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The Heater System

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  • The Heater System

    If you have a 6 volt Stock Truck then no reason to read further. However, if you have upgraded to 12 volts, there are some pretty important issues that should be addressed. There are two ways to address the Stock Heater under 12 volts. I will explain the difference between the two. It is important to understand there is a right way to do this and the other way. One way requires no voltage dropping and works just like modern systems. The other works by transferring Power to Heat. This is the function of voltage reducers. They merely transfer half of the power to heat which must be dissipated.

    1) Stock Heater using Stock Switch using 12 volt power - This is the most common setup. It requires using a voltage reducer. This is what I characterize as 'the other way'. This is because it transfers unneeded power to heat. There are several styles of voltage reducers. The simplest one is merely a 1 ohm, 50 watt power resistor like the one here:,15824.html
    Another option is to use this one which is a 1.5 ohm solution that works the same as the one above:,2375.html
    Another popular option because it uses electronics to help dissipate the heat:
    My personal opinion is they all do the same actual work. While the Ron Francis offering is more elegant, it's also $80 when the first on the list is only $17.99. Although if you do a little research, you can get much better prices. For example, the Speedway Motors offering of $17.99 is available at Mouser Electronics for as low as $4.27. It pays to do a little research.

    As important as getting the reducer in the mail is properly installing it. This is a wildly controversial subject but when examining using electronics theory and mathematics, the proper way to install the reducer is on the Positive side of things, and if using the stock switch, installing the reducer between the Ignition Switched Power and the BAT Terminal of the Switch. On a stock switch, if it is not marked it is the screw-type terminal lug directly under the fuseholder. Failure to put this reducer in this location will put twice the rated wattage on the on-board resistor located on the switch that is used to control voltage in Low and Medium switch settings. A common complaint is how hot the resistor on the switch gets. This is due to the placement of the reducer. Resistors are rated in Ohms, but they are also rated in Watts. A one ohm resistor that is rated for 5 watts will burn up very quickly if that 5 watt rating is exceeded. In this case, GM put a 5 watt resistor on board the switch expecting it to always see 6 volts. When putting 12 volts to this same resistor, it is being exposed to 10 watts and that number can be as high as 14. No wonder it gets hot, starts smoking and eventually fails to operate. By encompassing the entire heater circuit in your 6 volt reduction umbrella, you are lowering heat and enabling the system to last MUCH longer.

    If you purchased a 12 volt after-market heater switch, that issue becomes less important and it is okay to put the voltage reducer between the motor and the switch. If you have a burned out original stock heater switch or one that is questionable, you can modify your system to fix that switch and at the same time have a much happier resistor array for Low and Medium settings. I am working on that instruction but awaiting a 6 volt motor.

    Do not get me wrong, this system works just fine and will work as long as you own your truck. It is not the most optimal way to use power and there is a more elegant solution.

    2) Rather than transferring power to heat, we can install a 12 volt motor. This puts our voltages on even par and no power to heat dissipation schemes are required. This is more work but in the long run makes the most sense. Siemens happens to make a 12 volt motor that has the same shaft size and is just the right physical size to fit inside the shell of the original 6 volt motor. The reason this is a good thing is because there are no 12 volt motors that have the same physical dimensions, and GM made our trucks firewall to fit a specific diameter motor. Be sure to remove the small riveted plate in the very back of the 6 volt motor shell when you get that far or else it won't fit. You can read all about this particular modification here:
    Once you have your motor squared away and the heater is operating normally under 12 volts, you still have to address the stock switch. Because of the mounted resistor on board the switch, it is totally inadequate for the job. We want to make it more than adequate. To do this, you will be interested in reading the modification instruction here:
    By simply putting the two resistors needed for Low and Medium in the fan motors air stream we can keep things cool. This is how modern vehicles are made so it makes sense.

    I would very much like to hear from other aficionados concerning this issue. I would like recommendations on how to improve the design.
    Last edited by Deve; 10-07-2015, 08:56 PM.
    Deve Krehbiel