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O/T WW2 Jeep engine electrics


Roger King

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A friend of mine rebuilds the occasional Jeep engine (WW2 era, so real Jeeps). He likes to run them up on a test stand before sending them out; this is not a dynamometer, just a flying bedstead to check for leaks, rattles, misfires, etc.

The Jeep of this era is negative earth, but 6 volts and with contact breakers to trigger the coil. The problem is that a 6 volt battery has to be kept fully charged all of the time and even then struggles to turn a relatively tight new engine. It is very important that the customer's Jeeps retain the 6 volt system because almost to a man or woman the owners are much more fastidious about originality than most classic owners.

So, he has tried using a 12 volt battery to crank a fresh engine in order to raise oil pressure initially. This works very well, even if in theory he's overdriving the 6 volt starter; and as he says, on the test stand the starter gets very little use so he's prepared to do this and see how long it lasts (it belongs to the test stand and not the vehicle). Ultimately he could have the starter converted to 12 volts to but that would leave a 6 volt ignition system being fed 12 volts and presumably over loaded.

So after all that waffle, my question is this. Is it possible to use the 12 volt battery, but fit a resistor in the feed to the ignition to drop the voltage? All gauges, etc, on the test bed are mechanical so it is only the ignition that needs this feed.

I have done a very similar thing on my grandchildren's train set where each LED light is fed 12 volts, but via an approximately 1000 ohm resistor. That works, will this?

If anyone can give an informed answer - is it possible? what value and current capacity resistor (the ignition will not be a heavy load, unlike the starter) would be needed? where to obtain such a thing? Is this an insane idea?

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Can you use two 6V batteries in series? Take off at 6V for the ignition, use both batteries for the starter.

Anyway to feed 6V to the ignition just requires a potential divider - two big ohm resistors in series between 12V and earth with 6V taken off at the mid point.  I read the ignition coil needs about 5A so the resistors can be relatively low wattage - two of these?  Someone will be along in a minute to point out the error in my fuzzy memory of how stuff like this works - there might be too much resistance to get 5A to the coil.

Has someone in the Jeep world done this before? Or VW or another car that has 6V electrics.  What about the MGB? I remember that has two 6V batteries.

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My 1984 Mini 1000 had a 6v coil with resistive lead. The starter solenoid put 12v to the coil to boost the starting spark and once running would be on 6v.

I only found this out after I put the engine back in where I'd inadvertantly also connected the 12v coil feed that was still part of the loom. I got about 1/2 mile to work when it conked out - it had melted the heel on the points! 
 

It is possible and present on production cars as my experience shows. I was late for work BTW!

Ian

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If it's just to feed the coil then a ballast resistor or lead will do it as Wrightpayne suggests.

For 12v systems that's usually about 1.5 ohms, for the 6v system you just need a ballast resistor of about the same value as the coil resistance then you'll get 6v across the coil and 6v across the  resistor. 

 

 

 

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Thanks guys it seems that this is going to be quite easy as I'd hoped, but not really expected.

Yes, it's just the coil. This is only for the test stand which has no lights, wipers, etc. (actually not very much etc. seeing as it's a Jeep - oh yes, they have a horn too).

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It is important with a 6V system that your cables are thick enough, thicker than a 12V system. Equally important is to run the main negative earth direct to the starter motor to avoid current losses. Used to service the odd Jeep and Minerva derivative in military service and never had problems cranking, even a 1947 EMW i did some work on had no problem with cranking on 6V.

If i remember correct a coil with resistor is a 9V coil but is powered by 12V but only while cranking.

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Further to the above answers from Ian and orangepeel regarding ballast resistor circuits below is an extract from a Mini wiring diagram showing such a system. The coil (36) has 2 feeds, one that feeds the reduced voltage via the resistor wire (63) which is fed from the ignition switch (35) and the other is a 12v feed that is fed from the starter solenoid (3) that is only live while cranking the engine.  I may be able to find a Mini ballast resistor wire that I removed from a loom when changing to a full 12v electronic ignition

IMG_2242a.jpg.8094237ca930d2c560a7e213fbefc4ae.jpg

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I also have a pre-war BSA Scout (car) which is 6 volt positive earth. Use a Genius 5 battery charger which works on 6 and 12 volt and is actually used more on the Caterham. Other BSA owners who have converted their cars to 12 volt retain the original starter without a problem. Reliable electric starting had not been around for that long and engines of this age would start using a crank handle. As Elie says good thick cables and sound connections.

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