I was asked in another thread about if this was possible. Here is a simple diagram that will do just that:


Use: MC1741 or NTE941 or ECG941 part numbers for the ic. The VR is to adjust when you want the light to come on - note: this is fairly simple circuit and may trigger the LED as the fuel moves around in the tank. Someone can add in a capacitor/resistor between the input (fuel sender wire) and ground to add in a buffer of sorts - but I don't see this as being too much of a problem (as to why I did not stick on in).

enjoy...

[This message has been edited by Mickey_Moose (edited 01-18-2007).]

any more specifics on the ic 741? like where you can et it, and substitutes?

quote Originally posted by americasfuture2k: any more specifics on the ic 741? like where you can et it, and substitutes?

Original post edited to include cross reference # for the IC.

I know where I can get them locally, but as for where you can get them in the US, maybe someone else can help. Any electronics part store should be able to get it for you (it's a basic op amp).

[This message has been edited by Mickey_Moose (edited 01-18-2007).]

I had asked about that circuit quite awhile ago but I wanted to know what modifications were needed for the unit to light a bulb rather than an LED. I wanted to put a bulb in the "shift" position in the dash lights

It's a standard 941 op-amp. www.jameco.com has them far cheaper than NTE or ECG cross parts.

Most any op amp with the same general characteristics will work in the circuit. They should be less than a buck.


If you carefully drill your gague face you can pop a small round yellow led in a safe spot on the fuel gague and make it look almost 100% stock.

[This message has been edited by timgray (edited 01-18-2007).]

Where would you mount the circuit? Surely it can't go inside the tank? Yes, I know duh. lol

You guys are my heros.

I followed the link to Jameco. Looks like you could make this circuit for a couple of bucks. They have the op amp http://www.jameco.com/webap...0001&productId=24539 for 29 cents. One of these days, I'm gonna try building and installing this.

The LED is a 12 v one correct?

Dave, you'd mount this on the back of the instrument panel--you only tap into the fuel sender feed--right at the gauge.

quote Originally posted by Mickey_Moose: note: this is fairly simple circuit and may trigger the LED as the fuel moves around in the tank. Someone can add in a capacitor/resistor between the input (fuel sender wire) and ground to add in a buffer of sorts

Nice job.

This circuit uses a standard 741 operational amplifier (op amp) as a voltage comparator. You could also easily add a couple of resistors to introduce a little positive feedback into the circuit to add some setpoint hysteresis. This would eliminate most of the LED flicker due to fuel sloshing or fuel sender noise.

[This message has been edited by Marvin McInnis (edited 01-18-2007).]

THANK YOU .... added another "+" to your bar .....

Sweet. Someone needs to make these things and sell them!

The only problem is, my tank is actually empty when the gauge reads 1/4 full (no idea why) so I dont know if this would work with mine

quote Originally posted by domesticgal: The only problem is, my tank is actually empty when the gauge reads 1/4 full (no idea why) so I dont know if this would work with mine

Thats why the var. resistor is there, the circuit operates independently of what the guage (erroneously) reads.
------------------
Ol' Paint, 88 Base coupe auto.
Turning white on top, like owner.
Leaks a little, like owner.
Doesn't smoke, unlike owner

[This message has been edited by DtheC (edited 01-18-2007).]

I went to buy the parts this evening and they are rated by watts (!?) How many watts should the resistors be - 1/8, 1/4, 1/2, etc ?

1/4W should work...

quote Originally posted by domesticgal: The only problem is, my tank is actually empty when the gauge reads 1/4 full (no idea why) (

This may help fix that problem:
http://www.fieropride.com/tech/gas_guage.htm

quote Originally posted by 3800superfast: This may help fix that problem: http://www.fieropride.com/tech/gas_guage.htm

nice write up, if I hadn't already given you a plus I would now.

Great idea and write up, gets a + from me

quote Originally posted by DtheC: Thats why the var. resistor is there, the circuit operates independently of what the guage (erroneously) reads.

The circuit CAN read independent of the erroneous gauge. They are both getting the same feed from the in-tank sender. But the low fuel light circuit can be re-calibrated.

You probably are going to want to add some damping to the circuit. The in-tank sender moves around quite a bit and the stock fuel gauge does all the damping in the OEM set-up. Without damping this light is going to be blinking on and off quite a bit.

[This message has been edited by jscott1 (edited 01-19-2007).]

quote Originally posted by Phil: I had asked about that circuit quite awhile ago but I wanted to know what modifications were needed for the unit to light a bulb rather than an LED. I wanted to put a bulb in the "shift" position in the dash lights

...replace R3 and the LED with the bulb...

...others:

1/4 watt resistors are ok

You will probably want to mount this behind the dash or near the steering column or someplace close to the fuel gage so you won't have to run a lot of wire

As meantioned any standard op amp will work.

Does not matter if you fuel gage is off, you calibrate this using VR to set the point where you want to light to come on - or in other words, it does not care what the gage reads.

What kind of capacitor or resistor should be added in-line with the sender wire to stabilize the reading ? exactly what should I buy ?

quote Originally posted by PaulJK: What kind of capacitor or resistor should be added in-line with the sender wire to stabilize the reading ? exactly what should I buy ?

Done some thinking on this part and I would have to say to forget about adding in the resistor and capacitor to compensate for the fuel sloshing around, it as it would cause another problem. You would fix it so that the light did not come on under hard cornering, but without adding extra circuitry the light would come on every time you would start the car until the capacitor charged up, but I think that would be more annoying than it coming on under hard cornering (this would only really happen when the fuel was starting to get low to begin with).

However, if you wish, here is a modified diagram. Keep in mind this is just a 'simple' circuit and does not take some things into consideration (power up times, etc). Note: these are electrolytic capacitors have have a polarity.

[This message has been edited by Mickey_Moose (edited 01-20-2007).]

Bump, to add to my Fav's !

Here's more info inan archived thread.
https://www.fiero.nl/forum/A...050818-2-059238.html
I've had the old GM board(Buick Regal) working in mine for years! V8s' eat gas.... gas stations' are a required stop on every outing!
best,
Sandy

[This message has been edited by cowans (edited 02-05-2009).]

Thanks ! ++

i think i have a thread floating around somewhere that i made relating to this in every way possible.....*i think*

if so, its a floater! haha

EDIT: floater found....ew.... add empty your wallet (low gas) light to fiero

------------------
First LX9 Fiero GT, 1987
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
My Fiero Fuel Economy
Videos of My GT
Google Videos of My GT
MPG Display for OBD I

[This message has been edited by americasfuture2k (edited 02-05-2009).]

hmm....doesnt look like anyone has been biting at this one.

anyone made one? if so, how well does it work???

------------------
First LX9 Fiero GT, 1987
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
My Fiero Fuel Economy
Videos of My GT
Google Videos of My GT
MPG Display for OBD I

made one along with my Mpguino. they work great on the bench. gotta wait till spring to install and then probably early summer before I run through a full tank to see how they work.

I'm actually thinking of upgrading the light with a PIC to give it more buffering and two levels. 1 steady on at point A and a blink at a lower setting to really get the attention. Low gas light and a call a towing service light in one!

going off topic for a post....

since my altenator is the newer kind that takes a 5v constant from the PCM to start it charging, i made a 5v regulator out of a 7805 circuit. just to know that the circuit is working, i have a red LED at the voltmeter rally gauge. i was going to use a green one for something, but i dont remember what it is now haha.

back to our regularly schedualed post.....

------------------
First LX9 Fiero GT, 1987
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
My Fiero Fuel Economy
Videos of My GT
Google Videos of My GT
MPG Display for OBD I

tim, get the MPGuino and the low fuel light installed yet? id like to see some details and pics of your low fuel light.

------------------
First LX9 Fiero GT, 1987
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.....
My Fiero Fuel Economy
Videos of My GT
Google Videos of My GT
MPG Display for OBD I

I built the circuit that Mickey_Moose posted but I couldn't seem to get it to work.
I did some measurements and some quick and dirty mods to it and it seems to work good now for the (0 ohms = 0 volts = Empty, 90 ohms = +12volts = Full) gauge system that the Fiero has.
I put it in my 85 2M6 SE and it works pretty good. I am able to set it just about perfect with very little problems with false readings due to fuel slosh.

Thanks to Mickey_Moose for the original schematic.

Here is the schematic of my mods to his original circuit.


Any input into improving it or about redundant components would be appreciated.

Cheers,

Sidecar 2M6 SE

[This message has been edited by Sidecar 2M6 SE (edited 04-09-2009).]

quote Originally posted by Sidecar 2M6 SE: I built the circuit that Mickey_Moose posted but I couldn't seem to get it to work. I did some measurements and some quick and dirty mods to it and it seems to work good now for the (0 volt=empty, +12volt=full) gauge system that the Fiero has. I put it in my 85 2M6 SE and it works pretty good. I am able to set it just about perfect with very little problems with false readings due to fuel slosh. Sidecar 2M6 SE

Thanks for working on this. I have a low fuel light built into my HUD but I never use it, because I consider 1/2 to be empty anyway. Ever since going through two hurricanes when you couldn't buy gas I've been traumatized as I don't like driving with less than half a tank.

quote Originally posted by Sidecar 2M6 SE: I built the circuit that Mickey_Moose posted but I couldn't seem to get it to work. I did some measurements and some quick and dirty mods to it and it seems to work good now for the (0 volt=empty, +12volt=full) gauge system that the Fiero has. I put it in my 85 2M6 SE and it works pretty good. I am able to set it just about perfect with very little problems with false readings due to fuel slosh. Thanks to Mickey_Moose for the original schematic. Here is the schematic of my mods to his original circuit. Any input into improving it or about redundant components would be appreciated. Cheers, Sidecar 2M6 SE

Thought the Fiero gas gauge worked on a 0-90 ohm system

quote Originally posted by Phil: Thought the Fiero gas gauge worked on a 0-90 ohm system

It does. What on this drawing suggests otherwise?

hes talking about that 0v - 12v. that lost me as well. what fuel level does the light come on at? or is that adjustable to set at your desired fuel level?

------------------
First LX9 Fiero GT, 1987 | My Fiero Fuel Economy | MPG Display for OBD I
Youtube Videos of My GT | 87 GT Inside Test Run | 3500 Fiero GT Test Run 01
3500 Fiero GT Test Run 02 | 3500 Fiero GT Test Run 03 | 3500 Fiero GT Test Run 04
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.....

quote hes talking about that 0v - 12v. that lost me as well. what fuel level does the light come on at? or is that adjustable to set at your desired fuel level?

Yep, it can be adjusted to light up in about 1/8 of a tank increments from Empty to Full on the gauge.

The stock Fiero Fuel gauge circuit works as 0 ohms Empty and 90 ohms full as well as 0 volts Empty and +12 volts Full
*** (When the fuel gauge is connected and the circuit is powered up and ignoring any voltage drops across the Empty and Full coils in the gauge) ***

Here is why:

(Short Answer)

0 ohms at the wiper in the tank sending unit = 0 Volts on the sender to gauge wire. When measured between this wire and GND

90 ohms at the wiper in the tank sending unit = +12 volts on the sender to gauge wire. When measured between this wire and GND


(Long Answer)

If you take a look at the scan of the Fiero's Fuel gauge schematic below you will see that......

When the wiper of the sending unit variable resistor is at Empty it is at 0 ohms when measured to GND, it is also at 0 volts when measured to GND.

When the wiper is at Full it is at 90 ohms when measured to GND and because it is connected to the Empty coil in the gauge it is also at +12 volts when measured to GND

The Empty and Full coils in the gauge are connected in series across +12 volts and GND.
The center of this series circuit is connected to the wiper in the sending unit which slides across
a 90 ohm resistor coil that has one end connected to GND and one end open.

The Empty coil in the fuel gauge has one side of it connected to +12 volts and the other side is connected to the wiper in the sending unit.
When the tank is Empty the wiper is connected to GND, current flows through the Empty coil and the needle in the gauge moves to Empty.
The Full coil is shorted to GND by the wiper and no current flows through it.

This indicates..... Empty = 0 ohms and also 0 Volts at the wiper in the sending unit. When measured to GND

The Full coil in the fuel gauge has one side of it connected to GND and the other side connected to the wiper of the sending unit.
When the tank is Full the wiper is connected to 90 ohms above GND.
The current flowing in the Full coil is much greater than the current flowing in the Empty coil because the Empty coil is now in series
with the 90 ohm variable resistor in the sending unit in the tank and the needle moves towards Full.

This indicates..... Full = 90 ohms and also +12 volts at the wiper in the sending unit. When measured to GND

This is how the Fiero's Fuel gauge circuit works as far as I can understand it does any of this make any sense????.... at all!?!?

Cheers,

Sidecar

[This message has been edited by Sidecar 2M6 SE (edited 04-16-2010).]

quote Originally posted by americasfuture2k: hes talking about that 0v - 12v. that lost me as well.

My much shorter answer is that V = I x R (where V = voltage, I = current, and R = resistance)

Or Voltage is directly proportional to the resistance. So 0 - 90 ohms is equivalent to voltage....

However, you have a point that it's not 0 to 12 volts. The voltage reads some numbers between 0 and 12 volts.

[This message has been edited by jscott1 (edited 04-10-2009).]

quote My much shorter answer is that V = I x R (where V = voltage, I = current, and R = resistance) Or Voltage is directly proportional to the resistance. So 0 - 90 ohms is equivalent to voltage.... However, you have a point that it's not 0 to 12 volts. The voltage reads some numbers between 0 and 12 volts.

I was trying to explain it to myself too as I was writing. I guess it shows, eh I hope I didn't break any of Ohm's Laws

What do you think jscott1?
Is the Variable Resistor in the tank sending unit linear or logarithmic?
I'm guessing linear but I'm just a self taught hack. That's why I became so long winded in the post

quote Originally posted by jscott1: My much shorter answer is that V = I x R (where V = voltage, I = current, and R = resistance) Or Voltage is directly proportional to the resistance. So 0 - 90 ohms is equivalent to voltage.... However, you have a point that it's not 0 to 12 volts. The voltage reads some numbers between 0 and 12 volts.

ohm's law. and everything i sorta remember learning in A-school ( navy, im an electronic tech. with memory loss )

now that its explained how 12v plays into this, it makes sense.

im guessing this circuit is based off the 0 - 90 ohms. so it would only work with other fuel level senders that work off the 0 90 ohms. im hoping ithat is mostly other GM vehicles of the fiero's era. such as merlot566jka's 86 diesel burban.
------------------
First LX9 Fiero GT, 1987 | My Fiero Fuel Economy | MPG Display for OBD I
Youtube Videos of My GT | 87 GT Inside Test Run | 3500 Fiero GT Test Run 01
3500 Fiero GT Test Run 02 | 3500 Fiero GT Test Run 03 | 3500 Fiero GT Test Run 04
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.....

[This message has been edited by americasfuture2k (edited 04-10-2009).]

quote Originally posted by Sidecar 2M6 SE: Is the Variable Resistor in the tank sending unit linear or logarithmic?

The tank sending unit uses either a wire-wound resistance element or an equivalent ceramic element, so it's linear or nearly so. But the geometry of the float arm means that it moves through about a 90 degree arc from empty to full, so its movement at the sensor element is not linear at all with respect to fuel level. Adding even more nonlinearity, the shape of the tank itself means that fuel level, even when measured accurately, does not directly correspond to fuel volume.

It might be possible to describe these nonlinearities mathematically, but it's just not worth it for a typical mechanical fuel gauge. The only two places you really need a fuel gauge to be accurate are 1) full, and 2) a known amount (one or two gallons) before empty. If you want to be really anal about it, the best way to optimize gauge accuracy would be to empty the tank, carefully measure sender output as you add measured amounts of fuel, and then create a custom gauge face calibrated to your exact data. (If you measure carefully, you will find that many of GM's OEM gauge faces are not linear, probably for this very reason. It's a cheap and simple way to deal with sensor and/or gauge nonlinearities.) For an accurate digital fuel gauge, you would probably need to replace the OEM fuel sensor with something like a capacitance-based sender and then run its output through a calibration lookup table.

(Is this horse dead yet?)

[This message has been edited by Marvin McInnis (edited 01-18-2010).]

I'd rather go the route with a fuel flow sensor(s) and electronics. I had one of these on my Camaro back in the '80's and it was really nice. It could display instantaneous and average fuel economy, distance, fuel used or fuel remaining in gallons, etc. Sadly my unit's fuel flow sensor died and it wasn't the two-sensor unit anyway, otherwise it'd be on my Fiero right now. I still have it for sentimental reasons:



JazMan

quote Originally posted by Sidecar 2M6 SE: What do you think jscott1? Is the Variable Resistor in the tank sending unit linear or logarithmic? I'm guessing linear but I'm just a self taught hack. That's why I became so long winded in the post

EDIT - I should have read Marvin's answer first, he said the same thing I did earlier.

Every fuel gauge in every car I've owned has had very non-linear behavior

I suppose you could take some measurements on the bench and see how it responds. When I had one out I played with it and it was hard to tell one way or the other.

However, even if the sender itself is linear, the geometry of the tank is not symmetrical so some three dimensional integrals would be needed to see how movement of the sender corresponds to quantity of fuel in the tank. Given all the nuances I would say the likelihood of it being a linear equation is about zero.

So the bottom line is that the sender is going to respond in a non-linear way to quantity in the tank and you could maybe compensate electronically, but not with a simple analog circuit. You would need a lookup table to be able to calibrate it properly.

[This message has been edited by jscott1 (edited 04-10-2009).]