Low Fuel Warning Ohm's Specs? - Team Camaro Tech
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post #1 of 27 (permalink) Old Jul 12th, 11, 05:30 AM Thread Starter
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Low Fuel Warning Ohm's Specs?

Does anyone know at what Ohm's setting a factory 1969 Low Fuel Warning Module will activate and ground the yellow wire to activate the FUEL idiot light? I read here or on CRG that 67's Low Fuel Warning Module actually sent voltage to the idiot light, but 68-69's grounded to activate, but no one posted at what Ohm's for sure the switch activates. Some said 45 (aprox 1/2 a tank) others 22 or quarter and others 11 (aprox 1/8 a tank).

I understand the module regardless of year is a self contained unit and independent of the gauge settings and it sniffs the fuel tank sender lead for Ohm's signal strength and then acts on its own findings.

Everett in theory can you add an additional Ohm's resistor to the green or the red lead on the module to dial in a better "on" point by tricking it with a manipulated Ohm's reading? I say that since the vehicle was designed when:
a.) Premium Gasoline cost $0.35 a gallon.
b.) There were at least three Gasoline Stations - Full Service ones at that on any given street corner.
Now it is miles to the nearest self-service station and the additional hit on gas mileage from pulling a trailer of cash to pay for the fill the 18 gallon tank means you need to carry a full Gerry Can in the trunk just in case you don't make it.

I would like to be able to have the light activate at 22 Ohm's or about 1/4 tank. Maybe even mount a rheostat in the glove-box so on longer trips I could dial it back to half a tank warning if traveling out in the sticks.

Thanks in advance
Brian


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post #2 of 27 (permalink) Old Jul 12th, 11, 07:52 AM
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Re: Low Fuel Warning Ohm's Specs?

Mine starts flickering on corners around 1/4 tank, and comes on solid around 3/16 to 1/8 of a tank. The sender is a 0 to 0 ohm sender, 0 at empty, 90 when full is it linear, probably not because the float moves linearly (up and down) but the resistors in the gauge are in an arc, so the middle section of the gauge porbably has a greater resistance change per inch of float movement up and down, plus add the tapered shape of the tank into the equation and that really screws up any linearity. Your bet bet is to wait till your low fuel light comes on, disconnect the sender in the trunk and measure the resistance, then when it gets to the level you want it to come on measure it again. Add a resistor that equals the difference in the two readings to the low fuel sensor lead to the fuel gauge.

So if yours is set to activate when it sees 45 ohms, and you want it to go off when the level sender reads 22 ohms (1/4 tank assuming the sender is linear) you need to put a 23 ohm resistor in line to make it go off at 1/4 tank. If it activates to low you need to add a resistor in parallel to the low fuel light sensor lead so you are lowering the signal value the low fuel module sees, and thereby raising the level in the tank where the light comes on. Don't know what adding resistors to the low fuel module lead would do, if anything, to your actual gauge reading.

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post #3 of 27 (permalink) Old Jul 12th, 11, 09:30 AM
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Re: Low Fuel Warning Ohm's Specs?

You could do a s Mark suggests.
I would like to see a module schematic.

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post #4 of 27 (permalink) Old Jul 12th, 11, 04:40 PM
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Re: Low Fuel Warning Ohm's Specs?

If i am reading this correctly. putting a resister in line would go the other direction. You need to lower the resistance value to bring in light earlier?
Thinking on this, i think you have a delema. My car has single wire coming from tank, don't know how you would redue circuit without messing up actual guage reading either. My light flashes @ 3/8 tank. What MPG do you thing you get?
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post #5 of 27 (permalink) Old Jul 12th, 11, 05:25 PM
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Re: Low Fuel Warning Ohm's Specs?

Nope, just the opposite. The low fuel module is looking for a specific resistance value to switch the output to ground. Say its supposed to be 20 ohms (just under 1/4 of a tank), but it is coming on at a higher level, say 45 ohms ( 1/2 a tank). You have to fool the low fuel sender into seeing 45 ohms when the level gets to 1/4 tank (or whatever level you want it to come on at), which would normally be 20 ohms (or less) and the light would already be on. By adding a 25 ohm resistor to the green wire going to the low fuel sender it won't see the 45 ohms setpoint it need to turn on the low fuel lamp until the sender is putting out 20 ohms.

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post #6 of 27 (permalink) Old Jul 12th, 11, 06:14 PM
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Re: Low Fuel Warning Ohm's Specs?

Does the fuel sender unit in tank have less resistance as tank empties?
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post #7 of 27 (permalink) Old Jul 12th, 11, 06:36 PM
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Re: Low Fuel Warning Ohm's Specs?

Yes, 90 ohms full 0 ohms empty.

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post #8 of 27 (permalink) Old Jul 12th, 11, 07:13 PM
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Re: Low Fuel Warning Ohm's Specs?

I am still not convinced. To me if light needs 20 ohms or less from sender unit to be on. And say you want it to come on at a fuel level above this then the resistance value of sender is going to be above the 20 ohms which in turn will keep light off. Putting resister in line will add resistance farther above turn on ohms of 20 or less. I also think that if you place the resister somehow in series with the low fuel unit itself the correct current thought the designed circuit will be to low until you over come added resistance to turn light on. Tank sender resistance will have to be below normal value,say like 10 or less to create the added current level. My thinking design would be the sending unit would have to be made with less resistance according to level and place a resister in line with the fuel gauge only that would equal original value needed to read correctly. Lets say 1/2 tank was 20 ohms from new sending unit. Light would come on. Trying to have light come on earlier would need low fuel module redesign.IMO. I took one apart and there was transisters and diodes and some resisters. What ever is right you are making me think (ooch)

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post #9 of 27 (permalink) Old Jul 12th, 11, 07:17 PM Thread Starter
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Re: Low Fuel Warning Ohm's Specs?

Quote:
Originally Posted by red67L78
If i am reading this correctly. putting a resister in line would go the other direction. You need to lower the resistance value to bring in light earlier?

Thinking on this, i think you have a delema. My car has single wire coming from tank, don't know how you would redue circuit without messing up actual guage reading either. My light flashes @ 3/8 tank. What MPG do you thing you get?
Red67L78 the process would be to disconnect the green lead from the back of the gauge, run a lead from the post on the gauge to the variable Ohm's reostat or to a static resistor of a given ohm's rating then output that to the green wire on the Low Fuel Warning Module.

I think I was getting 8 to 12 city to highway when I last checked and was daily driving her.


Quote:
Originally Posted by Mark C View Post
Nope, just the opposite. The low fuel module is looking for a specific resistance value to switch the output to ground. Say its supposed to be 20 ohms (just under 1/4 of a tank), but it is coming on at a higher level, say 45 ohms ( 1/2 a tank). You have to fool the low fuel sender into seeing 45 ohms when the level gets to 1/4 tank (or whatever level you want it to come on at), which would normally be 20 ohms (or less) and the light would already be on. By adding a 25 ohm resistor to the green wire going to the low fuel sender it won't see the 45 ohms setpoint it need to turn on the low fuel lamp until the sender is putting out 20 ohms.
Mark that's what I was thinking would be possible.


Ok so its been a while since electromechanical engineering overview in school so follow my logic and see if I have it right.

The ohms added to the actual sender output = the real set-point of the module and drop a ground to light the light.

Example given:
Say the module has got a real set-point of 45 ohms 1/2 a tank, but I want to see it at 22 1/4 tank I would need to send 67 Ohm's to the Low Fuel Warning Module to prevent it grounding at 45 Ohm's. Later when the tank dropped to 1/4 and the sender sent 22 Ohm's to the gauge it would correctly show 1/4 full, but the Low Fuel Warning Module would see 45 Ohm's and drop the ground to light the idiot light.

So the more Ohm's in resistance added to the module delays the ground drop to activate the light.

But If the module has a true trigger set-point at 22 Ohm's and I want it to drop the ground at 1/2 a tank 45 Ohm's I would need to cut the resistance Ohm's the module sees from the sender by 50% - so tank sender is sending 45 for 1/2 full, module sees 22 and drops ground.

In theory running a second module rated at 22 Ohm's set-point in a parallel circuit connection would do the trick, but there must be another solid state device that would half the Ohm's. Maybe a rheostat set to 20 Ohms?

Ok my head hurts.


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post #10 of 27 (permalink) Old Jul 12th, 11, 08:07 PM
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Re: Low Fuel Warning Ohm's Specs?

Around 8-10 mpg is tough. My 427 engine got 10 mpg and you had to be careful. Low fuel light was a must. One of the best factory features for a 40 year old car. Now my 396 gets 13 to 15 mpg. Believe me it was a total supprise coming from a true 375 hp engine. My engineering bro said it had to do with smaller bore size difference and torq/hp rpm(3000) Last road trip was 300 miles and only had to fill up once after leaving home with full tank. Worked out around 14.6 mpg. Did not realize that 4 mpg better made driving so stress free. Also with premium price and not stopping as much is like a new day. Is there something you could do to increase MPG? I think that is the key for long trips.

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post #11 of 27 (permalink) Old Jul 12th, 11, 08:45 PM Thread Starter
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Re: Low Fuel Warning Ohm's Specs?

I did very little highway travel when she was a daily driver, mainly stop-n-wait and go traffic. When I did highway I'd say in wide open unpopulated areas I wasn't going 55. And whats the point of having Cowl Induction hood that works if you don't "test" is now and again?

She has never been tuned by me and I never did a real scientific calculation of my MPG, just loose est based on my wallet.

I replaced the original factory q-jet in 83 with Chevy service q-jet which is listed for both a 69 Camaro SS and a 69 Vette sure that has economy settings. LOL She's a L48 with M21 and 12 posi - could change things up but going to leave it as factory.


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post #12 of 27 (permalink) Old Jul 13th, 11, 03:15 AM
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Re: Low Fuel Warning Ohm's Specs?

By the posts I have been reading, I never had a low fuel module on the Camaro's Wife & I owned, I would guess the LFM runs in parallel to the fuel gauge and turns on a light.

The LFM acts like a differential amplifier, meaning two inputs, one input is 'set' for a reference, read as selecting a resistor or resistive network, say 25 ohms, stock shelf value, and this input side of the amp sets and conducts into a resisitve load. As fuel is used, lower resistance is sensed and compared to the set input. If input resistance is lower than the reference, amp changes state to the other input side and the light lights.

So, if this is the circuit used, one would have to change the reference (set) point internally of the diff amp to change state.

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post #13 of 27 (permalink) Old Jul 13th, 11, 03:54 AM
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Re: Low Fuel Warning Ohm's Specs?

American Autowire explanation:
http://www.americanautowire.com/faq/?p=1024

How does the 1967-69 Camaro low fuel indicator work?
Filed Under: Connections, Frequently Asked Questions, Tech Tips
The low fuel indicator is a small round plastic can containing four wires that is in the gauge package. It is not crucial to the operation of the gauges. In fact, many gauge clusters removed them when they were not functional. The module plugs into the fuel gauge for power, signal, and ground. There is a fourth yellow wire that comes from the module that goes to the dash through the console harness and the dash harness console extension to activate a light in the instrument cluster. However, here is where the problem arises.


The 1967 low fuel module operates by sending a 12 volt power signal on the yellow wire to a single contact bulb mounted in the dash cluster underneath the circuit board. The bulb has grounding tangs that ground the bulb through the instrument cluster body.

The 1968 low fuel module operates by sending a ground signal on the yellow wire to one of the 12 pin instrument cluster connector contacts. This subsequently lights a low fuel indicator bulb through the circuit board connection on the back of the cluster. This circuit board is different for a warning light versus a factory console gauge car.

The 1969 low fuel module operates by sending a ground signal on the yellow wire to a bulb located in the instrument cluster light pods at the bottom of the dash.

The low fuel modules are not interchangeable.

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post #14 of 27 (permalink) Old Jul 13th, 11, 05:17 AM Thread Starter
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Re: Low Fuel Warning Ohm's Specs?

Yes Everett there is a reference set-point in each module, and one would hope that the set-point was stamped or coded on each module but its not that I have been able to find or document. And everyone seems to have a different idea of when the light activates - frustrating. Now compound it with question how do you recognize at 67 version from a 69?

They are installed in parallel to the gauge circuit, and in theory you can install in any vehicle, anyplace you can access the tank sender tan wire to sniff the ohms. So regardless of your vehicle having optional gauges, the 69's all have the fuel light on the dash pod, but I think 67 and 68 you have to change the printed circuit to one that has optional gauges which may also involve switching from idiot light to gauges for everything else, since I think the oil idiot light space is used for fuel? But in 69 its easy, and for a non factory look 67 & 68 you could rig your own bulb and just put a small led or bulb on around the dash, or hook up a small chime to ding - but might drive you nuts. You could use either Low Fuel Warning Module - a 67 - sending voltage to bulb or - a 68-69 dropping a ground to a live line to activate the bulb.

There does seems to be a distinct line in the sand when it comes to original units of the period. They seem to come in two versions. One with thin gauge wires (what I have) and one with regular wire gauge. Some folks argue that the regular gauge is for Vette and Nova with larger capacity fuel tanks, thin wires for Camaro with 18 gallon tanks. One dropping the ground to activate at 45 ohms the other at 22 ohms. But which is which?

After market replacement modules seem to be all over the place in design and set-points as well. Ranges seem to be 1/8 tank to 1/2. It would be nice if someone manufactured a replacement module that had a screw set-point pot so it could be custom set by the owner.

I guess I could ask one of my more electrical mechanical engineering buddy's to design and build me a custom unit since I lack the design and electrical component device (i.e. capacitors vs didoes vs resistors - why and when you use one over the others etc) knowledge to build a module myself.

Anyone have any internal documentation on the originals design, markings etc? Anyone have an after market that came with paperwork with diagram or schematics?

How about a dog-and-pony show of what your modules look like?
Post a decent photo:
Post your year:
what you think your set-point is: i.e. 1/8, 1/4, 1/2 tank
(including those of you who say "never on" or "always on")
Does it flash or Blink with a predictable pulse or solid on when activated?:
(one or two second intervals, not to be confused with flickering as gas sloshes in the tank on cornering, acceleration, deceleration)
Original or replacement Module?:
If original any factory markings or date stamps?
If replacement make and model or vendor you purchased from if known.

That should cover a database


We could put something together as a trouble shooter here and send it over to the guys at CRG if we get enough documentation.

Brian


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post #15 of 27 (permalink) Old Jul 13th, 11, 05:29 AM Thread Starter
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Re: Low Fuel Warning Ohm's Specs?

Low Fuel Warning Module Database Entry

Post your year:
1969

what you think your set-point is: i.e. 1/8, 1/4, 1/2 tank
(including those of you who say "never on" or "always on")
1/4

Does it flash or Blink with a predictable pulse or solid on when activated?:
(one or two second intervals, not to be confused with flickering as gas sloshes in the tank on cornering, acceleration, deceleration)
Solid on

Original or replacement Module?:
Original

If original any factory markings or date stamps?
If replacement make and model or vendor you purchased from if known.
No markings, no stamps - has clips on top to mount inside the hole in rear of saw tooth gauge cluster carrier.
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