Fuel Guage

[curtis]

The fuel guage on my 100 has never read full when the tank is full.
I now find that when the tank is full it reads full almost all the time. Occasionally it will give an "accurate" reading and then as there is less and less in the tank it seems to stay stuck on empty.
Any ideas before I start pulling things apart?
Curtis

 

[Dave Russell]

Hi Curtis,
Try a couple of tests. Disconnect the wire from sender to gage, at the sender. Ground the wire coming from the gage. The gage should read empty or a little below. With the wire ungrounded the gage should read above full.If not, the gage or the wiring are defective. You can isolate the back of the gage & do the same tests to find if it is the gage or the wiring.

Measure the resistance from the isolated sender terminal to ground. with the tank full there should be around 70 to 80 ohms. With the tank empty the reading should be around zero to 7 ohms.

It may be easier to remove the sender for tests. Be prepared to replace the cork seal around the sender & there should be copper washers under the sender securing screw heads. Permatex "Hylomar HPF" is a good sealer to use on the gasket & screws. Also, the sender lid needs to be well sealed. I would suggest a short jumper from one of the sender screws to ground. Also make sure that the gage in the dash has a good ground.

Obviously there should be no fuel in the float. If there is, & it is brass, drill a small hole in it & drain. Solder the hole & submerge the float in hot water. Air bubbles will come out wherever the leak is. Solder the leak & retest.

As you move the float arm from empty position to full the ohmeter connected from sender terminal to ground should change smoothly from around zero to 70 ohms. Sometimes the arm can be making a poor ground contact on the sender body & cleaning or tightening the arm pivot can help. There is a small resistance wire coil that the arm travels over which gives the resistance changes with arm motion. It is pretty hard to repair the tiny wire but it can sometimes be done.

There is a procedure to adjust the magnetic shunts in the dash unit to get the overall calibration pretty close. Start here & follow it through:
https://mgaguru.com/mgtech/electric/fg_01.htm
The MGA gage is the same as the AH gage.

If you get a new sender (Moss is as good as any) do the same tests before installing things. There is a way to precisely calibrate the gage itself if everything is ok & the gage is just a little off. If the gage needs repair go here;
Margaret Lucas
Mo Ma Manufacturing - instrument repair
13211 Second Street. NW
Albuquerque, NM 87102
Ph (505) 766-6661 - Fax (505) 766-5419
momanm@aol.com

Maybe you can find someone on your side of the pond to do the same repair.
D

 

[curtis]

Now that's what I call an in depth response.
Thanks Dave. /ubbthreads/images/graemlins/smile.gif
Will have a go and report back

 

[GreenHornet]

If that doesn't work. Then do what I do...use a stick!
I have a bad sending unit in my Sprite and would have to drop the tank.
:Þ

 

[AndrewMawson]

A modern fuel guage is heavily 'damped' electrically so that it averages the readings to avoid the swishing effect. I did an experiment last year damping the sender unit by connecting a high value electrolytic capacitor across it. There was a marked improvement BUT the value of capacitor was so large (several 10's of thousand microfarads) that it was too large to locate in the tank dimple where the sender sits and I didn't want wires trailing all over the place.

/ubbthreads/images/graemlins/england.gif /ubbthreads/images/graemlins/cheers.gif /ubbthreads/images/graemlins/england.gif /ubbthreads/images/graemlins/cheers.gif /ubbthreads/images/graemlins/england.gif

 

[mbrooks]

It would be electrically the same to put the capacitor at the other end of the sender wire under the dash. Then it wouldn't have to fit in the tank dimple.

Rgds

Mike

 

[Dave Russell]

[ QUOTE ]
It would be electrically the same to put the capacitor at the other end of the sender wire under the dash. Then it wouldn't have to fit in the tank dimple.
Rgds
Mike

[/ QUOTE ]
Very true - I've always been a bit spooked by the prospect of a "mega" farad capacitor causing an internal spark in the sender that is large enough to ignite the fuel vapor in the tank. Probably not likely though.

I realize that many fuel pumps inside the tank have some brush arcing, but their design takes great pains to keep the pump "completely submerged" in liquid fuel. Not so with the top mounted gage.

Any opinions out there?
D

 

[shorn]

TWA Flight 800 demonstrated that sparks in fuel tanks can cause horrific results. Less fuel in a tank likely means more vapor and more potential for explosions. I don't think one can be too careful in eliminating any possible sparks.

 

[SargeVT]

Dave - I like your description on how to trouble shoot the persicky fuel gauge. I have visited the "mgaguru" site and purchased an old gauge to be challenged by. I presently use the Greenhornet's stick method for determining fuel level!

 

[Dave Russell]

Hi Sarge,
Yeah - I use a stick for "serious" measuring. On a level surface, one onch is close to two gallons on my particular tank. The gage gives me a general idea of when I need to get serious with the stick.
BTW, The reading on my gage varies by about 3/8 tank depending on whether or not there is a solid ground on the dash unit. The U bracket doesn't make good ground contact with the dash by itself.
D

 

[Keoke]

/ubbthreads/images/graemlins/savewave.gif
How the "U" bracket going to make ground contact with the dash Dave when the dash is Wood.---Keoke /ubbthreads/images/graemlins/devilgrin.gif---For the BJ8 cars there is a dedicated ground wire coming from the harness for each guage.Make sure each guage has one and includes an internal tooth lock washer under the head of the knurled thumb screw securing it to keep it tight.---Keoke /ubbthreads/images/graemlins/angel.gif

 

[Dave Russell]

Keoke,
I should have been more clear. I was refering to the gage that Curtis has, metal dash. If Sarge's gage happened to have the ground pigtail not connected, the gage will still read, but low by 3/8 tank or so. But you knew that.
D

 

[Stretch]

I soldered a flexible braided wire across the moving shaft. Been working great ever since.