Gas gauge and sending unit

[gonzo]

The gas gauge in my car registers beyond the "full" mark when tank is full; when there's about 4 Gallons remaining it starts to flutter and continues to flutter until the tank's emty. I've learned over the many years to deal with the gauge's behavior and just fill the tank when it flutters. However, my wife objects to this method.(Note: at times it's just easier to say "yes dear".)

So I purchased a new sending unit (OEM) and tested it before installation. The gauge moved smoothly and throughout the full to emty range when the sending unit float was moved up and down. (Oh, and the original sending unit also passed this test.)

With the new sending unit installed and the tank very full (within an 1/2 inch of the lip), the gauge registers beyond the full mark. Analogous behavior to the original sending unit.

Is the gauge and sending unit now working correctly or will I have to wait until the tank is half full, or perhaps empty, to learn the answer?

GONZO - Optimistic even when running on fumes.

 

[Dave Russell]

Hi Gonzo,
Check out this reference. It is for a MGA but the gages are the same. You can check & calibrate yours pretty closely. Two things to be sure of, the sending unit is firmly grounded, add a jumper, & the gage itself needs to be well grounded. The mounting bracket often doesn't make good contact with the dash metal. Add a ground jumper.
https://mgaguru.com/mgtech/electric/fg_01.htm
D

 

[Skip & Lynne]

Hey Gonzo I spoke with Margret Lucas at MO MA the other day as I was sending in my speedo and tach for overhaul and she had me send in the fuel gauge as well, something about a transistor or something that causes problems with the gauge itself. I just took her advice and sent it in to her. You might want to give her a call. Skip

 

[Keoke]

/ubbthreads/images/graemlins/savewave.gif S&L, Transistors in the fuelguage circuit ?? NAH---Keoke /ubbthreads/images/graemlins/nonod.gif

 

[chuck1006]

Hi Gonzo,
To Dave's point, I had an accuracy issue with my gauge and found the gauge was not grounded well enough through the dash. I added a ground wire from the bracket to a good ground location on the steering bracket. I already had a ground wire on my sending unit. I didn't have flutter but it wouldn't hurt to add the ground wires evn if you have to get the gauge repaired.
Chuck

 

[Skip & Lynne]

I`m not sure about the transistors; my degree is in mechanical/ metallurgical engineering not electrical but I would call Mrs. Lucas just to be safe. she knows more about what she is talking about. Skip

 

[Keoke]

/ubbthreads/images/graemlins/savewave.gif Skip, Just an old EE here---Cheers Keoke Still No Transistors!

 

[Dave Russell]

Skip,
From another old retired EE & farmer. Many other cars used a newer version of the gage which may have electronics in it, or associated with it. NOT the Healeys. You can see from the reference on MGA's that I posted above that there are no transistors. It is strictly a balanced bridge circuit. This is why it is not sensitive to varying supply voltages.

BTW - I recently discovered that a poor gage case to ground connection would cause the gage to read quite a bit too low. This ground connection then became obvious in the referenced schematic.
D

 

[Skip & Lynne]

Hey guys as I said I am no E/E but I just got my tach and fuel gauge back from Mo Ma and they replaced a shunt inside the fuel gauge. I knew that there was something inside that she said needed replacing but I wasn`t sure what it was. Would a bad shunt cause erratic readings. Skip

 

[Dave Russell]

Skip,
At the bottom of the page I referenced, there is a schematic diagram of the gage system. The "shunt" is the part referenced as "upper winding". If it is defective the gage definitely won't work correctly.
D

 

[gonzo]

Thanks you all for your input. After taking everything into consideration and trying, unsuccessfully, to locate 2 x 68 ohm resitors to perform the calibration, I decided to pack the gauge up and send it to Nisonger. Now it's in more capable hands anyway

I'll report back in about 3weeks on this particular project. Back to the hammer! GONZO.

 

[gonzo]

As a follow-up to my original post, the fuel guage has returned from being serviced (recalibration) - never noticed before that the color of the face is silver tone unlike the other guages on this Healey which are gold tone -with suggested ground leads attached, is still behaving the same and not functioning correctly.

Question: Is there supposed to be a correct orientation for the fuel sending unit? Should the lead face forward or to the back of the car? I suspect that the float arm is grounding on the tank resulting in erratic readings.

Look forward to suggestions. Thanks GONZO