Fuel guage erratic

[tvoyt]

I am having issues with my Faria fuel gauge and my Rochester 8341-7000 sender unit. The gauge almost always reads full, but sometimes jumps to half, then back to full.

The interesting thing is that with the ignition key off (no voltage to the gauge), I get a reading of 30 ohms. With the ignition key on, I get 0 ohms. I would have guessed it would be the exact opposite. Does this make sense to the community?

Also, I've checked, cleaned, and reseated all connections and still have the problem. I think the next step is to replace the sender (if I can find one).

 

[toy4x4]

Re: Fuel guage erratic

Are you checking the resistance of the sender to ground? Also about how much fuel is in the tank?

 

[Silvertip]

Re: Fuel guage erratic

No it doesn't make sense but probably because you NEVER check resistance on a POWERED circuit. You check resistance of the sender by disconnecting the pink wire from the sender. Then measure from the sender stud to ground which is the metal shell of the sender. You check VOLTAGE at the "I" terminal of the gauge with the key on. It should measure 12 volts. If not, you either don't have a ground at the gauge or 12 volts is not available. The sender should measure 240 ohms with an empty tank -- 109 ohms (approx) with 1/2 tank and 30 ohms with a full tank. The metal shell of the sender also needs to be tied to ground.

 

[tvoyt]

Re: Fuel guage erratic

Thanks for the notes below. You are correct, i was incorrectly checking resistance on the powered circuit. I was trying to do so at the gauge itself rather than the top of the sending unit (I didn't have the floor boards lifted off - just being lazy). The tank was about full so the 30 ohm measurement seems about right, but I was wondering why the gauge was not reporting correctly when the switch was on.

I am going to pull the sender unit out and replace with a new one (I found a place that has original equipment). Before i pull it, i will check the resistance on the old unit at the top of the unit itself. I am getting a 12 v reading on the circuit. My first thought was that I wasn't getting power to the gauge, but I was. Then I started reading earlier threads about resistance. I will update the thread once I replace the sender unit to see if that indeed was the source of my problem. Thanks to toy4x4 and Silvertip for the input.

 

[Silvertip]

Re: Fuel guage erratic

Thanks for the notes below. You are correct, i was incorrectly checking resistance on the powered circuit. I was trying to do so at the gauge itself rather than the top of the sending unit (I didn't have the floor boards lifted off - just being lazy). The tank was about full so the 30 ohm measurement seems about right, but I was wondering why the gauge was not reporting correctly when the switch was on.

I am going to pull the sender unit out and replace with a new one (I found a place that has original equipment). Before i pull it, i will check the resistance on the old unit at the top of the unit itself. I am getting a 12 v reading on the circuit. My first thought was that I wasn't getting power to the gauge, but I was. Then I started reading earlier threads about resistance. I will update the thread once I replace the sender unit to see if that indeed was the source of my problem. Thanks to toy4x4 and Silvertip for the input.

You don't need to pull anything apart UNTIL you disconnect the PINK fuel sender wire AT THE GAUGE and measure resistance to ground. That is the same as measuring directly at the sending unit. If you get NO reading, then the PINK wire is open or the sender is bad. NOW you can tear up the floor.