Ok I know gas gauge problems have been talked over for ever . I have tried the search feature and have not gotten the information I need . The truck is a 56 F250 , stock gauge , aftermarket wire harness , Bob Drake sending unit sold by Midfifty . When I turn on the ignition the gauge goes to full . I have checked the gauge out of the tank , 10 ohms full , 94 ohms empty . Gauge has 12 volts going to it . checked wire from sending unit to gauge , 0 ohms when ohm meter probes hooked to each end , There is no voltage regulator for the gauges , can't find one . Am told 56 was first year for 12 volts going to gauges , also none shown in wire diagram . If gauge was grounding out through the dash , gauge would show full with sending unit wire unhooked , It does not . Now I have read in my search on this site that the ohm reading on the sending unit should be 10 ohms empty 73 full . That is opposite of what Midfifty is telling me . At this point , I have hit the brick wall . So tell me what I need to do .
THATS one of the problems , The information I am getting is all over the place . who do you believe . I have a 56 car shop manual and it said if you put 3 volts to the gauge it would read 1/2 full , 4 volts will show 3/4 full . at that rate 12 volts would show full twice . I checked three other vendors and they all show 56 sending units as a stand alone item .
You probably found this too, but it confirms that '56 gauges and sending units are unique.
"In 56 when Ford went to 12 volt systems the same pulse sender design was used but redesigned to work on 12 volts. In 57 for some reason Ford went to 6 volt gauges again and began using the constant voltage regulators (CVR) to drop the voltage for the instruments. Starting in 57 all sending units were changed to the resistance type.
So 56 gauges are very unique being 12 volt and not generally compatible with other years."
It gets more confusing , I called Bob Drake , as the sending unit I bought from mid fifty is a bob drake with part number 99a-9275 . Their on line catalog shows this fits 35-56 ford trucks , but if you go to the 56 site they have a sending unit as part number B6C=9300 . The person I talked to had know Ideal why they had 2 different part numbers or if they really where different . Does anyone know for sure what the empty and full ohm readings should be ?
Ok Dave, I just went out and checked the sending unit on Missy Green. The wiring and gauges are original AFAIK. Last time out I filled the tank and it has probably a gallon burned out. My reading with a good Fluke meter was erratic, between 150 and 90 ohms. I believe there is a heater strip in the float itself that the meter battery is heating, thus the fluctuation. BTW, wire to the gauge was disconnected for the reading. Frustrating, ain't it.
Edit: And as I told you in our PMs, my gauge is no longer accurate. Gauge is reading 1/4 tank with those readings.
Ray my sending unit reads 94 at empty , your seems to be 90s at full . Ross the wiring is the same ,but 56 was a 12 volt gauge , in 57 they went back to 6 volt . so I don't know if the sending unite is different for 56 , All the vendors show a 56 only unit but nobody can tell me what the difference is .
I believe there is a heater strip in the float itself that the meter battery is heating, thus the fluctuation.
For those having issues with your stock fuel gauges and new tanks, it's because there are two different "languages" being spoken. Most modern fuel gauges and senders are simply paired ohm meters and variable resistors; the gauge measures the resistance of the sending unit circuit which changes with the fuel level.
However, our stock Ford trucks used a different language, named The King-Sealy principle. These sending units supply a full 6V through a heater wire in the gauge in the form of a square wave with the duration of the "on" being the variable determining the gauge reading. The longer the duration, the more fuel in the tank. It is this full 6v of current which heats the bi-metallic strip in the stock gauge that causes the gauge to read via a small gear train. Most (if not all) after market sending units are simply a variable resistor which sends a constant closed circuit of varying resitance to the gauge. Therefore, the sending units signal doesn't heat the bimetallic strip in the gauge as much making the calibration of the after-market sending units and the stock gauges nearly impossible to match up very well due to the differences in the principle of the signal.
Modern gauges are easy to pair as long as the sending units full and empty ohm readings match that of the modern gauge. Some senders and gauges are even highly adjustable in this regard.
I should note that my knowledge goes for the stock 48-52 trucks, but I think the next generation, minimally through 55, used 6v gauges with resistors.
So, can we conclude that reading the resistance of the sending unit accurately is not possible with a VTOM due to the heating coil?
Affirmative - the correct proceedure for testing the stock sender is testing against a known good gauge. Expanding on that idea leads to this: For example, if you have a funny oil pressure reading, test your oil pressure gauge. If the gauge passes the test, the sender is suspect (provided your wiring has not deterioated).
All 48-52 stock gauges (fuel, temp, oil pressure) can be tested with 2 D Cell batteries. 1.5 volts does 1/2 swing, 3 volts full swing.
Originally Posted by raytasch
Anyone else notice the 6 volt battery is wrong polarity in the schematic shown above?
The image ALBUQ F-1 posted is for 48-52 trucks which were positive ground. Polarity does not matter for the King-Sealy gauges.