Testing the fuel gauge

[wkilleffer]

Awhile back, I asked the forum about a problem I was having with the fuel gauge on my 1974 MGB. Time and financial crunches kept me from being able to do anything, and the topic has since dropped off the forum. Now, I can do something, but am not sure what to do.

My fuel gauge has not been working for awhile. After starting the car, the needle will rise up to a point between the two lowest markings on the gauge and stay there. Initially, I thought the float on the sending unit might have failed, but on a really hot day, the gauge came to life and gave a reading that seemed more or less accurate based on the mileage since the last fill-up. But it didn't last too long.

It did it again last weekend. It was a hot day that day. Once again, the reading seemed to be as accurate as those gauges ever are. Something I've noticed both times is that it seems a little balky when it does this. The needle's movement isn't smooth, whereas the temp gauge needle always moves smoothly. It's like a connection's bad or the gauge is having problems.

The temp gauge works just fine. Both gauges are plugged into a voltage stabilizer that might be original to the car.

I've seen a method that's used to test the operation of the gauge that involves some kind of connection to the wire that plugs into the sending unit.

Any suggestions would be appreciated.

Thank you,
-Bill

 

[wkilleffer]

Just to tell what I've done so far:

When I ground the wire that plugs into the sending unit and start the car, the gauge quickly moves up to F. Remove the ground, and it goes down.

I used a small handheld multitester to check the resistance of the sending unit. The scale's hard to read below 100 ohms, but it looked like the reading was between 70 and 80 ohms. The fuel tank is close to full with less than 20 miles since the last fillup. The leads are too short for me to keep them connected and shake the car. The digital multitester has long leads but got confused during this test.

Does this point to a bad sending unit or some kind of bad connection?

Thank you,
-Bill

 

[bob67bgt]

I would think the problem is in the tank. If the guage moves to full when you ground the green/black tracer? wire back at the tank then the system in the car is fine. Sounds like a sender problem to me. Maybe the sender is not grounding properly also. Remember its better to work under the car than under the dash! Bob

 

[Bruce74B]

Run it down below a quarter tank, jack up the right side and pull the sending unit...I bet your plastic float is perferated somehow and is fillin up with gas, giving you a bad reading. If not, the sending unit should be replaced IMHO

Bruce