Gas Tank Sender Unit - Got lucky

[Drbrown]

Have a '47 for abt 2 years. Gas gauge never showed empty, always pointed to just below half when tank was FULL and the pointer always rose UPWARDS as the gas got used !!! This is a 12v system with volt reducer at gauge.

Bought new MidWest sender unit (BD Box) in March. Today reviewed threads re trouble-shooting gauges, sending units and adapting BD units. Leaving existing sender unit in-place, momentarily grounded wire coming from gauge, to frame, and "wa-la" the gauge pointer first dipped to empty then rose to near-full as it should have !!! Removed much dirt around the sender unit and what may be an original sender w/Part # 99A-9275. The two-piece cork float was gas-logged - attached a string to the cork and it sank in gas. Sender has tabs that limit the maximum amount of float-arm movement. Sender body is finished with a galvanized looking finish in good condition. Moving the sender arm up and down, the gauge reads correctly. Tested sender: 10 ohms at "full"; 0 ohms at "empty" (had to use analog tester; digital went crazy). Could not get top off of unit so flooded it with electronic spray cleaner and let dry (didn't seem to make any apparent difference in gauge reading). Replaced cork float with BD's snap-in-place plastic bulb (exist arm in good cond); trimmed/used BD gasket. Reinstalled unit and works fine so far; will see as I use gas up.

I expected the main problem was at the gauge. Got lucky.

Photo #1: Tank sender hole.
Photo #2: On left, top-side existing/reused sender with new float bulb; On right, unused BD sender & arm; Center bottom old gas-saturated 2 pce cork float.
Photo#3: Under-side of existing/reused sender unit showing part #.

[40cpe]

The spec on the sending unit is about 10 ohms full and about 73 on empty.

[JSeery]

Is the existing sender unit an original one? If so, it is not a resistance unit, it works with points. The replacement units are resistance devices.

[Drbrown]

40cpe: 73 ohms empty ? That's strange. Both my digital and analog meters said "0" ohms with float at bottom (empty) position. Could the difference from the spec be because I have negative ground system instead of the stock positive ground ? At least the gauge reads correctly now.

Jseery: After some 68 years I doubt the sender unit is original. There was a fair amount of dirt in the recess on the tank-top where the sender is located, so its been in there for some time. As the sender may relate to the tank's origin, outside and from what I can see inside, the tank looks to be clean and rust-free. I don't have a gas line filter and the glass bowl on the mechanical pump does not show any sediment. As well, I was told by previous owner that the tank had been replaced. Based on threads I read, I avoided installing the BD (resistance-type) replacement I had bought because of the issues discussed about trying to adapt resistance units; many threads recommended to just replace the float if possible.

[JSeery]

It will be "0" or no reading, LOL. Points are either "open" or "closed", so zero or nothing. The 10 - 73 numbers refer to the resistance units. This is assuming you have an original style sender. What does it read in the full position?

[Drbrown]

Sponsored Links (Register now to hide all advertisements)

Jseery: To confirm, the high and low movement of the float arm is limited by metal tabs on the sending unit.

Both the analog and digital meters read "10" with the float arm fully raised to the top (full tank level).

As I slowly lowered the float arm, that reading decreased until it reached "0" at the bottom (as you say - no resistance detected).

The only other thing I noticed was that at the underside of the sending unit housing, the upper end of float arm rotated and seemed to act like a cam, rotating another short lever that extends down from the housing in a sideways motion.

[JSeery]

Sounds like an original style sending unit.

[terry]

I'm wondering if the cork float is original for your year. Maybe the cork one was a quick solution for a problem of the previous owner and it was never coated. When corks were used in earlier years, they were given a varnish coating if I remember right. Terry

[Drbrown]

Jseery: Top looks same as your pic's but the bottom of the round can-shaped extension underneath is completely open - see my third pic.

terry: I doubt after 68 years this sender unit is the original. Someone could have installed an NOS replacement at any point in time since, or as you said replaced the cork. The cork was coated with a varnish-looking substance but then the wire arm was pushed thru the cork to hold it, creating a point for the gas to seep in. Regardless, after all those years sitting is gas, AND especially after being exposed to ethanol gas, the coating developed weak spots and gas soaked in. It's fortunate the metal parts and workings are in good condition.

[Jack E/NJ]

Originally Posted by JSeery

Sounds like an original style sending unit.

Do you have a narrative that describe how that system works. Seems like the points must vibrate or cycle on-off at different frequencies at full, intermediate and empty tank positions. Thanks.

Jack E/NJ

[JSeery]

Originally Posted by Jack E/NJ

Do you have a narrative that describe how that system works. Seems like the points must vibrate or cycle on-off at different frequencies at full, intermediate and empty tank positions. Thanks.

Jack E/NJ

That is how they work. All of the early Ford gauges worked in a similar manner, they were current devices. The current went from the gauge through the sending unit (now there is a miss-name) to ground. There are bi-metal strips is both the gauge and the sender, the current through the sender heats up the bi-metal strip and causes it to move (flex) which causes the points to open. As the strip cools the points close and current flows again. The points open and close (vibrate) and the rate of this opening and closing controls the current flow through the gauge and it's bi-metal strip moves to match the sending unit (this is related to the average current flowing through them). So, the gauge follows the sender and displays the result.

Different senders use different means to read various engine conditions, ie. fuel level, oil pressure, temperature. A cam arrangement is used in some to apply pressure to the bi-metal strip to change the current required to open and close the points. Or the bi-metal strip itself is moved by engine heat, ect.

So, the vibration of the points in the sender controls the average current in the system and the average current is what is being displayed by the gauge.

Note: Jack, were you interested in a particular gauge or just how they work in general? There is an operational description of the fuel gauge system in the shop manuals if you are interested I can repeat that.

[Jack E/NJ]

Originally Posted by JSeery >>>Jack, were you interested in a particular gauge or just how they work in general?>>>

Mainly the principle, thanks. Though I seem to recall the temp sender system might've been produced in two different flavors. One with one sending unit. The other with two units --- possibly for overheating warning. What about those? Thanks again.

Jack E/NJ

[JSeery]

Yep, Here is a drawing of the temp system and just as well throw in the oil system while I'm at it.

[Jack E/NJ]

Thanks. So I guess that double-terminal sender is mainly there to get the driver's attention by pegging the temp guage if it gets too hot. And with a bit of modification, I suppose that pegged-gauge could even be made to switch on an idiot light. 8^)

Jack E/NJ

[JSeery]

Because of the design of the flathead cooling system it is possible to have an overheat issue on one side and the other side show normal. Normally both sides are somewhere in the same range (they do run different). This system allows you to guess-a-mate the engine temperature and still have a backup if the other side starts overheating. I think the on/off sender is set somewhere in the 206 degree range. These original gauges are not accurate, if you are wanting more accurate readings you need to install quality aftermarket gauges.