What temp sender should I use?

R

runmikeyrun

Senior Member
Hi guys,

I have a 42 Ford truck that I have the gauges wired up, but I have an early 60s Y block motor in it. I’m trying to get the proper temp reading on the gauge, however it’s reading full cold as soon as I turn the ignition on.

The sending unit is unknown, but almost certainly not the one that came with the original flathead. It reads 2 ohms cold.

I’m thinking the gauge is only going to read correctly if I use a 42 sending unit, correct? I’ve read up on the bimetal sending units etc. Would just like to confirm before I order one.

Thanks!


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The 42 temperature gauge should be on the H side without power and as soon as the ignition is turned on it should go to the C on the gauge and as the engine warms up the needle will go to the N. And you should use the original 42 sending unit for the best reading. The gauge should read around 10 ohms without power that is when the points are closed but the gauge does not work on resistance.
Here is a picture of the cluster without power. 60 MPH and 100 MPH.
BILL WELLS
 

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Thanks Bill! My gauge does indeed look like that without power, and when turned on it goes all the way to cold. However, it never moves off cold. I will look for the correct sending unit. Will the threads be the same size as other sending units? Reason I ask is the flathead has been replaced by a Y block and not sure if the threads on the block are the same.

Thanks again.


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Bill, Which Flathead sending unit should he be using? The one with the single terminal, or the one with the 2 terminals?
 
Should be using one terminal sender V8's up to 1953 I think 6ylinder is the same as
well as Lincoln up to 1951. I have 1949 F8 with Lincoln and it has only one sender unlike
its small brother V8 that used the two and one terminal. How that works in normal
operation the one terminal sender signals the dash gauge. The two terminal is only a
switch - if the engine temp gets too hot this switch opens and the dash gauge goes to
hot just like it does when ignition is off. Then it cools down the two terminal closes and
the circuit is complete Through out my life I find the temp gauge was always a problem.
 
The single terminal is the one to use if it will fit. the dual terminal unit is just a temperature sensitive switch that allows the indicator to go to full hot when the bi-metallic switch closes at overtemp.


Your next best choice is to use a temp guage from a model that used the same temp sender as engine used in the late 50s to early 60s. This would require modification of the gauge cluster.


Most of the flatheads used pulse type senders for oil psi, fuel quantity, and coolant temperature. The gauges are not a lot different than the later units since they all use a heater coil on a bi-metallic strip to operate the indicator but the indicator doesn't function with a variable resistor. It functions by a variable voltage & current input so the calibration is completely different. Later units of the late 50s & 60s use resistance senders with a voltage regulator to keep the input voltage lower for less heat generation.
 
rotorwrench said:
Most of the flatheads used pulse type senders for oil psi, fuel quantity, and coolant temperature. The gauges are not a lot different than the later units since they all use a heater coil on a bi-metallic strip to operate the indicator but the indicator doesn't function with a variable resistor. It functions by a variable voltage & current input so the calibration is completely different. Later units of the late 50s & 60s use resistance senders with a voltage regulator to keep the input voltage lower for less heat generation.
Click to expand...


Makes sense. I used a modern replacement oil pressure sending unit that was recommended on another post somewhere and it works fine.

Sounds like the original flathead’s sender is giving the dash voltage and current on a logarithmic scale and not linear like modern resistance based sending unit.


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The senders have a bi-metallic strip with a heater coil and a little set of points to open an close the circuit depending on how much deflection is going on with either the diaphragm (fuel qt & oil psi) or temperature on the bi-metallic strip (coolant temp). If you do a search on this forum for King Seeley, you can find cut away diagrams of some of the system components that show how the principle works. The systems were relatively well sealed from the elements and fuel fumes and the like. This makes them last a lot longer than the rheostat type senders used in later eras. They would be expensive to manufacture them like K-S did.
 
Agree with all above use of one-terminal OE sender remarks. My OE two-terminal sender sensor misbehaves sending my '47 stock gauge to "hot" during warm-up so I by-passed it. After adding a good quality aftermarket mechanical gauge it showed that the OE gauge and sender are pretty accurate.

Re thread size match-up '42 to a Y Block ? I have no experience w/that but options might be (1) if OE Y Block threaded hole smaller, redrill and rethread hole to receive '42 sender .... use a magnet to gather metal shavings during drilling/threading, (2) if Y Block threaded hole larger, try to find a short-length adapter that will still allow the sensor bulb to reach into coolant, (3) plug the existing sensor hole in the Y Block and use the once-popular Tee-in-radiator-hose adapter fitting in the upper radiator hose where it attaches to the OE Y block thermostat housing, or (4) last resort: plug the Y Block hole and have your radiator guy install a threaded fitting for your '42 sensor in the top-tank of you radiator .
 
I am guessing that the threads might be different so you may have to use an adapter and the one terminal sender would be the one to use as others have said, also check that you have a good ground on the sending unit when screwed in the block, some tend to use a sealer and causes a ground problem.
BILL
 
Thanks a lot everyone! I appreciate the input. I will have a look at the block today and figure out what size the threads are and go from there.


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About sealing threads. A friend of mine who is an EE gave me some very thin non-adhesive copper tape about an inch wide that is perfect for sealing these. I don't know where he got it, but it's out there and it's perfect for these. I can ask if anybody cares.
 
I use a thick plumbing paste that still allows good contact but doesn’t end up inside the engine.

For the heck of it, I wired a potentiometer in series with the sending unit to see if there was any way to make it work. As you guys said (and I assumed) it didn’t work, it needs the on/off oscillation of the bimetal sender. At one point in the pot’s travel it went full Hot and then slightly backing off it went back to full cold.


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If a person leaves a several threads clean on the tip of the NPT threaded fitting, it will seal around the outer portion of the thread better and leave plenty of metal to metal contacts. An NPT fit is like a wedge so it it tighter than most thread fits. The bore of the female thread portion needs to be good & clean too.
 
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