I new the title would get someones attention. :p I am doing a power steering upgrade but first I am putting in a electric 12 volt fuel pump. Instead of hooking up the power straight to just one battery to get 12 volts, does anyone know if the 'hockey puck' sized round metal box near your fuses will be sufficient to power a fuel pump.

The box is a resistor of sorts and is 12 volts on the output side. I'm wondering why I haven't heard of anyone using this for a 12 volt item? Or have they?

I know a 24 volt fuel pump would have been easier but I saved $ by getting the 12 volt. My plan was to just use 1 battery and after the pump is brought up to pressure, I figured it would shut down...IT DOESN"T. The little motor continues to vibrate at a slow but steady rate. I need this hooked up to the ignition. The box described above is controlled by the key. Any thoughts?

OK so I hooked up a 12 volt fuel pump to the line running to the distrubutor from the resistor. It tested 12 volts and so far there is no problems running the pump. It seems this was too easy to find a 12 volt component to tap off of on a 24 volt system. Why would you need to reduce the 24 volts to 12 for just one component? I have the radio and CB still hooked up to just one battery.

I haven't torn apart the 'magic hockey puck' yet but the Mercedes diagrams I have seen describe the magic puck as a voltage regulator. Depending on how Mercedes went about designing this regulator it may be just fine... or you may have a longevity Issue (usually accompanied by a ozone/burning smell). Just to be on the safe side I would use a separate block loading resistor to run your fuel pump. My guess is that a 2 Amp 6 Ohm block resistor would be about right. If you give me a Current draw reading from the pump off of a 12V source I can tell you the exact resistor size to use.

Note... the radio and CB are dynamic loads (current draw changes) so a simple loading resistor would not work for them.

Another option to give you a ignition controlled 12V source would be to make your tap between batteries as you would normally (with a fuse I hope:)) and use a relay controlled from the 24V Ignition.
to do this using an ordinary Bosch or 'Bosch style' (Tyco, potter and brumfield, delco, nashua...)

Most of these relays have a pin-out printed on them and all their pins are generally labeled on the bottom by each respective pin.
Pin............connection
85.............24V ign
86.............80-100 Ohm resistor to ground
30.............your devise here (fuel pump)
87.............Power tap between batteries (after fuse)
87A...........N/C (:grinpimp:'unless you knows whats yous doin, dont play here!)

Yeah inside the hockey puck is a round ceramic disc wrapped by a spring (resistor). Pretty simple. So you think I should hook up a block resistor before where I tapped the circuit? Refresh my memory how exactly do I find the current draw reading for the fuel pump? If this helps with longevity of the pump, I'm good with that!

Ok... I would not worry about the longevity of the pump so much as the longevity of the resistor(regulator) which has a specific value based on the load of our gauges. So... the resistor is designed to provide a specific voltage drop relative to the load, meaning that changing the load will change the output voltage resulting in an inaccurate gauge reading. Also the change in load may exceed the limit of the resistor (burn it out).
Now... not knowing the ampacity of the resistor I don't know if you will experience the latter problem but to be on the safe side I would use a seperate block resistor for your fuel pump.:homer:

To take a current draw reading set you digital multimeter to Amperage. Then ground your pump to your 12V test power supply. Next connect the ground (black) terminal of your meter to the positive wire for the pump (this wire is usually red) then connect the positive terminal of you meter to the positive terminal of the test supply. The pump should start running and you will get a reading on your multimeter. ;)

Note... some multimeters have a separate positive terminal position for taking current readings; If yours does you will need to make sure you are connected to this port.

to calculate resistance take voltage (12) and divide it by amperage (A).
12/A=I
I is the impedance of the resistor/device. So your load resistor will have an Impedance of 12/A and a Ampacity (current rating) of A.:grinpimp:

Or you can just tell me what you get and I will tell you what resistance to use.:flipoff2:

Or, you could go to a Napa and get a 24v electric pump...hehe I did that on one of my older trucks as a backup and it worked great.

I have never actually researched a 24v fuel pump from Napa for a carbureted vehicle but what I had understood was that the pump available (from Napa) required a fuel pressure regulator.
Do you have a part number for the one you purchased?:homer:

From what I understood, the only Napa 24 volt pump was too much pressure thus needing a regulator. The Napa 24 volt pump plus the needed regulator is more work plus MORE money, which I have a shortage of... I'm almost sure running the 24 volt pump/regultor was over triple the price of the cheap 12 volt pump running the already perfect pressure.

That was kinda what I was alluding to.