94 SL2 IP Cluster Fuel Gauge Resistance
- Thread starter tim2at429
- Start date
Derf
Moderator
The fuel gauge is essentially a small, sensitive ammeter. It has an electromagnet (a coil of wire) and a needle attached to a pivot. When current flows through the coil, it creates a magnetic field that interacts with a permanent magnet, causing the needle to move.
The amount of current that flows through the gauge is what determines its reading. This current is controlled by the resistance of the fuel sending unit.
Because of the operating principle, its internal resistance (equivalent) isn't constant in the same way a static resistor is. Its impedance will vary slightly with the position of the needle and the magnetic field.
What issue are you trying to address?
The amount of current that flows through the gauge is what determines its reading. This current is controlled by the resistance of the fuel sending unit.
Because of the operating principle, its internal resistance (equivalent) isn't constant in the same way a static resistor is. Its impedance will vary slightly with the position of the needle and the magnetic field.
What issue are you trying to address?
Hello Derf,
I am wanting to test the fuel gauge. When using a adj. resistor wanted to be sure it would handle the load. So by Ohm's law rule of thumb I will at minimum 5 watt resistor. 33 ohms @ 12v = 4.4w .
33 ohms full
I am going to be dropping fuel tank as the fuel pump has stopped working. I have voltage to the trunk connector so I am good to that point.It is functioning as designed and turns off after 3 sec. I need to check from trunk to top of tank and move on from there. I am also looking for the right body control ground location.
I am wanting to test the fuel gauge. When using a adj. resistor wanted to be sure it would handle the load. So by Ohm's law rule of thumb I will at minimum 5 watt resistor. 33 ohms @ 12v = 4.4w .
33 ohms full
I am going to be dropping fuel tank as the fuel pump has stopped working. I have voltage to the trunk connector so I am good to that point.It is functioning as designed and turns off after 3 sec. I need to check from trunk to top of tank and move on from there. I am also looking for the right body control ground location.
Derf
Moderator
That is an effective resistance at a single point in time. The current is what indirectly deflects the needle of the gauge via magnetic interaction. I'm not sure where 33 ohms comes from. If that is what you measured at the highest across the gauge, remember that that is a single gauge reading at a single deflection of the needle.
Deflection is not going to be a linear function of effective resistance. If you are simply trying to determine if the gauge responds to the incoming current from the sender, I suppose this approach will give you either it moves or it doesn't move, but there is no simple way to ensure the gauge is reading correctly. I'm not sure if that is part of your goal, just tossing it in.
Also, when you replace the fuel pump and sender unit (do both and save yourself a whole lot of trouble --- sender is 31 years old), I believe the arm on the sending unit may need a small adjustment for the gauge to read correctly. It's hard to tell with a 31-year-old gauge, but my vehicle has always read incredibly high for the last 10 years. I replaced the fuel pump and sender and it behaves exactly the same. I have since read that the sender needs to be adjusted in order to move freely and not contact the tank on the bottom. I think my float must be barely touching a baffle or similar when I fill the tank and it's sort of gets stuck there. The tank will be full for the first actual half of a tank, then it nose dives consistently towards empty. I did not perform any adjustments. Again I don't know if it's the gauge but just passing this on. I believe if you search on it for Saturn fuel pumps it's discussed somewhere on one of the forums.
Deflection is not going to be a linear function of effective resistance. If you are simply trying to determine if the gauge responds to the incoming current from the sender, I suppose this approach will give you either it moves or it doesn't move, but there is no simple way to ensure the gauge is reading correctly. I'm not sure if that is part of your goal, just tossing it in.
Also, when you replace the fuel pump and sender unit (do both and save yourself a whole lot of trouble --- sender is 31 years old), I believe the arm on the sending unit may need a small adjustment for the gauge to read correctly. It's hard to tell with a 31-year-old gauge, but my vehicle has always read incredibly high for the last 10 years. I replaced the fuel pump and sender and it behaves exactly the same. I have since read that the sender needs to be adjusted in order to move freely and not contact the tank on the bottom. I think my float must be barely touching a baffle or similar when I fill the tank and it's sort of gets stuck there. The tank will be full for the first actual half of a tank, then it nose dives consistently towards empty. I did not perform any adjustments. Again I don't know if it's the gauge but just passing this on. I believe if you search on it for Saturn fuel pumps it's discussed somewhere on one of the forums.
Derf
Moderator
You can also purchase an ammeter or a multimeter with a current input and put it in series with the signal coming from the sender. The current is what drives the magnetic field in the gauge, not the voltage. Since you will be starting with an empty tank, once it is reinstalled, simply add a small amount of gas say 2 gallons to the tank. That should be enough to move the float and change the current. If the gauge is functional, you should see the deflection away from empty and a current on the display of the multimeter in probably milliamps or fractions of an amp. I can't remember whether empty or full has the largest current. You can look that up. To truly test the gauge, you would need to remove it from the circuit board which I highly do not recommend ss the information you will gain is not worth it
The sender is suppose to supply 33.6 - 37.6 ohm's to the gauge on full. Which should be the highest current sent back to the gauge.
I have not been able to locate a fuel pump with sender as of yet.
My 02 SL gas gauge will only drop down to around 1/8 tank.It has been like that for years. I always fill up at the 300 mile trip meter. The last time I checked I was able to get 340 miles on a tank, getting to old to walk now.
I have not been able to locate a fuel pump with sender as of yet.
My 02 SL gas gauge will only drop down to around 1/8 tank.It has been like that for years. I always fill up at the 300 mile trip meter. The last time I checked I was able to get 340 miles on a tank, getting to old to walk now.
Derf
Moderator
See attachment for fuel pump with sender.
https://www.rockauto.com/en/catalog...fuel+&+air,fuel+pump+&+housing+assembly,10147
https://www.rockauto.com/en/catalog...fuel+&+air,fuel+pump+&+housing+assembly,10147
Attachments
My son was finally able to come get the fuel tank removed. The Original Bosch fuel pump would try and run but just would not spin up. 31 years old and then car was parked for 8 years guess it finally stopped. A shame it was working good a month ago but better it stopped working in yard than out on road running errands. The fuel send checks good on bench so maybe next weekend get it all back together.
Derf
Moderator
If you plan to keep the car and are in the rust belt, consider replacing or at least purchasing the tank straps while they are still available
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