Gas gauge calibration?

[Ed]

When I fill up, my gas gauge barely reaches the "F" and after about 2 miles it reads 3/4. I've also taken it below the "E". When I fill up the most I could ever put in is about 13 gallons (capacity is supposed to be 16 gallons). My question is; How do I calibrate my gauge to be more accurate?

[AxtellZ]

i've been told you can remove the sending unit and simply bend the wire arm thats inside the tank to "adjust the reading". Haven't tried it, but supposedly you just unscrew the big knurled nut where the sender is mounted in the side of the tank towards the front of the car, and pull it out. Of course you'll want very liitle gas in it at the time:rolleyes: HTH.

[EScanlon]

Before you go pulling sending units, and everything else, why not check the condition of your tank.

Mine had a nasty dent that apparently got put into it from someone backing up over a concrete divider. It was bad enough that the bottom of the tank AND the drain plug actually allowed a couple of gallons to sit in there below where the pick up tube could get to them. I didn't find this out till I took the tank off and was able to look through the filler neck hole.

It may be that this is NOT the case with yours, but it is a simple visual check and might save you some very tedious work.

2¢

[Nigel Mulvey]

I have a fuel measurement problem that I would appreciate any suggestions on. When I fill the tank the guage reads 3/4 at empty it reads 1/4 so there's a discrepancy at both ends of the scale.

[EScanlon]

It sounds as though your gauge was replaced at some time.

The fuel gauge is nothing more than a modified ohm meter. It measures the resistance in the coil which is what the sending unit is.

Check the back of the gauge. (Been a while since I've been there) but if I recall, there is a small access hole by which you can adjust the travel of the needle based on the resistance presented by the sending unit. Use a plastic screwdriver to adjust this.

If in doubt, remove the gauge, open it up, look inside and figure out what that access hole does and then do what it is obvious it does.

[Daniel]

I recommend starting with the connectors at the fuel tank sending unit.Make sure they are clear of oxidation and are making good connection.There are two connectors.Use an alligator clip type jumper wire to hold/short the two together and ground the other end of the jumper.The gauge should go to full max. and hold there .Next,if needed remove the sending unit(of course low fuel,safety in mind)and make sure the travel of the sending unit arm is not being limited by rust/corrosion and that the float is not holding fuel and causing false readings.It works like a toilet float.You will see the contact path on the unit.Again it should be clean and smooth travel.Hook up your wires and test your gauge readings.I can't remember if you need to ground the sending unit housing,but you'll know.I would try to make any corrections from the sending unit.There is a set screw that allows you to move the float arm without having to bend it. Once inside the dash gauge you would have no way to verify your changes without reassembly and reinstallation.Also they put heavy duty adhesive inside the gauge to lock the factory setting.It can be difficult to break loose,this would be a last resort.Put a little oil on the rubber seal at the sending unit to aid in reinstall.It stops it from binding and tearing Have fun!! Daniel

[EScanlon]

The Fuel Tank Sensing Unit has two wires connected to it as Daniel pointed out. However, just by connecting the two together you will have grounded the signal coming from the fuel gauge and you should read above FULL. By disconnecting the lead, it should read below empty.

If by connecting the Yellow and the Black Wires at the sending unit together your gauge does in fact read higher (More towards "Full") the gauge appears to be working properly and may just need calibrating. Now check with the Yellow wire disconnected. If it now reads BELOW what your "empty" indication is, then you do in fact have a gauge that either needs calibrated to the ohm range in the sending unit OR you have a sending unit that is unable to move it's full travel in one or both directions.

The sending and the fuel gauge are mated together. When you replace one or the other, you usually have discrepancies. Sometimes tolerable and other times not.

Whether it was the sending unit or the fuel gauge is a moot point at this time. I mentioned in the prior posting that it more than likely was your fuel GAUGE because the Ameter that is in the top half of the gauge is known to go "bad" and people replace it readily. The problem is that unless they replace the fuel gauge below (swap out their original with the replacement one) they will more than likely find themselves with a gauge that doesn't read right.

Now, you can either calibrate the gauge to the fuel sending unit and assume that the sending unit is in fact going full travel. Worst case? You read empty with a larger than normal amount of fuel in the tank.

OR

You can remove the sending unit from the tank, to see IF it is in fact not moving the whole amount of travel allowed the arm. This is the low end possibility. More than likely you will discover that the sending unit is in fact moving the whole distance, and the problem lies in the gauge.

The main reason I'm saying it is the gauge and not the sending unit is simple. The sending unit is incredibly simple, and very unlikely to fail mechanically. It is as simple as a toilet arm mechanism with a variable resistor on it. Remember slot cars and their controllers? The controller had a sliding contact that contacted this coil of wire. The farther up the contact the less resistance, the more current you got throught the coil. The lower down the contact, the more resistance and hence the less current you got through the coil.

There are two different postings here.

BOTH appear to deal with end point readings, which is a calibration issue; hence gauge.

[pparaska]

Originally posted by EScanlon

...The main reason I'm saying it is the gauge and not the sending unit is simple. The sending unit is incredibly simple, and very unlikely to fail mechanically. It is as simple as a toilet arm mechanism with a variable resistor on it. Remember slot cars and their controllers? The controller had a sliding contact that contacted this coil of wire. The farther up the contact the less resistance, the more current you got throught the coil. The lower down the contact, the more resistance and hence the less current you got through the coil.

And for just that reason, the sender is the part the usually goes bad! The friction between the finger and the windings on the sender is what can cause bad readings. I've seen this on several of the units. The gage has only friction at the pivot points, and the movement is imparted only through magnetic field fluctuations. A few years ago, the sender was still available new from Nissan, not sure about now.

If anyone is interested in how to calibrate a gage of this type, check out the "gage calibration" link on my home page.

[EScanlon]

Point taken.

In my car, my ameter was giving an intermittent short signal to the tester my mechanic hooked up. We replaced the instrument only to discover that my "FULL" tank of gas, now was reported as Half Full.

Ran it this way for a couple weeks to check. It ran through to empty pretty much ok, but Full was half way up the gauge.

Took my old gauge, and transplanted the old fuel gauge into the new ameter/fuel gauge instrument. Voila! Fuel gauge now reported a Full tank as "FULL" and not as half. So, what was different? Not the sending unit, but the gauge. Are the two items somehow mated? It appears so.

What's your take on that one?

[pparaska]

I'd think that they are all the same, within a given set of years.

I've tested 240Z and 280Z fuel gages and they seem to respond very similarly to the same sending unit. I measured the extent that several 240Z sending units had for full motion of the float, while out of the tank and they all were very close to 88 ohms Empty, and 8 ohms Full. No telling what the true Full reading will be until I've filled the tank (car not on the road). I'm waiting until I get it on the road, fill the tank, and make a resistance reading on the sender before I finalize what resistors to use to calibrate my Ford type gas level gage.

The senders do vary a bit due to manufacturing tolerances, but I doubt that they'd actually match gages to senders, as there seem to be no markings even on a brand new sender to differentiate a high ohmage one from a low one.