I may have a solution. I was going to post my findings in a new thread but I guess I will let the cat out of the bag; This is a "bit" long - so skip to the bottom for the PUNCH LINE, is so inclined.
I was troubleshooting my "Low Fuel Level" idiot light and came upon a failure that is very interesting. But let me digress a moment and breifly explain the workings of the fuel sensors; Two sensors, one which provides a variable resistance (via a float) to drive the dashboard needle, and one which lights up the idiot light. The latter works on the principle that the low fuel sensor conducts (i.e. low resistance, or short) when not submerged in fuel and does not conduct (high resistance or open) when in fuel.
My low fuel idiot light had been lit for about 100,000 miles. Ignored it until me and (unfortunately) the wife-e-poo ran out of gas in a (steep sided) canyon highway at rush hour (insert female-generated expliatives HERE). Did I mention NO CELL SERVICE ? AT DUSK...... CRAP - now I have to fix the damn thing soon.........
A few days later......Pulled rear seats, pulled cover plate, disconnected connector and found short on the low fuel sensor connections (I believe pins 1&5 - check FSM). Since the tank was 1/2 full this indicates a bad sensor -or so I thought. Ordered new sensors from C-Dan.
NOTE: both the Low Fuel Light sensor and the Variable Tank Level sensor come together as one part. Fast forward 1 week, installed new sensor and gosh darn if the low fuel warning light is still lit.
Well at this point there is a short, but where ??? Again, even with a new sensor, at the connector going to the fuel tank, a short is indicated across the low fuel light sensor connectors. POSSIBILITIES 1) C-Dan gave me a bad sensor?, 2) The 10" of wire or the connector outside the tank is bad?, 3) A short inside the tank?
>>>> SAFETY WARNING - Don't try this next "in truck" trouble shooting in your garage, or with a truck you would dearly miss, should it (and you) explode in a lethal mix of electricity and gasoline. I took a bunch of precautions (did it outside, covered up the big hole in tank where pump frame bolts, opened all the truck doors, used special insulated test leads......)
To find out I removed the sensors from the pump frame (this is the tank coverplate through which the wires for the sensors and fuel pump pass). I connected the fuel sensor (using micro grabber test leads) to the connector side leading to the dash. With small pail of fuel I observed that the sensor worked as designed - with sensor submerged the light went out. Possibility 1) dismissed (C-Dan's part is vindicated !). That leaves the 10" wire, the connector, or something inside the tank. Using VOM I determined the cause.......
PUNCH LINE: There are five penetrations through the lid of the pump frame. These seal fuel from getting out - but allow the electrical wires to get into the tank. Using a VOM (Volt-Ohm Meter) I determined that THE PENETRATION TO THE TANK FOR THE (high side) FUEL LOW WARNING LIGHT WAS GROUNDED TO THE PUMP FRAME (lid), thereby effectively grounding the Fuel Low Warning Light and causing the low fuel warning lamp to stay lit !
I slightly twisted the offending penetration terminal back and forth 10 degtees and the ground went away! Upon reassembly, after a few days driving, the ground reappeared, indicating that the electrical penetration has deteriorated. I have oredered a new Pump Frame from C-Dan.
TAKEWAYS: 1) This same failure mechanism could occur on the terminal that is used for the varible fuel level sensor (fuel gauge) - hence making the indicated level incorrect. 2) I do not believe that this type of grounding would leave you stranded, i.e. it would not affect the Fuel Pump, which also uses the same type of electrical penetrations into the gas tank, but a a higher amperage. The reason for this is I believe the failure mechanism applies to low amperage circuits such as what the fuel sensor operate on.
Hope this helps you guys with fuel sensor problems - and keep you out of trouble with your wife-e-poos / significant others.
job.
