 28 - 29 volts which is acceptable though this test was carried out with known to be good working regulators. So unless you have the knowledge, skills, and an accurate voltmeter, it is a good idea to replace like for like as the best option.

WHAT SHOULD THE VOLTAGE SETTING BE?

The correct setting will always be a compromise somewhere between 28 and 29 volts. The advantages / disadvantages of operating at the low and high ends of this range are explained in table below and further information is given on the battery page.

 

VOLTAGE LIMIT	28 volts	29 volts
ADVANTAGES	Maximum service life; the battery remains cool during charge; the vehicle can operate in temperatures exceeding 30°C	Faster charge times and the battery is less likely to suffer from sulphation.
DISADVANTAGES	Slow rate of charge and sulphation of the negative plates will occur if the battery is not regularly given a top-up charge.	Battery will be seriously over charged at temperatures >30°C and 'wear' of the positive plates (known as corrosion) is increased.

 The actual performance of a factory set Paris Rhone regulator can be seen in the graph below which is taken from MAT 3422-3 (the detailed repair guide for the 24 volt M201).

Compared with the performance of a modern electronic control system, the electromechanical regulator was a rather crude device but one can see that the voltage at a low load was regulated to around the quoted 28 volt mark and that the current limit operated around the quoted 22 amp limit. The zone is, I think, temperature dependent with the higher voltage at low temperatures and reduced voltage at higher operating temperature. The significant voltage drop with increased load is almost entirely due to the resistance of the anti-parasitic filter unit that is included in the base of the regulator rather than any defect of design of the regulator itself.

As long as your regulator is operating within the range shown (i.e. 27.5v - 28.5v ) with just the ignition system drawing a current then it is within the original design tolerances and there isn't a major issue though at the low end your battery will certainly need a top-up charge from time to time to prolong battery life. The actual setting for a Paris Rhone regulator given in MAT 3422-3 is 27.6 volts with a 10 amp load having allowed the regulator to warm up by running the charging system for 30 minutes with the lid removed from the regulator. So there you have it.

If you are having on-going problems with the charging system on your M201 that you can't solve then get in touch, I have a test rig and some spares and may be able to help. I am also working on the design of an electronic replacement given that replacement regulators are increasingly difficult to obtain.