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Technical 6 volt overdrive on 12 volt system

Discussion in 'The Hokey Ass Message Board' started by Hot_Rod_Hellbilly, Dec 22, 2018.

  1. Hot_Rod_Hellbilly
    Joined: Feb 17, 2013
    Posts: 17

    from Texas

    I have been looking for information on this. I just upgraded the 6 volt system to 12 volt on my 53 Merc. I still have the 6 volt overdrive solenoid and relay, I have been reading some people have good luck with running the 6 volt system on the 12 volt. I bought a ceramic ignition ballast resistor and installed it on the line from the ignition switch to the relay and the overdrive isn't working. Did I hook it up right or do I even need the resistor? I'm not the best with electrical so any information would be helpful thanks
    chryslerfan55 likes this.
  2. Blues4U
    Joined: Oct 1, 2015
    Posts: 3,316

    from So Cal

    Hard to troubleshoot from 2000 miles away, but I'm not sure just installing an ignition ballast resistor in circuit will drop the voltage down from 12 to 6. There is a way to determine the voltage drop using ohms law, but you have to know the current draw. Still, this isn't the best way to do it. I'd prefer to use a voltage limiter for a single application. But if you want to keep gauges and other items operating at 6 volts it may be best to run a dual voltage system using two 6 volt batteries hooked in series. Feed a 6 volt panel off the first battery, and take your 12 volts off the 2nd. We did this often with 24 volt systems in trucks and heavy equipment, using two 12 volt batteries in series (or 4 in series/parallel). I don't see any reason why you couldn't do it with a 12 volt system in a car.
  3. Draw us a diagram of every part of the circuit you created because "ceramic ignition ballast resistor and installed it on the line from the ignition switch to the relay" isn't enough to go by. There should be more to it.
  4. when did Merc go to 12 volt? '54? is the 12volt solenoid interchangeable?
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  5. It'll be easier to replace the relay with a 12V version; a 'standard' Bosch SPDT relay will work. The solenoid may be a different matter... Some have had success running a 6V solenoid on 12V, some haven't. If using a dropping resistor to reduce voltage to the solenoid, you'll need to know the current draw @ 6V and the internal resistance of the solenoid. The current draw will tell you how many watts the resistor will need to be rated for so you don't burn it up, and the resistance number will give you the value needed for the resistor in Ohms; the wrong value will mean high or low voltage to the solenoid. The resistor should be installed between the relay and the solenoid.
    BurntOutOldMechanic likes this.
  6. oldsman41
    Joined: Jun 25, 2010
    Posts: 1,071


    No resistor on mine 51 merc no trouble in 6 years. I do believe they make 12 solenoids but they are pricey.
  7. Hot_Rod_Hellbilly
    Joined: Feb 17, 2013
    Posts: 17

    from Texas

    Yeah like $400 pricey haha
  8. G-son
    Joined: Dec 19, 2012
    Posts: 504

    from Sweden

    With a (mostly) resistive load it's easy to pick the resistance of for resistor to drop the voltage to half, the resistor needs the same resistance as the load. 10 ohm load? 10 ohm resistor. You'll get half the voltage over the resistor and half over the load.
    There seems to be resistors sold for reducing voltage w/o taking the actual load into account, that's nothing but a recipe for disaster. If the load has lower resistance than the resistor it will get less than half voltage, if the resistance is higher it will get more, possibly way more than half voltage - you may end up running 6V items on nearly full voltage from the 12V battery if the parts are too incorrectly matched.

    Basically: resistors CAN work if you get the right one, and the load is a suitable type (lightbulb, solenoid, relay coil). They are less suitable for electric motors since they draw different amount of current at different rpm, it will have to start at very low voltage (since it draws the most current at 0rpm) and that may be a problem for some motors - they'll simply not be strong enough to start moving at such low voltage.
    One downside to doing this is that the resistor will burn off as much power as the load uses. Not a problem if you've got a big alternator and not much to run with it, possibly a problem if you have a small generator/alternator that barely keeps up with demand.

    Using double 6V batteries in a 12V system and running 6V items off a single battery (or doing the equivalent in a 24V system) is not a very good idea. When you charge the two batteries in series they get the same amount of charging current, but if you run some equipment off just one off them you will just drain one off the batteries. The result is overcharging one battery to keep the other full. Evidently, it DOES work, but it will reduce battery life.

    Electronic voltage regulators like the 78**-series has been cheap for a long time, they put out a stable reduced voltage but can usually only handle 1-2A, they can be good for gauges and similar little things. probably costs 2-4 dollars (with the capacitors you should run them with). These has the same low efficiency as a resistor, but they give a stable output voltage. This means they can handle a varying load better.

    Today there are cheap step down/buck regulators available too. They can have an efficiency over 90% so way less losses than the other methods, and there are different sizes that can handle different loads - some relatively large ones.

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