Fuel Warning light comes on early

[webrest]

Hi All,
Have recently noticed that my fuel warning light activates earlier than it used to,
does anyone have any experience with this problem and if so what do I do to rectify it.
Light comes on midway between 1/4 and 1/2 way point.
Realize this is not of earth shattering consequence but it still annoys the crap out of me.
Model is a 1985 911 3.2
R. Dave

[88-diamondblue]

The float in the sensor is hanging up. The gauge will show more fuel in the tank than there is. Your low indicator light coming on would be correct. I have the same thing happen on mine.

[webrest]

Thanks Michael,
I will pull it out and see what is going on.
You have told me what I needed to know.
i.e. That the fuel light is always constant.
R.
Dave

[webrest]

Ok, thought some may be interested in the rig I set up for getting a real time look at what the fuel gauge was doing out of the vehicle.
I first checked the parameters as per the the manual [see chart attached] and that appeared to be fairly close to spec, a little short on ohms at both ends but near enough. I still wanted to see what was happening though so I decided to set up a test rig and observe first hand what was going on.
The first thing to do was replicate a fuel tank and it occurred to me that the simplest way of doing so was a length of 50mm PVC pipe cut to a slightly longer length than that of the sender and fit an end cap to it. Voila. A petrol tank. I then tapped in a barb at the bottom and fitted a length of fuel line so I could control the amount of fuel in the chamber and watch, when wired up, how the gauge responded to flow. It would also show when the low fuel light would activate and at what level on the meter.
Turned out to be a worthwhile exercise as it did demostrate that the components are working correctly. If it still reads wrong in the vehicle I can only assume that there is something else going on. It did conclusively prove to me that the light is the main thing that is to be believed as it cannot be wrong. [Not that I doubted you Michael] Having pulled the sender apart and checked out the way it activates it can never be wrong.
I realize that some might figure this may be a bit anal but those sort problems are always in front of you when you drive the car and to me at least, are a constant source of annoyance. Can never rest until I get get to bottom of the problem. It may never marry up properly whilst in position but doesn't really matter the main thing is that I now fully understand how it works and what is going on with it.
Dave

[aj88cab]

Just curious, with all those exposed electrical connections...did you really use gasoline as the test fluid

Nothing anal about your testing (well maybe a little), but we have all spent weekends drilling into the details of how and why some system or component does (or does not) work.

[RoninLB]

Originally Posted by webrest

Ok, thought some may be interested in the rig I set up for getting a real time look at what the fuel gauge was doing out of the vehicle.

probably best post on this in archives

[webrest]

Don't worry Andrew didn't use fuel.
Probably wouldn't have mattered much if I did all the connections were covered. No real risk of spark. Don't forget it is working this way in your car all the time.
Dave

[ossiblue]

Great write up Dave! Given what you've done, you might check out the dash gauge itself. Clearly, the float must be working if the light is going on so probably the needle gauge needs a looking into.

[webrest]

Hi LJ
The system is working properly, all reads as it should.
That is the actual gauge out of the car in the image.
The only thing that I can think of is that it may have issues in the vehicle wiring that is creating a little resistance pushing the meter up a bit.
Dave

[dshepp806]

Can you meter this wiring resistance that you suspect? (gauge to sender wiring checks...ohms). Maybe connectors along the path.

If "continuity" is the ticket for engaging the light,..and it's happening early,..maybe something could be providing LESS resistance to the gauge (grd) ...Am I reading this backwards?

BTW: NOT anal, at all,..quite smart.

Thanks,

Doyle

[webrest]

Hi Doyle,
I'm sure it won't be the light, that is either on or off. The guage is something of a different nature that can vary.
I will take a reading of the resistance values at the sender itself and then I will take further readings at the dash connections to see if there is any variance. If there is I will know that is why I am receiving incorrect level indication on the level meter.
If it has higher resistance at the dash the meter would be reading low[not the scenario I would expect in my case] and if the resistance is of a lower value than the one taken directly at the sender it will mean that the meter will read at a higher level.
So to cap off I'm expecting a lower resistance level reading and if that is the case it will just be a matter of following through the wiring in the car and diagnose why.
Dave

[88-diamondblue]

Dave,

Really great write up. Interesting that there could be issues between the sensor and the gauge. I had installed a new sensor when one of the wires broke inside of the old one. The new one would read as I had 3/4 of a tank and the light would come on. It would not do it all the time though. Just got to where I would see the light on and fill the tank which was about 2 gallons less than capacity.

[webrest]

Well the results are in.
All is working spot on.
Checked the continuity at the sender 67.2ohms and 67.3 ohms at the dash terminals, so no resistance errors there.
The next thing thing to do was put the gauge back in the dash and see what it reads.
The meter indicated exactly as it should. The only thing that I can assume is that there was a build up on either the the resistance wires in the sender or the terminals. They were all cleaned so that could have been the answer to the faulty readings.
What ever the reason all is well with the world again.
Hope it was all of some interest and help.
Dave