12 volt

probably weak starter, But another thing with these old 6V. trucks is if your timing is off they crank reaaaalllll slllloooowwwww too, so check that as well,

When I was getting my SD-240 running for the first time, before I discovered the timing was 30 or 40 degrees out of whack (can't remember which way), it would crank over fine on 12 volts until I dribbled a little gas down the carb. Then it would slow down for several revolutions until it got rid of the gas, then speed up again until I dumped more gas in the carb.

I've seen gauges in the K series fail with just a quick "jump start to get it going" from a 12 volt vehicle. L-R series are simler in constrution of the guages to the K.
Sometime in the mid 50's two things happened... The std for vehicles went from 6 volt to 12, and King Seeley changed how the guages worked. Niether of these changes can be pinned down to a spefic date (before X/X/XXX things were one way, After y/y/yyyy they were another) so a little caution is needed.
The later King Seeley system was used right up until the end of the light line (in the 80's) and was also used by Ford and Chry ( thru at least the mid 70's). While the faces on a guages of Ford and Chry differ from the faces used by IHC, the guage itself can have the faces swapped. If I were going to change a six volt King Seeley vehicle, I would install the COMPLETE gauge system from one of these later (12 volt) cars, allowing the use of easyer to find senders and parts to the older vehicle, and still retain the look of the orignals.
The later system is compatable with 12 volts.
Haveing said that, I have never felt the need to change from six volts.
Because componants for the old 6 volt stuff is no longer being made, nor readly avaible, I consider ANY loss of componant life unacceptable.
The early guages are hard to come by in working order, but senders are even harder to find. These early senders are not a flat resistace type used in the later system but have a resiatace "card" with a pulseing set of contacts biult into them. For this reason no modern flat resistance sender will work and give the same readings as the orignal senders.
I can not comment on Bruce Frank's experiance other then to say, I have not seen what he has in his truck, nor has it been my experiance that 6 volt gauges survive on 12 volt for even a short while, nor do they give correct readings while they are trying to funtion on 12 volts. Electical theroy doesn't back up his assertions either.
For all the reasons I have stated, I plead with people who just can't wait to crank up an old truck, and have to do it with a 12 volt battery, to reach up behind the dash an snip the feed with to the guages and disconeecting the voltage reg and genny before they do, thus saveing the old stuff while satisfieing their curosity if it will start.
What you do is up to you, but so far the number of people who have had burned guages out number those how have not by several factors of 10.

INFO ONLY, This is a good read, I took a "S" 153935R91 regulator hooked it up to a 6 and twelve volt with a 6 and 12 volt bulb and gas gauges worked fine for testing as CB89 suggested in one of his posts. It is pulsed about 3 times a second,checked with a digital meter, hard to read, 0 to 5---0 to 7, all over, I do not have a sweep meter. So, am posting 2 gauge sets, PNs will tell what they are. Everybody can decide for them selves. Look close at the PNs.

Cornbinder is correct...had that same problem when I first bought my truck. Running battery ground to the starter mounting bolt does the trick.

Help a confused guy out...
I have two questions:

1st:
If current doubles when Voltage doubles....then why does wire gauge drop? Power doubles too according to Ohm's law.
So resistance in 12V light bulbs must change (increases) as we change to 12V to cause less power and less heat in wires.
Wires size tends to depend on the load.

http://www.ohmslawcalculator.com/ohms_l ... ulator.php

2nd:
What are considered the circuits in automotive wiring harnesses? How are they typically broken down or combined?
Some can say a circuit is everything on the same switch...or could be everything that is on the same fuse ....can be two different things.
I tend to thing each circuit has it's own fuse....but may have multiple switches involved.

Headlights, taillights, brake lights, front parking, turn signals, wiper motor, heater, fuel gauge, temp gauge, amp gauge, oil press, license plate light, rear lights, radio, interior lights, horn,

Ok, 1st when voltage doubles the current is 1/2 FOR THE SAME WORK (watts) BUT if you double the voltage thru the same resistance (componant) the current thru that componant will double.
In other words, takeing a headlight for example, a 55 watt headlight will draw 9.1 amps @ six volts. If you hooked the same (6 volt bulb) to a 12 volt sourse it would draw 18.2 amps (for a very short time), but would be useing 110 watts. If you replaced the bulb with a 12 volt 55 watt bulb, it would draw 4.58 amps @ 12 volt for 55 watts. If you connected it to 6 volts, it would draw 2.2 amps for 26 watts.
The accutall watts will likely be slightly less as the resistance in the filiment changes with temp.
So things on 12 volt use half the current for the same (work) output, but require a higher resistance in the componant itself to keep the work the same. Therfore a smaller conductor size can be used on 12 volt then 6 volts for the same job.
As far as circuits go, it is up to the desiger of the system which ones he or she combine. Sometimes it just make sense to group things together or seperately.
For example, it was common when fuses were used, to have each headlight on a seperate fuse, so blowing one fuse would not cause a total lack of headlights. With auto-resetting circiut breakers, it is less of a problem and both headlights are on one breaker now.
Things that are all controlled by switched ign feed (wipers, heater etc) are sometimes combined on one circiut while things that operate with or without the key (hazard lights, parking lights etc) may also be combined.
Another factor is the fuse or circiut breaker must be such that it will protect the smallest item on the circiut (and its assorted wireing) so a 20 amp fuse must not be used on a circiut that has 18ga wires (which aren't able to handle a 20 amp load) or a motor that would burn out if locked but not draw 20 amps (like a wiper motor). The total load of all componants on a circiut must not be such as to overheat or trip the protective device (fuse or C/B).
Ideally, each item would have its own fuse or C/B such that it could be tailored to the item, with just the right trip rateing. That is why on newer stuff you see so many fuses. It come down to cost and simplisity. 6 fuse system was common for many years.

CB89, I have always envied your storehouse of knowledge, especially on electrical stuff! Good to see you're still here as I try to check on the forum more often!

John

Regardless if you (OP) keep it 6v pos ground or 12 neg ground, I'd like to suggest moving the wiring from all going to the switch, to an installed fuse block. At the very least, it will allow you an easier way to troubleshoot a wiring problem down the line. My 2 cents.

Also a good fire safety tip Havi. Blow a fuse rather than melt a wire and cause a fire.

When I do my 12V conversion, it will be total; no voltage reducers, no half measures. I have a new rebel wiring harness, with fuse panel, and I will replace every gauge, every motor, every bulb and every switch. When I got my '56 S-120, it had about ten different voltage reducers, and the harness had been hacked and spliced and was all faded tho one uniform color.

John