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.
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.
view of this I came up with a simple cost-effective way of testing
the dynamo without removing it from the jeep.
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.
TESTING THE DYNAMO
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.
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
The lamp should build up to full brightness
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!
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.
The embedded .mov
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The above test works well with
the dynamos that have been in use until recently but may fail to work
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
||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
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
||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.
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.