HI ,
I designed a Board to make most of my 75 Camaro Gauges interfaced to a digital lcd and some 7Segments ,
but unfortunately i'm facing some problems now.
Here is the schematics and pcb i am using :
(sorry the pictures are too large)




The Fuel level sensor is a 5 to 90 ohms pot. and i have seried it with a 50 ohm
resistor as you can see in the schematics above. but the problem is when i am reading the adc in home every thing is OK
and if i short circuit the input of ADC to ground it will read 45 ,but when i am using it in the car if the Fuel sensor is
10 ohms it will read 55,45,35,20,10 !!! . i even tested it with a multimeter and verified the voltage is stable. this occurs both
when the engine is running and when the engine is not running but the switch is open .(the board powers on whenever the switch opens)

and the second problem is if i connect the Generator to the board ,stable 5 volt boost up to 6.2 volt !(when the engine is running)
but this doesn't happen when i connect the Oil pressure sensor . i checked the Gen signal with multimeter
and all i see is that it connects to ground if the Generator is not running and it is float when the Generator is
running.

Some explanation about the board and schematic design :
-i have read the ADC grounding and ground loop here but don't know my design is a good one or not !.please tell me.at least i used a .1u .
-i add the 5.6 v zener diodes to input of the controller for the protection if +12-14 volt signal has applied.
-the pullup values for the Oil and Gen is 47 ohm and i used it because i doubt a 1-100k ohm will
make a good load .

Now i'm confused ! and beacuse i disattached the 30 year old gauges from the car and i have to carry
my multimeter everywhere and check my fuel level !.

Any Help or tip would be appreciated.
Farshid J.H.
(Sorry for my bad english)