THE SIX VOLTER

A tale of gloom and doom by Chris Ryle

July 1999

Those with Paris built cars will be all too familiar with the vagaries and idiosyncrasies of a six volt system. The flaccid response of the starter motor struggling to turn the motor over compression by compression, and the headlights which give out less luminance than the side lights of the modem motor.

The love affair with 6 volt systems is as deeply ingrained in the American psyche, as pastrami on rye or baseball. The American army was so engrossed with 6 volt systems that US Ordnance decreed that all vehicles should be equipped with standard 6 volt sealed beam lights. This love affair entered a sticky patch during WW2 when Diamond 'T' tank transporters were equipped with Hercules diesel engines.

A 6 volt motor will not turn an engine of 9 litres capacity with a 20:1 compression ratio no matter what the capacity of the battery. Meanwhile back at Fort Benning, Captain Six Volt of the US Ordnance was not to be outdone, and while relenting to a 24 volt starting system retained 6 volt lighting, entailing the most complex of parallel series switching systems to charge the four 6 volt batteries.

However the Captain met his nemesis in Korea where due to the excessively low temperatures vehicles with 6-volt systems virtually had to be set alight before they would start. Post war the French army which was equipped with American WW2 surplus very wise opted to change the electrical systems of these vehicles to 24 volts.

I recite this brief history to illustrate both the limitations of 6 volt systems and the lengths to which some people will go to retain them. Perhaps the Achilles heel of 6-volt systems is their greed for current. To generate the equivalent power to 12-volt systems, the former has to pass twice the current of the latter - thus a 36-watt bulb at 6 volts consumes 6 amps, compared to 3 amps for a 12-volt bulb.

6 volt systems demand cleanliness of all conducting surfaces and their associated terminals, together with the appropriate cross section of cable to carry the current if significant voltage drops are not to occur.

This makes 6 volt systems rather more unforgiving of neglect than their 12 volt counterparts, and requires batteries of greater capacity, (amp/hours) and the appropriate system to charge them at a higher current.

Starter motors and dynamos of 6 volt systems are likely to need their commutators and brushes inspected more frequently than their 12 volt counterparts due to the heavier currents, although this is compensated for to some extent by the greater cross section of the carbon brushes employed.

Due to the slower cranking speed also demand that the timing accurately and that the points and associated circuitry is up to scratch.

Heavier current in a system greater heat, particularly where significant resistance is encountered. This in turn generates greater resistance.

This phenomenon accounts for stop lamp bulbs play up, drawing as they do higher currants and encouraging their contacts to oxidise, resulting in poor conductivity.

A few words on ancillary electrical equipment - at one time it was easy to acquire car radios working off 6v, these have become rarer as fewer 6v systems survive. Some radios manufactured in the 1960's such as the HMV hybrid valve and transistor models, had internal links which could be altered to cater for 6v. It is unlikely you will find a radio/cassette designed to work on 6v, but small 6v to 12v steppers are available, which will do the job nicely. I understand that some of the car radios manufactured by Blaupunkt which run on 6v also covered the FM frequencies. Other sources of 6v goodies are Renault 4's up to about 'H' registration, which can at least provide a 6v, coil, and may have radios.

Fusing - since 6v systems pass greater currents the potential danger of an electrical fire is greater, although slightly mitigated by the greater carrying ability of the wiring. French built cars are not equipped with fuses, although it is tempting to install one in the wiring harness where it passes through the bulkhead onto its point of supply at the ammeter. The only argument against this is that it is not selective repeatedly blow on a dark night, further progress is difficult. Selective fusing of discrete systems, as on the Slough built cars is preferable but not easy to achieve on Paris cars without modification of the wiring harness. Perhaps the best compromise is to isolate the feed to the lighting switch on the steering column and fuse this supply, since in my experience it is the lighting circuits which are likely to cause trouble

Having run a number of different vehicles over the years with 6v systems I am inclined to the view that they are more capricious than their 12v counterparts, but generations of motorists struggled manfully with them, no doubt wielding the starting handle when the need arose.
So the watchwords are - Cleanliness is paramount, and size (of conductor) does count.

Chris Ryle

Last update: 19/Nov/99	Contact us

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