Testing M201 Marchal and Paris Rhone Dynamos Dynamos

  The 24 volt M201 and earlier 6 or 12 volt jeeps converted to the 24 volt standard by ERGM are fitted with either a Marchal or Paris Rhone dynamo (opposite). According to MAT 2835 (1961) new Hotchkiss 24 volt jeeps were fitted with a Paris Rhone dynamo and regulator. Whether there was a switch to using the Marchal type before the end of production in 1966 is unclear but there is evidence on some un-rebuilt examples to suggest that this was the case. What is certain is that the Marchal type is more commonly found on 24 volt jeeps so it is clear that in rebuilding earlier jeeps to the new standard ERGM placed the contract for dynamos and regulators with Marchal. Both types were designed to the same output specification but the internals are not identical and were designed to work with a regulator made by the same manufacturer.

When the charging system stops working it can be difficult to work out if the problem is with the dynamo, regulator, or even the cable that connects them together. You can puzzle this out by substituting each in turn for a good part that is known to be in working order but this assumes that you have access to all of these, is very time consuming and even expensive if you go out and buy all of the parts.

In view of this I came up with a simple cost-effective way of testing the dynamo without removing it from the jeep.

The following procedure involves working with a running engine. It should only be attempted by a person familiar with the risks and who is competent to do so. There is also risk of shock from back emf as is the case with any component containing a coil!

The test rig was made from a few simple parts: a 24 volt 21 watt bulb, a 20 ohm wire-wound resistor, 33 ohm wire-wound resistor and some insulated wire. I used a festoon bulb because I had one to hand but a standard indicator bulb will do just as well. I also used two 10 ohm resistors connected in series to make the 20 ohm resistor, again because these were to hand.

I know it wasn't very pretty but the point is that it was made quickly and almost anyone familiar with electrics can construct it.


Remove the cable connecting the dynamo to the regulator.

Connect the red lead and the wire end of the resistor on the black lead to the dynamo socket as shown in the diagram. The ends should fit into the sockets. 

Check that there are no stray strands of wire on the end of the red lead that could touch the casing.

Run the engine at about 1000 rpm. Keep hands, clothes etc well away from the rotating fan & belt while doing the next bit!!! 

Touch the green wire on the field connection. (hold the wire by the green plastic not the bare end – there could be a strong back emf when the wire is removed – you don’t want a shock!)

The lamp should build up to full brightness quite quickly.

As soon as it does remove the green wire – the dynamo is ok. 

Note - There will be a spark as the green wire is removed (normal) 

Switch off engine.


Do not be tempted to join the dynamo +ve terminal to the field terminal. The dynamo is quite capable of producing in excess of 100 volts and a huge current will flow through the field winding possibly damaging it. Never do this!

Hold the green wire by the insulated plastic as you touch it onto the field terminal to avoid a shock from the back emf when the wire is disconnected.

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The above test works well with the dynamos that have been in use until recently but may fail to work on dynamos that have been in store for years. The reason for this is that the residual magnetism in the pole pieces of perfectly good dynamos tends to drop over time if they are not used.  As the test rig was designed to isolate a fault on a charging system that had only recently stopped working it met it's purpose but be warned, any M201 dynamo, either NOS factory rebuilt or second-hand, may fail to operate at all with a perfectly good regulator until the pole pieces have been remagnetised / polarised by connecting the field to +24 volts briefly a few times. When installing a replacement dynamo (new or used) it is good practice to polarise the dynamo before connecting it to the regulator and running the engine.

There seem to be plenty of dynamos about for sale at the moment if you need a replacement, the Marchal type being possibly  the most common. Some people are convinced that the Marchal and Paris Rhone generators are not directly interchangeable and that you must replace on a like for like basisl. I personally haven't found this to be necessarily the case. One of my jeeps has a Paris Rhone generator working with a Marchal regulator both marked 'A' (more about the stickers in a moment).

Many of the dynamos for sale have been removed from rebuilt M201 engines (photo opposite) that have been subsequently fitted to 6 volt Willys or Ford jeeps. These are painted in a characteristic blue/green paint though dynamos taken from an older series of rebuilt engines sold at auction during the 1990s were a less colourful army green.

  The dynamos fitted to these rebuilt engines have a plastic cap fitted over the connection socket to protect against physical damage (the socket is made from a fairly soft alloy).

It is advisable to remove the plastic cap and and inspect the socket for damage before buying as it could be hiding something. If you are buying over the internet (e.g. ebay) ask for a photo showing the socket or at least a written assurance from the vendor that it is in good condition.


The dynamo above was in good order but another from the same source had been damaged despite the plastic cap. A socket that is slightly damaged (as in the photo below) can be carefully put back into shape with a bit of patience but any worse than this and I would reject the dynamo. The damage may seem slight but the plug is a close fit and the collar of the socket must be perfectly circular.


Even with a perfect socket it can be difficult to get the plug end of the connecting lead to engage properly. It first pushes in about half way then is fully engaged by twisting the collar till it clicks. I have found that a bit of WD 40 works wonders.


  The two types of dynamos can obviously be distinguished by their data plates but if these are missing then the end plate furthest from the pulley can be a useful guide. It is cast with reinforcing ribs on a Paris Rhone dynamo and the socket is mounted on a round base whereas the Marchal dynamo has a plain endplate and the socket is mounted on a square base. Some examples of both makes were modified with a steel protective ring welded to the body of the dynamo to prevent damage to the socket. This cannot therefore be used to identify the manufacturer. The example opposite has a ribbed end plate and circular post on which the socket is connected and is therefore almost certainly a Paris Rhone dynamo.

Factory rebuilt dynamos and regulators were coded with vinyl stickers. I have not come across any documentation that explains the labelling system under which dynamos and regulators had stickers attached marked 'A', 'A B', 'B', 'B C' or 'C'. One finds both Marchal and Paris Rhone regulators marked 'A' so it is not to identify the manufacturer although many believe that to be the case. Certainly, on an M201 fitted with reconditioned units the letters match and I believe the origin to be in the fact that both OEM and ERGM issued these as factory matched pairs with the coded stickers. I have seen a crate of three factory fresh sets but at the time did not note the labels (curses!). Photos of examples are given on the Regulator page.

- This page should be read in conjunction with the page on regulators and the page on batteries.
- How to dismantle a dynamo is dealt with on the next page.
- If you are having problems with the charging system on your M201 that you can't puzzle out then get in touch, I have some spares, a test rig, and may be able to help.