1934 RP Saloon, original 6v set up.
Charges merrily on 'High' charge, but shows discharge on 'Low' charge.
Have now located a supplier to replace the resistance wire, but don't know the one to buy?

In simplest terms, you need to make up a field resistor of approximately 3 ohms and which can safely carry a current of 2 amps. I say 2 amps to give a margin of safety. The field current at a nominal half charge should be about 1 amp, but I would opt for a fair margin of safety from overload.

The actual resistance figure of 3 ohms is also nominal, and could be "adjusted" slightly to give you your personal requirements for "half charge" - I have simply worked on the basis of a nominal field resistance of about 3 ohms.

The complications are:

If you have earlier "C35A" Dynamo and a "CF" type Cut-Out, with resistor located in a box on the Dynamo, then it is no more complicated than the above

If you have the set up where you have a later type Dynamo, and a "CFR1", or "CFR2" type of Cut-Out, then you will find the resistor/s cunningly concealed in the nether regions of the Cut-Out.

Here beginneth the first complication. There may be two resitors, in series, making up a total of 3 ohms.

The "split" resistance was used in other vehicle applications to provide an additional charge rate, but in A7 service we need the total 3 ohms.

The second complication is that not all resistance wire is easy to solder, and if you need to solder the resistance in, then you will either have to be sure that the type you buy is happy to be soft soldered, or otherwise, the only way I know of handling the recalcitrant types, is to "tin" first with Silver Solder, and then use ordinary soft solder.

So, if you have a choice of things, try and get some readily soldered wire if you need to solder the resistor in.

I hope the above is of some use.

Good luck

Barry R.

Many thanks, much appreciated. I will delve further.

The science is all a little beyond me but I restored the half charge on a Chummy I no longer own by fitting an aluminium cased 25W resistor bought from Maplins. Cost about £1-99. I fitted it behind the dash panel but what I wired it to I can't remember - presumably in the wire from the half charge position on the switch panel. It worked fine though and, as far as I am aware, continues to do so to this day.

Perhaps Barry or others might tell me how I connected it as I did follow some instructions that have long gone!!

STEVE

Hello Steve and Co.,

The simplest place to install the type of resistor you have mentioned Steve, in the case of an earlier car fitted with the SM5 Switchbox,would be to connect it across the D+ and SH terminals on the switchbox terminal panel.

I don't have a later wiring diagram at my fingertips, but a similar arrangement could be applied to the later "keyed" control switch.

For what it is worth, I personally would prefer not to mount extra bits and pieces in that area. I would prefer to either replace resistors where they were originally fitted, or else mount them in a suitable spot under the bonnet where they can be "got at" without the trauma of disturbing the maze up behind the dash.

The real problem with using resistors from an electronics supply house, is that they come in a myriad of types and sizes, and may be difficult to physically fit in a sensitive and workmanlike manner.

I would think that a couple of ten watt rated 1.5 ohm wire wound resistors could probably be found which would readily lend themselves to do the job in a CFR2 Cut-Out.

Just for the record, anyone trying to purchase a three ohn resistor may well have to accept either a 2.7ohm or a 3.3 ohm item, as resistors tend to be sold in "preferred" sizes. Either will work of course, but the half charge rate would be slightly higher with the lower value resistance.

I have always managed to scrounge suitable resistance wire from old heater elements, jug elements etc etc.


Regards

Barry R.