Ford 3000 Temperature Gauge / Sending Unit

NCMichael

Member
I have a 1974 Ford 3000 (diesel) that after a few minutes of running the temperature gauge always reads in the red and beyond the max scale. I have just replaced the water pump, thermostat (192 deg F) and radiator. I have also tried two different temperature sending units both with the same results. The tractor runs at expected temperatures based on IR thermometer readings. The fuel gauge works properly and I feel I don't have any electrical issues like voltage stabilizer or regulator problems. The temp. gauge responds to differences in resistance but I am thinking either there is something wrong with the gauge or it does not match my temperature sending units. Here are some resistance readings from the sending unit to battery negative terminal. Tractor cold - 600 ohms and gauge on full cold. Gauge at lower end of red - 42 ohms. Gauge at upper end of red - 40 ohms. Tractor running at normal operating temp. after thermostat open - 28 ohms. Can anyone tell me what the ohm reading should be at normal operating temperature and at the lower end of the overheating (red) level? I cannot find those numbers anywhere in manuals or online. Thanks.
 
IMHO, 9 times out of 10 this issue is caused by a faulty voltage stabilizer. You say that you "feel" you don't have any issues with it, but how do you know for sure? Have you done any testing on it, and if so, what exactly did you do and what did you find?
 
The output of the voltage stabilizer reads about 10 volts. Of course it is hard to read using a Fluke meter vs. something more advanced like a scope which I don't have access to. Also, the fuel gauge works perfectly which I wouldn't think would be the case if I had voltage issues. Can you give me any suggestions on how to further test the voltage stabilizer? Thanks.
 
Have any idea just what that voltage stabilizer is? Is it a zener diode setting the panel operating voltage at an upper limit, or a temperature variable resistor varying the load on the incoming power in a voltage divider network to balance things out.

I think I understand that both of those indicators are Iron core electrical coils just like you would have in a D'Arsonval meter movement for an electronic instrument, not iron vane (mechanical) type used on bellows operated meters.

I have had few problems with these panels, but the VS has always intrigued me as to just what is it?
 
The voltage stabilizer output should be a constant on-off-on-off, much like a turn signal flasher. And that's basically all it is, to answer the other poster's question, just a turn signal flasher essentially.

Easiest way to test these is with a 12-volt test light. Test light probed to the output terminal should flash on and off. Give it about 30 seconds or so to warm up first. If it doesn't start flashing after about a minute, it's faulty.

THAT SAID, if it does not flash, and you really do have a constant 10 volts output, maybe you have some sort of an aftermarket replacement. If there's 12 plus volts going in, and 10 going out, it's doing something.
 
Now you have my curiosity aroused. What then you have is a 50% duty cycle device.

Brainstorming....50 % duty cycle on a 12v supply could simulate the heating effect of a 6v powered circuit, like using 6 volt designed instrumentation circuity with a 12v system. It's a crude attempt to make the conversion (seems to me) but that instrument panel wasn't used on 6v tractors and the 2" instruments used on them were stand alone like on my '63 2000D, so where's the 6 volt application?
 
Hobo, NC - Thanks for the warning about the constant 6V source burning up the gauge. I was thinking about going this route for better gauge accuracy but will stay way from that now. Any idea why that burned the gauge as in theory that would be equivalent to the 12V at 50% duty cycle which doesn't cause harm?
 
I think that the constant switching on and off adds some of the features of AC to the circuit, and since the gauge itself is basically a coil, the AC component of inductive reactance adds itself to the mix and increases the overall impedance which helps cut down on total current, but with a steady 6 volts the circuit won't have that AC component, so more current will pass, thus causing things to run hotter.
 
Here are some pics of the inside of my voltage stabilizer.
a159946.jpg

a159947.jpg
 
Here is an update. I retested the voltage stabilizer with my digital Fluke meter and was reading the same voltage out and what was going in. I know that isn't the correct instrument to test with but that is what I had available and I thought before I was seeing a lower voltage on the output. I also tested the VS output with a 12V light bulb and did not see any flickering - it was steady. In hindsight, I don't think I left it on for the 30+ seconds recommended warm up time. Thinking I had a bad VS, I purchased a new one and installed. Got all the same results with it. Measured a good ground to the VS and instrument cluster but added a wired to the negative side of the battery for sake of completeness. No change. Hooked up a potentiometer and the temperature gauge responded although it read higher on the gauge at higher resistances than what Hobo,NC reported in an older post. I think I am back to my original thought that somehow the temperature gauge is just out of calibration. As luck would have it I had exactly two resistors on hand that when paralleled provided almost the correct resistance offset to make the gauge work properly. I hooked them up in series between the sending unit and the gauge and now it reads just slightly low but much better than pegged out hot all the time. At some point, I will get the proper resistor(s) to make the gauge read accurately at normal operating temperature. Thanks for all the guidance as without it I would have surely burned up my temperature gauge.

If anyone can explain how that coil of wire wrapped around the metal strip in the VS causes the contact point to open please do.
 
If anyone can explain how that coil of wire wrapped around the metal strip in the VS causes the contact point to open please do.

That metal strip is most likely a bi-metallic strip, made of two different types of metal, and the coil of wire around it heats up as current flows through it, and as the bi-metallic strip heats up it bands away from the contact point because the two different metal expand at different rates as they heat up, and when it bends away from the contact it breaks the circuit and then when there is no current it cools back down and bends back toward the contact and once it makes contact current starts to flow again and the whole cycle starts over again.
 
It is entertaining to read all the speculation as to how the voltage stabilizer works! :wink:
 

I don't know does the temp gauge work thru the VS. Fuel gauges are the only thang I have seen it affect, now a temp gauge is added. :?:
 
(quoted from post at 00:15:55 05/12/17) It is entertaining to read all the speculation as to how the voltage stabilizer works! :wink:

JMOR, you are correct. I was only speculating, but I have a fairly good grasp of electricity and electronics, and there are only so many different ways to skin a cat. Can you please educate me on how it does work?

I had ruled out the coil causing a magnetic field that moved the contact to open like a relay because someone had said that it takes 30 seconds or so before it starts switching on and off, but a magnetic field would begin to work immediately, so I thought that it must be heat that causes it to open and the easiest way to get heat to move a piece of metal is by using a bi-metallic strip like in a home heater thermostat.
 
I'll start by saying I am not an electronics guy , basic tests and change parts then move on to something mechanical .

I have a 3000 diesel with non working gauges , fuel reads 1/8 tank when full , new temp sending unit does not work , so I may tinker with it someday but it is low on the list . I read these posts and retain some of the knowledge , lol .

Without needing an in depth reply ( y'all can technically debate it for fun ) , in this case posted , could you swap the fuel and temp wires , since the fuel works correctly to see if the error is the fault of the gauge ? Or is the resistance too far different ?

Thanks - Ken
 
Hey I have a new aftermarket guage and temp sending unit and mine is doing the same thing. I read somewhere that the resistance on the aftermarket sender is diff than the OEM. I think a OEM sender might solve the problem.
 
(quoted from post at 23:45:00 05/12/17) Hey I have a new aftermarket guage and temp sending unit and mine is doing the same thing. I read somewhere that the resistance on the aftermarket sender is diff than the OEM. I think a OEM sender might solve the problem.

If you got the new aftermarket gauge and sender from the same place they should be compatible with each other. If not, contact them and ask for a sender that is compatible with the gauge that they sold you.
 

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