I find myself looking at something I no longer remember much about. I just got a 54 f750 that runs and drives. That sounds good. It has an 8 volt battery that is bad, the battery is hooked up backwards (neg, ground). It has an alernator that has 2 wires on of which one just goes to a light hung under the dash which is lighted only when the truck is not running and the key is on. The truck will not start on the 8v battery but if I hook up a 6v charger and put it on full boost the truck will start right up and run fine. The question is what should I do with this and why does it work at all. If I put in a new battery and hook it up correctly will I mess up the alternator. Did someone do this on purpose or did they just do it wrong. Thanks for any help.

 

I have never seen a 6 or 8V +grnd alternator (doesn't mean one doesn't exist, but it wasn't for auto use, all were 12V - grnd by the time alternators came around), so it seems to me they have deliberately wired it as neg grnd and are using the light bulb as a load to reduce the alternator output so as to not boil the battery?
Are you sure it's an 8V battery? My guess is that it's a 12V battery and the bulb is being used as a voltage drop for the instruments. Where does the second alternator wire go? By two wires do you mean 2 wires hooked to the output terminal?

 

My two cents:

8 volt battery is dead and needs jump to start truck.
Light under dash is a load to excite the alternator into charging.
Betting that the other wire runs to Battery.
Truck converted to neg. ground to run alternator.
Any amp. or volt gauges working in the truck?

 

Sounds like a GM alt set up.

Barry

 

2 wire alternator and the other wire does go to the battery. Battery is deffinately an 8v. Amp gauge works backwards, the oil and water gauges have been replaced with mechanical ones and the fuel gauge reads high all the time. 6v alternators are available from hot-rad/restoration suppliers but seems to be no such thing as a positive ground 6v alt.. thanks again for your help.

 

switch the wires on the alternator to get it to read properly or reverse the direction of the wire if it's a loop type gauge. Fuel is likely a broken wire or failed sending unit.

 

After having expierence with 8 volt batteries, the first thing I would do and have done is get rid of it. If you think that the battery is installed backwards, or polarity reversed the amp guage will show discharge. Check to see that all the battery cables are the heavy guage type like as in welding cable 0/1 guage at least. This is overlooked by many guys that are operating on the standard 6 volt system. They go down to their local auto parts outlet and buy the 12 volt type, which are no where near heavy enough and cause resistance.
You can do a quick check on this by cranking your engine a few times, then feeling the battery cables. If their warm or hot then you either have a bad connection or the wrong type cables. As far as 6 volt batteries go I have no problem with them, and I buy mine at the local Tractor Supply Outlet, 925 cranking amps. If the battery you have has 4 caps on top its an 8 volt, 3 its a 6 volt, 6 its a 12 volt unless its a maintenance free one, pop the caps and if there are 6 ports its a 12 volt.
On my 53 with the factory O/D 239 Flathead, runs just fine on the 6 volt, and after a good drive, if I have to stop for gas or what ever shut the truck down for a few minutes or 1/2 hour it starts right up.
Again I've been there with the 8 volt batteries, and wouldn't thank you for one. Just one more item here you have to set your charging system to put out 9.2 th charge the 8 volt. Whereas the 6 volt requires 7.2, didn't mean to get into a story here but I hope this may answer your question.

 

With a voltmeter, you can check the output of your alternator to determine what if it is a 6 or 12 Volt. 6 Volt alternators are produced for those that want the reliability and output of an alternator, but don't want to convert the gauges, wipers, radio, blower, lights, etc. Those dash ammeters are inductive, so once you get it set up the way you want it, if it reads backwards, just turn the wire around in the loop so that it passes by in the other direction.

The indicator lamp that is hooked to the alternator, draws current through the field circuit, so that the alternator begins to output as soon as the engine starts. A generator can't do that.

I concur with the suggestion to jetison the 8 Volt battery. Run it at 6 or 12, whichever your alternator is built to be. If you don't have a voltmeter, you could take the alternator to an auto parts store that has a test setup. They will typically test it free; you will find out the voltage and whether or not it is healthy.

 

I think SteV8e has hit it; I'd say the chances are just shy of 100% that the alternator is a 12v Neg Grnd. Also that the PO didn't reverse the run of wire thru the ammeter (for Pos grnd to neg grnd conversion) so it reads backwards. There aren't many of the 6v alternators around and I'm sure they'd have a big tag on them identifying them as 6v. Shuck the 8v battery as suggested and stick in a nice fresh 12v. See if the coil is a 12v type with a ballast resistor (if it still has points). Re-route the loop of wire thru the ammeter so it reads correctly. You may have to run a resistor on the fuel gauge circuit. They may make 12v sender units for '53-'56's that would cure the problem.

 

thanks to all. Have put in a new 6 volt battery and the alternator is puting out 6.8 to 6.9 at idle. Still have to reverse the amp gauge wires and move the negative battery cable from the frame to the starter.

 

C-700, if memory serves me correct your charging system should be puting out 7.2.
Just for curiosity sake rev up the engine just a little over idle and see if this is what in fact it does.

 

will do that

 

I used to have two tractors that were 6v, a Farmall Cub and an International Crawler, I put 8v batteries in them and ran them for years, never did nothing but swap the battery. Ofcourse tractors don't get the day to day use a automobile does, but they do run for several hours at a time.

 

Each cell of a fully charged battery is about 2.1 Volts, so your six Volt battery will show about 6.3 Volts without the engine running. As 5 Star noted, if it is a 6 Volt system, it should be charging at about 7.2 Volts (at least 7.0).

Compare the voltages between the engine running (at least 1500 RPM) and the engine cut off. If they are the same, you are not charging, you are just measuring your new battery.