The Stovebolt Site Forums The Shop Area The Electrical Bay gas guage with reducer

i have a reducer from one of the online parts stores.it is the light duty one. has bare wires on each end . i have two questions .1. when connecting it to my gas guage how do i prevent the bare wires from shorting to the body? 2. if the gas guage just measures resistance changes why do i need it at all? there have been so many posts about this i not sure just who to believe. could someone settle this once and for all

Cliff,

I bought my '49 pickup when I was an undergraduate electrical engineering student umpteen years ago. As a project for a measurements class, I setup a '49 fuel gauge and sender and then measured the current into it as a function of the voltage applied.

The system (gauge + sender) is designed to read correctly independent of the voltage applied to it. If it wasn't, then the gauge would be moving everytime you revved the engine and the generator voltage went up.

The only question then is how much voltage can be applied before it does burn out the coils. I can't remember the exact numbers from my test but the conclusion was that the system didn't draw enough power at 16 or 18 volts to even worry about.

The summer after that class, I rewired my '49 and converted to 12 volts. I ran 12 volts through the '49 gauge without problems for over 20 years. I did have one fuel gauge problem. The sender in the tank finally broke (mechanically) after 45 plus years and I had to replace it.

I've posted this here before and lots of folks have horror stories about how their gauge fried on 12v etc.. If you hook it up wrong, even at 6 volts, it'll cook in a heartbeat. That's why the divider between the terminals on the back of the gauge.

I'll have another look for the paper I wrote for the class, it has all the readings, graphs and conclusions on it.

I was curious about just what was the difference between the 6v and 12v gauges so I went out and did a little measuring just now.

Since the fuel sending unit is simply a resistor that varies between 0 and 30 ohms on Chevy's we'll assume that the sender is at the 0 ohm end and then we can figure the power (heat) generated in the gauge itself.

'38 truck fuel gauge - 30 ohms between the supply terminal and sender terminal

'64 Chevelle fuel gauge - 47 ohms between the supply terminal and sender terminal

Using Ohms law Power (watts) = (Voltage * Voltage)/resistance (ohms)

'38 gauge at 7 volts = ( 7 * 7)/30 = 1.63 Watts
'38 gauge at 14 volts = (14 * 14)/30 = 6.5 Watts
'64 gauge at 14 volts = (14 * 14)/47 = 4.17 Watts

That's the numbers. I ran my '49 without a dropping resistor and plan on doing my '38 the same.

many thanks for all that useful info...

Cliff,
Why not simply install a Runtz reducer? I'm sure you must have read about it after doing a search? I'd be happy to show you what they look like, assuming I can find my spare. Installion is pretty straight forward.