QuestionQUESTION: I have a 6 volt system and the head is from a 57. The temperature sending unit in the head is a 12 volt sending unit. Is there anyway to make this work? Possibly a resistor.
ANSWER: The sending units for a 55 might fit in the same hole - why don't you just get the right sender for it?
If you cannot find one from the auto parts store (It is Ford part number 1A-10884 and fits just about all Fords, Lincolns and Mercurys from 1948 to 1955), try C&G Ford parts in Escondido CA - I think they will have it.
Adding a resistor isn't going to work - you might find a value that would make it read right at one place on the gauge, but it wouldn't work over the whole temperature range.
Dick
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QUESTION: My problem is the hole is not the same size the sending unit on the 55 is very much larger than the sending unit on the 57. I have this small opeining with a 12 volt sending unit on a car that is a 6 volt system and a 6 volt gauge.
ANSWER: Yes, I understood the nature of the problem, but I was not aware of the size difference.
If you have access to a correct 55 sending unit, there is a way you can figure out what to do, but it takes two instruments: One, you need a thermometer that can read fluid temperature in the range of 100 to 220 degrees - that is easy, you can buy a "candy thermometer" at any housewares department of a super market - it looks like a giant fever thermometer - they are cheap and very accurate. The second instrument you need is an Ohm meter. If you have one or can borrow one, make the following measurements:
Measure the resistance of the sending unit from the electrical terminal to the outer threaded portion at 3 temperatures: Cold (tap water), 150 degrees, and 212 degrees (boiling). Do this by putting a pot of cold water on the stove and heating it up, making the resistance measurement at each temperature. Do this test on the 55 sending unit and on the 57 sending unit. If the numbers are the same within 30% or so, just go ahead and connect the 57 sending unit to your dash gauge as is - the reading will be close enough that you can use it as is.
If you wish, measure the temperature of your cooling fluid in the top of your radiator when the car is at full operating temperature, and make a note of where it shows on the dial - that will keep you safe if you use that as a reference reading as you drive the car.
If the resistances are more different than that, post a follow up to me and I'll try to come up with a compensating circuit idea for you.
There is a fair chance the readings will be close enough that the 57 sender will work just fine, as the 57s have a voltage reducer in the gauge circuit that sets the gauge voltage near enough to 6 volt that the difference in reading will be well within normal gauge tolerance.
I hope this is helpful to you,
Dick
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QUESTION: Thanks Dick, one problem I have is the guage barely moves now with the 57 sending unit. It actually registers H when cold. After I start the car it goes to C and stays there. I do not believe comparing the two is going to do anything for me because it appears the 57 sending unit does not function properly with the 6 volt system.
AnswerStriking out here! The problem is, the later Ford head uses a 1/4X18 pipe thread sender, and those are not used on any car I can find that takes a reverse curve resistance/temp sender. The early Ford type which was used in your car is very unusual, in that it goes from low resistance when cold (I don't have the exact number, but it is probably about 10 ohms) to a high resistance when hot (probably around 75 Ohms). Since the books don't show the resistance for the early senders, I am only guessing. If you have one, you can measure it at two temperatures - cold and boiling, and I can probably give you the best choice resistance to put in the one I recommend you try below.
I have looked at all Temp senders from NAPA - (about 150 of them) and there are only two that have the reverse curve that you need, and they are both 3/8 pipe thread senders. For what it's worth, they are TS 5004 and TS 5008, and neither of those will fit into your head. You could get an adaptor at the parts store that would accept the male end of the 3/8 pipe sender, and step it down to the 1/4 pipe male end that you need to thread into your head. If you do this, pick the TS-5004, because the resistance range is closer to what you need. You may still have to hide a 100 ohm or so resistor somewhere - not in series with the sender wire, but rather from the sender wire to ground, because the gauge will read a bit high if you don't do that. You'll have to experiment a bit to get the exact right reading on your gauge, but 100 Ohms is a good starting point. If you can measure one of the older senders for me at cold and hot temps, I can make a better choice.
If you do not have one of the older senders, you can get the same information by doing the following: Get a 250 Ohm potentiometer from Radio Shack, and connect the center terminal to the wire that connects to your dash gauge. Connect one of the other terminals to ground, preset the potentiometer to the center of it's range, and then turn on the key. Be careful not to move the potentiomter all the way to either end, just move it in very small increments until you have adjusted the potentiometer so that the dash gauge reads right in the middle of the normal temperature range, disconnect the wires and measure the resistance which you have set the potentiometer to. The gauge will react slowly to the resistance changes, be sure to let it stabilize before you move the potentiometer further. Avoid going to either extreme of the potentiomter, because it is possible to damage the gauge if you run too much current through it.
This test will give us the right resistance for the sender and resistor for normal temps. We'll have to settle for what the gauge reads at other temperatures, but at least it will be right at normal operating temperature. It will probably have significant error at other temps, but I don't see any way around that (other than finding the right head for your engine!).
I know this is long and complicated, but that is the situation we have here - that is why I usually refuse to help if an engine has been modified! If I hadn't had to solve this same situation myself once, I'd have just ducked this one!
I hope this is helpful, and that you are able to provide the info we need to finish up this conversion.
Dick
