Hi
I have a fuel level sensor that shows 250 ohms when empty and 33 ohms when full. It runs on a 12 Volt DC Circuit. (14-15 actual volts from engine alternator)
I would like to shut off the power to a 12 VDC coil on a relay so the engine shuts down. This would happen when seeing 230 ohms from the level sensor.
Operating temperatures are what you would see in Buffalo NY. So very cold and very hot. The parts would need to be resistant from the environment, although will be covered and will not see direct weather.
It will be subjected to minor vibrations from the engine.
Longevity is key to eliminate breakdowns.
Can anyone suggest a circuit diagram.

 

Since the fuel level sensor is connected to something else (you say it runs from 12Vdc), then we need to know what the sensor voltage is when the sensor is at 230 ohms.

 

Since the fuel level sensor is connected to something else (you say it runs from 12Vdc), then we need to know what the sensor voltage is when the sensor is at 230 ohms.

OK. So I will push the sensor to the bottom of the tank manually and measure the incoming voltage and then take a reading of the outgoing voltage. Correct?

 

I am guessing that the fuel level sensor is part of a fuel gage system and so at some point in the circuit there is a voltage that changes as the fuel level changes. THAT voltage can then be used to initiate the shutdown. And diesel engine vibration is not minor, it is persistent. And the rest of the system also vibrates. So a comparator circuit with a relay output is what you want.
But a diesel engine does not stop when you switch off the spark, so it will take more than just a relay.
And it could be useful to know what the application is. A CAT "D-6", or an irrigation pump, or a backup generator. Or a Mercedes?

And the quote is backward.Avoiding breakdowns leads to longevity.

 

I am guessing that the fuel level sensor is part of a fuel gage system and so at some point in the circuit there is a voltage that changes as the fuel level changes. THAT voltage can then be used to initiate the shutdown. And diesel engine vibration is not minor, it is persistent. And the rest of the system also vibrates. So a comparator circuit with a relay output is what you want.
But a diesel engine does not stop when you switch off the spark, so it will take more than just a relay.
And it could be useful to know what the application is. A CAT "D-6", or an irrigation pump, or a backup generator. Or a Mecedes?

The shut off system is simple. It is tied to a closed circuit which involves the temperature switch, oil pressure switch and the fuel level sensor. They are all tied in a series circuit. Once broken, power to the coil that keeps the fuel solenoid feeding the injectors running is closed as well as other circuits operating off this same coil.
Presently the fuel level sensor is the only thing I need to get working to open when the fuel is low.
This is a Cat C1 that used to be a welder, and had come with electronic circuits from the OEM that controlled everything. I had the generator rewound and got rid of all the welding circuits, so now it is just an AC generator.

 

Somewhat just out of curiosity, but what is the purpose of shutting the engine down versus just letting it run out of fuel? Are there things that will have issues if left powered even after the engine quits?

 

Somewhat just out of curiosity, but what is the purpose of shutting the engine down versus just letting it run out of fuel? Are there things that will have issues if left powered even after the engine quits?

It depends. With some smaller diesels it will get air in the fuel lines and the only way to get it running again is to loosen the lines at the injectors and purge the air out. It's also better for your injection pump to always have fuel in it versus running it dry.

 

Elaborating on post #47, it can be a pain to purge the air out of an injection system for some engines. That can also be true for gasoline fueled cars of a few years ago, back when they had carburetors. A very simple choice could also be adding a float switch to he fuel tank, in addition to the fuel level sensor.
But to use the fuel level sensor you will need to monitor the voltage with some sort of comparator circuit. A system with two comparators can also provide a low fuel warning prior to shut down, allowing a refuel before power loss.

 

In all reality now that fuel injection has become common on just about everything over 20 HP it's just plain a good idea not to let an engine run out of fuel whether it gas or diesel. Anything with a pump needs the fuel for cooling and lubrication. Gas isn't much of a lubricant, but it's better than bare metal. The only exception pump wise would be the old style mechanical diaphragm type pumps that ran off the cam shaft.

 

I am guessing that the fuel level sensor is part of a fuel gage system and so at some point in the circuit there is a voltage that changes as the fuel level changes. THAT voltage can then be used to initiate the shutdown. And diesel engine vibration is not minor, it is persistent. And the rest of the system also vibrates. So a comparator circuit with a relay output is what you want.
But a diesel engine does not stop when you switch off the spark, so it will take more than just a relay.
And it could be useful to know what the application is. A CAT "D-6", or an irrigation pump, or a backup generator. Or a Mercedes?

And the quote is backward.Avoiding breakdowns leads to longevity.

A spark? in a diesel engine?

 

but what is the purpose of shutting the engine down versus just letting it run out of fuel?

The fuel system will have to be re-primed before it will run again, and it is probably not so great having it run erratically immediately prior to running out of fuel.

The shut off system is simple. It is tied to a closed circuit which involves the temperature switch, oil pressure switch and the fuel level sensor. They are all tied in a series circuit. Once broken, power to the coil that keeps the fuel solenoid feeding the injectors running is closed as well as other circuits operating off this same coil.
Presently the fuel level sensor is the only thing I need to get working to open when the fuel is low.
This is a Cat C1 that used to be a welder, and had come with electronic circuits from the OEM that controlled everything. I had the generator rewound and got rid of all the welding circuits, so now it is just an AC generator.

I think you just need a normally closed relay in series with all the other devices in the loop.
Does the sensor connect to anything else? such as a gauge? If so, what powers the gauge?
You have to avoid the situation where there is no power to the gauge when the engine is not running, and your circuit sees it as empty and thus won't allow the engine to start.

 

A spark? in a diesel engine?

That was included to make a point. THERE IS NO SPARK FOR A DIESEL ENGINE.

 

The fuel system will have to be re-primed before it will run again, and it is probably not so great having it run erratically immediately prior to running out of fuel.

I think you just need a normally closed relay in series with all the other devices in the loop.
Does the sensor connect to anything else? such as a gauge? If so, what powers the gauge?
You have to avoid the situation where there is no power to the gauge when the engine is not running, and your circuit sees it as empty and thus won't allow the engine to start.

There used to be a fuel gauge that had some circuits in it that opened the main relay which controlled the fuel solenoid to the injectors. That is all gone.
I will have a 12 volt supply that goes to the new fuel gauge. The sensor will then connect to that.
I just need something that is going to "open" so that it shuts off power to the main relay.
I have attached a drawing showing somewhat of what the circuit might look like. (I am not an electrical person, so the drawing is probably not up to speed)
I will need a time delay relay to close the circuit for 30 seconds when the engine starts due to the oil pressure s/w being open.
The fuel sender will need to open when the ohms get to 230

 

There used to be a fuel gauge that had some circuits in it that opened the main relay which controlled the fuel solenoid to the injectors. That is all gone.
I will have a 12 volt supply that goes to the new fuel gauge. The sensor will then connect to that.
I just need something that is going to "open" so that it shuts off power to the main relay.
I have attached a drawing showing somewhat of what the circuit might look like. (I am not an electrical person, so the drawing is probably not up to speed)
I will need a time delay relay to close the circuit for 30 seconds when the engine starts due to the oil pressure s/w being open.
The fuel sender will need to open when the ohms get to 230

Haste often does seem to make waste. At least in this instance.

 

So I will push the sensor to the bottom of the tank manually and measure the incoming voltage and then take a reading of the outgoing voltage. Correct?

Yes, if the incoming voltage is the battery voltage and the outgoing voltage is the voltage at the sensor.

 

There's probably room for improvement, and part selection is not my strong point so I left out part numbers (I just go for overkill on everything myself). If this were my project I would do something like so...


The key on power would come from the normal on / off switch, key, etc.
The starter activate signal would be from your start button / start position on ignition, etc.
The fuel gauge connection would be from the connection between your fuel gauge and sender.
R1 would probably be better as two resistors acting as a voltage divider, but you'll need to find your cut off voltage in order to calculate the values for that.
Q1 is some variety of NFET.
U1 is some form of comparator. I didn't draw it, but you'll probably want some form of hysteresis added to keep the relay from cycling on and off as the fuel level hits the cut off level.

Relay 1 would energize the cut off solenoid while the engine is cranking. Normally oil pressure switches have a threshold low enough that the engine should build enough pressure while cranking to activate the switch, but I'll admit I don't have experience with them in a diesel application. I don't know if they are set up for a higher threshold due to how a diesel uses oil compared to a gas engine or not (piston cooling jets comes to mind... if it even has them).

You could make it a one relay system that cuts off the fuel when the relay is energized, but in my opinion that would leave a condition where if the circuit fails for some reason and the relay is never energized then the engine would run out of fuel.

Feel free to berate, optimize, or otherwise comment.

Edit...

I also just realized I forgot the protection diode at Q1.

 

Q1 is some variety of NFET.

You show the symbol for a JFET.
That should be an N-MOSFET.

 

There's probably room for improvement, and part selection is not my strong point so I left out part numbers (I just go for overkill on everything myself). If this were my project I would do something like so...

View attachment 307917
The key on power would come from the normal on / off switch, key, etc.
The starter activate signal would be from your start button / start position on ignition, etc.
The fuel gauge connection would be from the connection between your fuel gauge and sender.
R1 would probably be better as two resistors acting as a voltage divider, but you'll need to find your cut off voltage in order to calculate the values for that.
Q1 is some variety of NFET.
U1 is some form of comparator. I didn't draw it, but you'll probably want some form of hysteresis added to keep the relay from cycling on and off as the fuel level hits the cut off level.

Relay 1 would energize the cut off solenoid while the engine is cranking. Normally oil pressure switches have a threshold low enough that the engine should build enough pressure while cranking to activate the switch, but I'll admit I don't have experience with them in a diesel application. I don't know if they are set up for a higher threshold due to how a diesel uses oil compared to a gas engine or not (piston cooling jets comes to mind... if it even has them).

You could make it a one relay system that cuts off the fuel when the relay is energized, but in my opinion that would leave a condition where if the circuit fails for some reason and the relay is never energized then the engine would run out of fuel.

Feel free to berate, optimize, or otherwise comment.

Edit...

I also just realized I forgot the protection diode at Q1.

Looks good. I will do some research.

 

If the circuit uses an opamp to drive the mosfet the rat of change will be slow enough tat no protection diode will be needed.

 

I suspect feedback from the in-tank sender may vary with temperature and/or battery voltage. Test to find out, but if this is the case then you might get better results with a binary on/off type of fuel level sensor