looked it up, so far wioki has said it lights up when it was at 4 gallons. going by the common GM 90Ω at full, it would mean it comes on at (i cant think of the math right now to figure it out)Ω . its really simple. its ona paper film kinda ribbon cable and not the plastick kind. it has 3 resistors and a diode. i have yet to look at it more to figure out what all the parts are. but it was connected to all the electrical hookups for the guage itself. im guessing power, ground and signal. pics will soon follow...

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First LX9 Fiero GT, 1987 | My Fiero Fuel Economy | MPG Display for OBD I
Youtube Videos of My GT | modernize your fiero with technology!
If you can't fix it with a hammer, you've got an electrical problem
Fiero's are people too. We pay just as much attention to them, if not more than our loved ones
screw paying those bastards. im not going to become rich by paying for things.....

since i seem to be getting so many PM's about my hitch, ill answer right here
the trailor hitch/bike rack hitch jack, lifting and resting points...

aaaand PICS!

Do you know how this is wired in? Would be a cool update for sure.

i know it was attatched tot he 3 terminals that drive the needle. ive figured out which one was ground. that lead from the gauge travels down to the diode, thru the diode and the the cathode of the LED. now to figure out which is (+) and signal. easiest way to be sure would be to pull of a schematic of the 78 LeSabre fuel gauge and see what terminals are what. anyone have an unlimited alldata account or mitchell on demand?

Looks like it just works as a voltage divider with the tank level sender. Voltage gets high enough to overcome the diode drops and power the LED, and it comes on. About as simple of an indicator circuit as you could build, only problem is there's no hysteresis so as gas sloshes back and forth the light will be going on and off, on and off.. Probably starts dim and gets brighter as it hovers around the diode's forward voltage too..

Would be better to set it up with a comparator so you could build in some hysteresis, and make it adjustable.

Never seen a paper printed circuit before, cool!!

EDIT: nevermind, i drew it right.

[This message has been edited by americasfuture2k (edited 03-15-2011).]

Why use two when one will do? I mean why did GM put two 280 ohm resistors in parallel when one 140 ohm would have done the same thing??? Nevertheless, thanks for posting this. I think I could build one easily enough. You'd have to make sure to keep total current through the LED at 20ma or lower. You could use it with just about any car if you knew the fuel sender's resistance. Thanks again for posting this.

PS I know GM probably wanted to use standard resistor values but a company with their bucks could have had zillions of 140 ohm resistors made cheaply.

quote Originally posted by starlightcoupe: Why use two when one will do? I mean why did GM put two 280 ohm resistors in parallel when one 140 ohm would have done the same thing??? Nevertheless, thanks for posting this. I think I could build one easily enough. You'd have to make sure to keep total current through the LED at 20ma or lower. You could use it with just about any car if you knew the fuel sender's resistance. Thanks again for posting this. PS I know GM probably wanted to use standard resistor values but a company with their bucks could have had zillions of 140 ohm resistors made cheaply.

Maybe it had to do with the wattage? As in finding a 140 ohm that would handle the wattage.

quote Originally posted by starlightcoupe: Why use two when one will do? I mean why did GM put two 280 ohm resistors in parallel when one 140 ohm would have done the same thing???

Simple... wattage. 2 in parallel = 2 x wattage. Why? Likely affect the machine to make it... Easier to load the machine with same diameter and max length part. That way all component get loading on same spool. Need only 2 pick machine at max...

quote Originally posted by Riceburner98: About as simple of an indicator circuit as you could build, only problem is there's no hysteresis so as gas sloshes back and forth the light will be going on and off, on and off.. Probably starts dim and gets brighter as it hovers around the diode's forward voltage too.. Never seen a paper printed circuit before, cool!!

Was made to be cheap... late 70's many part cost more. Paper flex is likely cheap to...
Blinking light for gas is a plus... annoying but you won't ignore it. Many will run out even with light on...

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

quote Originally posted by theogre: Was made to be cheap... late 70's many part cost more. Paper flex is likely cheap to... Blinking light for gas is a plus... annoying but you won't ignore it. Many will run out even with light on...

me and my brother (merlot566jka) were thinking of a way to do this. i still want to buy and copy one of those ones commonly used in the grand nationals. then either work with that circuit, the one i posted or build one from scratch with a 555 timer to make it blink faster the more empty you get. the GN one already gets brighter as resistance in the tank lowers. i was even saying make it work the dome light. give yourself or your passengers a seizure, but hey, at least you know you need to fill up haha

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First LX9 Fiero GT, 1987 | My Fiero Fuel Economy | MPG Display for OBD I
Youtube Videos of My GT | modernize your fiero with technology!
If you can't fix it with a hammer, you've got an electrical problem
Fiero's are people too. We pay just as much attention to them, if not more than our loved ones
screw paying those bastards. im not going to become rich by paying for things.....

since i seem to be getting so many PM's about my hitch, ill answer right here
the trailor hitch/bike rack hitch jack, lifting and resting points...

[This message has been edited by americasfuture2k (edited 03-16-2011).]

terminal left to right.... sender, g, + is my guess.

What you thing is G above is likely wrong... Is obvious yes but wrong.

My guess again... Second diode is a backup so LED fails in short it can't effect gauge.



If LED out is ground... 52 ohm goes to sender


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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

[This message has been edited by theogre (edited 03-16-2011).]