6 volt fuel tank sending unit

[Guest imported_mossiyjack]

Can someone tell me what the ohm value is for a tank sending unit in a '35 Dodge DU. Or if anyone has one that needs a new home or is a dipstick my final option?

Thanks,

Just JACK

[Rusty_OToole]

If you can't find a Dodge sending unit you might consider a Ford.

Ford antique suppliers sell a sender that fits from the 30s right up to the last 6 volt models. It is quite reasonable in price.

I know of one guy who modified one to fit a 6 volt Chrysler product. He said it has the same resistance value as the Chrysler products. He had to modify it to fit his tank but it wasn't a big deal.

[Guest simplyconnected]

My Ford 6-volt sending unit has contacts with a bi-metal strip. It is impossible to say any ohm value because it makes and breaks with current. More gas, pushes the contacts tighter. Empty tank means contacts are totally apart.

I hope this doesn't confuse the situation. I couldn't believe Ford would put a contact so close to gas. I don't see any char witness marks, so this original piece must have been ok all that time. The current limiting fuse is funky, but that's no biggie. - Dave Dare

[JFranklin]

they can be rebuilt, google can find you a rebuilder

[Guest elmo39]

also the sending unit is not voltage consious (hope i've spelt that right) 6V ,12V no difference

[Guest simplyconnected]

Power starts at the gas gauge, then goes to the float (sending unit) where it 'sinks' various cycles of ground. Current heats the bi-metal strip, and opens the grounding contacts. As the fuel level goes down, less mechanical pressure is pushing the contacts together, allowing the contacts to remain open longer than closed. Fully grounded shows as 'full,' and open contacts show as 'empty.' on the gauge.

Notice the snail cam in the picture, for initial adjustment. Temperature differences play hell with these old units. You can imagine, in very cold mornings, the gauge will show more gas than usual, and very hot days will have an opposite gauge reading. Modern Fords still use the very same 6-volt gauges, with a voltage regulator. If you need a reg., get it from any Mustang dash.

New sending units are rheostats, 10-100Ω range. I like that arrangement much better, and they work with the old gauges. I'm still not comfy with the old way of opening and closing contacts around gas fumes.

As you may guess, I am going with the new rheostat. They come cheap, and they are usually housed in stainless steel. I bought a new one, on sale, for $15. It has a brass float, and is compatable with any type fuel. - Dave Dare

[Guest DodgeKCL]

Very interesting. But I'm still a bit confused. If closed contacts are going to allow current to flow and open contacts will stop the current,what holds the charge in the inbetween times to hold the needle on the gauge at the required reading? Is there a cap across the gauge or do the points pulse so rapidly the needle averages out the pulses and stays put according to where the current flow leads it? This is the first gas tank sending unit I've come across that didn't use a rheostat to simply vary the current flow to ground as the float moved up and down.

[Guest simplyconnected]

The fuel gage is spring-loaded to 'rest' on 'E,' under no power. The float arm is a cam. When the tank is actually empty, the cam relaxes pressure on that electrical contact, from the bottom. When full, the cam exerts full pressure, keeping the contact closed all the time.

Any time current is flowing through the contact, it heats the bi-metal strip, which tries to open the contact. If the tank is half full, the contact opens and closes in equal time increments, causing the gauge to read 'half.' Remember, all the current going through the float unit also goes through the gauge (they are wired in series). These gauges are s-l-o-w to respond, and they average the on/off time. There is one very small cap at the sending unit for spikes.

So, with a slow gauge, a few things happen. Going over hills and hard corners, might not show any deflection. Filling the tank won't 'register' for a little while. The system averages contact-on/off time which turns it into an average current reading just like the voltage regulator for your charging system.

I won't get into mechanical regulators, but I often wondered how they stopped the battery from over-charging. Simply put, when battery voltage goes past 7-volts, a relay energizes, opening generator field current. As power is used, that contact closes and opens as needed, and it seems like voltage is smoothly held at that level. - Dave