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I have a 52 F1 and I installed a brand new 67mustang gas tank along with the sender for the same tank. I've converted the truck over to 12v but didn't convert the gauges, other than bulbs. My truck ran out of gas and the gauge shows 1/2 tank. Anybody else have this issue or know of a fix, besides filling up at 1/2 tank?
I have a 52 F1 and I installed a brand new 67mustang gas tank along with the sender for the same tank. I've converted the truck over to 12v but didn't convert the gauges, other than bulbs. My truck ran out of gas and the gauge shows 1/2 tank. Anybody else have this issue or know of a fix, besides filling up at 1/2 tank?
I am shocked that my F100 teachers have missed this so I am going to give you a bump to page one. I think it's a matter of the difference in the compatibility range between the sending unit and your gauge more than 6 vs. 12volt. I could be wrong. Hopefully someone who knows will catch his the second time around.
Several things come to mind, I'm not sure what ohms the factory gauge worked at Vs. the sender you are using. It should not be too difficult to research that on your own if someone here doesn't chime in soon. I do know there is a voltage converter available to drop the gauge voltage back to 6v if you don't already have one.
The easiest way to handle the situation is to disconnect the wire to the sender and insert a variable resistor in its place. Tie the wiper connection of the resistor to the wire from the gauge and one end of the resistor to ground. Turn on the key and read the gas gauge. Turn the variable resistor until the gauge reads "E". Use an ohmmeter and read the resistance across the variable resistor. Insert the resistor back into the system and turn it until the fuel gauge reads "F", once again read the resistance across the variable resistor. Now you have the range, in ohms, that the gauge expects to see from the fuel tank sender. If the sender that you currently have installed is not in this range then find one that is and your problem is solved. As far as 6v vs 12v, there is no difference when it comes to ohms. Use the values you measure and get the appropriate sender.
It actually does matter, 6v vs 12v, because the stock gauge uses a heater to move the needle. The stock sender doesn't send a continuous voltage to the gauge, it makes-and-breaks. So comparing to stock sender resistance won't help. Do as Charlie says with a variable resistor. I wouldn't expect it to be real accurate, stock gauges on 6v weren't.
That gauge is a hokey setup...get rid of it and get a modern gauge with a d'Arsonval movement. Either that or put a turn signal flasher in the circuit.
OK, here is a diagram...not as clean as the one that Ross posted but basically the same except for the fuel tank circuit. Nothing needs to be done with respect to the supply voltage to the gauge...that is all taken care of with the change in resistance within the fuel sender unit. This is a very simple circuit and if you can connect a variable resistor in place of the fuel tank sender then you can determine the resistance range you need in the sender unit.
There are good nuggets in all of these and when pieced together, you have the full picture.
As Ross points out, the original setup was made by King-Seeley and used a bi-metallic trip and heater elements to make/break a full 6V to the circuit. It was a pretty stable needle and reasonably accurate.
Converting to 12V must be addressed.
Perhaps the simplest is to leave it all as-is and simply put in a 6V CVR (constant voltage regulator). There are several designs - Ford themselves used a chopper-style and these are available at the junk yard or new from NAPA. They tend to be about $30+ and will run all the gauges when you convert to 12V. Runtz used to make dropping resistors, but now theirs are electronic - although I've never held one in my hand. They're also pricey. A 6V Zener diode will work, but it will be a bit crude. Personally, I prefer the L7806 CVR available for less than a $0.50 at dozens of places. While one would run all your gauges, I prefer to run one to each gauge....they're cheap enough. The advantage with these is they actually make your gauges more accurate by providing a steady-state 6V regardless of whether the alternator is putting out 14.7V or 11.6 on your battery-only - the gauges always get 6V.
Modern sending units can be used with the KS gauge. All you're doing by varying the resistance is controlling the current through the gauge unit and bi-metallic strip - the same effect the on/off heat/bend-make/break contact original KS sender did. Current is heat and you can turn on/off a full 6V to effect an average heat value, or you can control the resistance to maintain a steady state heat value (current).
Finally, what CharlieLed said is accurate. The trick might be finding the correct sending unit. You can still control the heat (current) through the gauge regardless of whether or not it's 6V or 12V using variable resistance. Most of the aftermarket sending units I"m aware of are made to work with 12V, but the gauges being converted with a 6V regulator.
You can figure it out as described by CharlieLed in his first post and then go look for a sender. Or find a sender that is the correct range, but lower overall. In other words if the range between full and empty is 130-240 and your sending unit is 30-140, you can insert a 100ohm resister in series because resistance, in this instance, is cumulative. This too could prove tricky to find the right combo.
The easiest is to convert a stock setup using a 6V CVR - less thinking, figuring, and experimenting - and if electrics aren't your strong point, this may be easier. But it's not as intimidating as it sounds...
I think the issue may be that the Mustang sender has the opposite range from Full to Empty -- i.e., low resistance at Full vs high resistance at Full with the stock. I have to wonder if the Mustang gauge can be adapted to the stock cluster? "Julie" did that on here a long time ago, so did Tip49. https://www.ford-trucks.com/forums/9...ml#post9198161
As Ross points out, the original setup was made by King-Seeley and used a bi-metallic trip and heater elements to make/break a full 6V to the circuit.
Please explain your comment regarding "make/break"...I see no contacts in the schematic. There is always a "full 6V to the circuit". What makes the circuit work is that there is a resistor network that varies the current drop to the gauge.
The insulated grounded contact in the sender makes/breaks with the contact on the bimetallic arm.
To what end? From the way see the circuit the "insulated contact" is merely a wiper arm that adds/subtracts resistance in the heating coil circuit...no opening or closing here. From the standpoint of the gauge, the rest of the circuit is a resistor network...as the fuel level changes the resistance in the network varies. As the resistance varies the voltage drop across the heating coil in the gauge goes up/down, the corresponding heat generated causes the bi-metal to react and move the gauge needle.
There is no wiper. Current flows thru the heating elements, which causes the bimetallic strip to move away from the contact. With a high fuel level, the contact is closer to the strip, so the strip has to get hotter before it will break contact. Longer "on" period means the heating element in the gauge gets hotter, moves the needle further towards "F".
Thanks Ross for the excerpt, I would have never gotten that from the schematic. What an arcane way of implementing a rather simple concept, Rube Goldberg would have been proud. And I thought that the old Sun tach senders were crazy!
To the OP...get rid of this gauge and fit a "modern" gauge in its place that will work with a "modern" sender. I could design a circuit to emulate the OEM sender but why bother when you should be able to find a replacement gauge.
