Gas Gauge / Sending Unit

Are you sure it is not strictly a mechanical thing?

Is the tank OEM or aftermarket?

In is the tank mounted in exactly the same plane as on the newer model?

It is not in the same plane as original. I have had the sending unit out and made sure that it does work and is functioning as intended. It does seem that the only problem is the resistance difference between the unit and gauge.

to increase or decrease the Resistance in the line you could put a variable potentiometer in line ie cut the wire to the gauge and put one wire on the centre tag of the pot and either one of the other tags connected to the other cut wire, you could the vary the resistance, By the way as i have never tried tried this i would suggest you connect the potentiometer up temporally at first, also if it works when you have the setting correct you could measure the pots resistance after you take it out of circuit and replce the varible pot with a fixed resistor.

just an example do not use the values here.

Potentiometer, single turn, 5 kΩ, linear taper (Radio Shack catalog # 271-1714)
Small "hobby" motor, permanent-magnet type (Radio Shack catalog # 273-223 or equivalent)

For this experiment, you will need a relatively low-value potentiometer, certainly not more than 5 kΩ.

LEARNING OBJECTIVES

Rheostat use
Wiring a potentiometer as a rheostat
Simple motor speed control
Use of voltmeter over ammeter to verify a continuous circuit

SCHEMATIC DIAGRAM

ILLUSTRATION

CONNECT WIPER TERMINAL TO GAUGE AND EITHER ONE ONLY OF THE TRACK TAGS/ TERMINALS

Standard Variable Resistor Photograph © Rapid Electronics

Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available.

CR4 Admin: Modified Post

Copyright Violation: Reduced copied text according to copyright law. Please see Section 13 of the CR4 FAQ about posting copyrighted material.

http://www.kpsec.freeuk.com/components/vres.htm

i would suggest you try a 500 ohm varible pot first

Thanks for the great explanation and examples. This is definitely looking like the way to go.

Thank you for your time and expertise.

Just to add to my first post, after reading some of the other replies i though i had better add this.

The readings from most fuel gauges are non linear, in that the reading is not continuous as you might expect the sender unit if opened will i am sure reveal the resistance track is different at one end to the other.

So adding a fixed resister will not provide and accurate representation of quantity of fuel.

So the using another's response it would be wise to mark gauge empty with a reserve amount left then half fill and mark that then full etc.

although it may be better to get correct sender unit

Couple thoughts:

1) I there any chance of replacing the gage with one that matches the year/model of the sending unit?

2) I like the idea of using the pot, although I think I would use a small trim pot (range 0-25Ω to start), because of the values you gave in your question are small and the difference is also quite small.

Okay...three thoughts

3) You may want to make the adjustments to the zero or empty side of the gage with the tank empty or as close to empty as you are comfortable. You may never get the range or "span" correct and it should be more important to know when you are about to run out of gas than when you are full.

No chance of replacing the gage as it is the original but updated to modern specs (except for the resistance).

I think the potentimeter is probably the best way to go.

I like the 3rd idea of setting it at zero with a reserve left in.

Thanks

Back when I could afford the addiction, I purchased a gauge tester from Snap-On. One of the things it could do was dial a specific amount of resistance into itself. Then the tester was connected between the gauge and ground. It came with a chart to tell you exactly how many ohms it would take for a specific make-model-function gauge to read at each line that the factory put on each gauge. If you know what the new sending unit came out of, I may be able to tell you what the two different fuel sensors used as a range for proper gauge readings. Just adding additional resistance or lowering resistance will not give good results. If you originally needed 25 to 137 ohms resistance for normal gauge movement, and the new sender runs from 10 to 50, it would seem that adding 87 ohms to the wire would give you a full reading. But at half full, you will have 25 on the new pick up plus the added 87, which will give you 112 ohms. That will read well over 3/4s. A better solution is to find a sender that has the correct electrical resistance, and fitting it to the pick up that fits your new tank. I will go look for the gauge tester and get back. Good luck.

Bob,

Since this was a "custom" installation done to the truck by another individual I don't have any idea what the sending unit is from.

Thanks,

Chuck

I haven't played around in this area for many, many years, but it should be quite easy to fix.

The problem as I see it is that the gauge appears to be designed to work with 6 volts and using it on 12 volts, will probably cause too much current to flow for the good longterm life of the display unit.....especially when the tank is full.

As the gauge unit is 36 ohms, may I suggest a resistor in series also of 36 ohms as a minimum, as someone else suggested a pot would be cool BUT a) it must be Linear not Logarithmic, b) Wirewound is a must as the currents that flow are too high (up to 170 ma) for a normal carbon potentiometer. You can look here and see that externally they look the same as any other Pots:-

http://www.megauto.de/html/wirewound-potentiometer.html

I would suggest buying one of say 200 Ohms value, to allow for some adjustment and/or a "reserve" of fuel.

Take the sender and the gauge out of the car, let all fumes dissipate first and test using a 12 volt battery or power supply and an inline fuse of say 1 amp max, to get everything working well.

Afterwards put the pot in a sealed plastic box to keep it away from moisture, even if in the car itself.....but place it where you can still get at it for final adjustment(s).

I found some good infos here:-

http://mgaguru.com/mgtech/electric/circ_f1b.htm

Someone else also suggested a poti first, but as far as I remember, did not specify fully the type needed......its important that its a wirewound model......and that its Linear. (Most wirewound are linear anyway.....!

Best of luck.

I did the resistor thing to a '50 Studebaker(6volt) guage on 12volts. It works, but the span doesn't work right. The potentiometer idea seems to work best. Set it near zero with just a little fuel (so you don't run out) is best. It will probably show 3/4 tank when full.... I rewired my truck to 12V and it still is the same way.

also remember on a truck that old, it's best to just put in a measured amount of fuel or risk the tank overfilling and coming out an auxiliary (sending unit etc) hole... Been there, done that.

Fuel spilling on the ground is not fun!

P.S. There are companies that will sell You a 12V setup, but may be expensive.

First, I have made many conversions over the years. Generally I have found that the gauge needs to be modified by adding a matching resistor in series with the gauge. It is important to supply the correct current to the gauge for it to operate correctly.

The original gauge, (as has been stated in several other post) needs the correct resistor in series with the gauge. i.e. if it indeed has 36 ohms then it will need a 36 ohm resistor. Next, determine what the current, 6V/36 ohms = .167 amps. The next is to determine the wattage needed for the resistor, .167 amps*6Volts = 1 watt. I would suggest putting in at least a 2 watt resistor. Higher wattage resistors will not affect the outcome. This would mean that you need 36 ohms at 2 watts. This might be hard to come by but any resistor value close to 36 (34-38) would give you very close to the same results. This will give you the correct voltage across the gauge.

I would try this before venturing out into other realms.

If this does not give you satisfactory results, get yourself a pot rated at least 2 watts and variable from 0 to 500 (or more) ohms. replace the sending unit with the pot and then record the fuel gauge readings and the resistance across the pot for several different values. This will give you an idea of the resistance needed at the tested levels. Next test the sending unit at different levels and record the values at those levels. Compare the two charts and make needed adjustments.

Thanks to all who have taken the time to respond to my post. I guess I should have clarified in my original post that even though the gage is original it was sent out to be updated with modern instrumentation and converted to 12 volts at that time. The problem started when I was asked what ohm's the gauge should be calibrated for and through misinformation on my part (and others who shall remain nameless) told them 36 ohms. I am trying to solve this without having to take the gage out and sending it back for recalibration. I have had the sending unit out and have no idea where to start with that route since we don't know what or where it came from.

Chuck

What would the normal setting be for the gauge

or get a reading from a 12v gauge that works to give us a starting point.

ie if a normal working gauge is say 30 ohms and yours is 36 ohms then you would need a potential divider network to bring the setting down to 30 ohms

but if yours is 36 and it needs to be 40 then a 4 ohm in line seems to be the answer.

My answer is the same, only you won't have to add the resistor in series to the gauge. Start with the pot, ( of at least 5 watt ratting) and record the resistance and fuel level indicated. Then compare the resistance at various "levels" of the sender.

cb-

I have a '50 Chevy truck also, but I have not yet moved the gas tank out of the cab. If you had it to do over again, how would you do it? Where would you place the tank? What tank would you use? Could the in-cab tank be used out of the cab safely?

Also, you you have a web site or blog for your truck project?

Bill

I would definitely do it over again but I would put more thought into the type of tank and the placement. Mine sits behind the rear axle between the frame rails and the filler runs above the frame rail on the passengers side and into the rear fender. This arrangement works well except for the fact that it is difficult for tank removal as it has to drop out on an angle and then sideways for the filler tube to clear the frame rail. If you has access to a hoist it isn't a bad job but doing it on jack stands sucks. I have looked at aftermarket tanks and the problem I have with them is the filler comes into the wood of the bed which I don't really care for (personal choice). I'm not sure if the in-cab tank could be used due to the filler location.

No I don't have a web-site for the truck as it has been "finished" for a few years (never finished are they). It is running a GM Crate 350 with a 350 trans. It has the front stub frame from a 1979 Firebird along with the Firebird rear end.

Do you mean that the tank is farther to the rear than the axle, or in front of it....

You wrote "Mine sits behind the rear axle between the frame rails and...." I take that as farther aft....

It has been general practise for around 15 years (maybe even more) never to place a fuel tank farther aft than the rear axle on safely grounds......BMW had a disaster about that time, or maybe a tick earlier with rear tanks.....some US makes it seem still ignore this simple point!!!

Its fine until you get rear ended, or you run over a guard rail or similar.......

See also:-

http://www.autoweek.com/article/20091110/CARNEWS/911109993

This comment below is from Wikipedia:-

Placement and Safety Gas tank in an unsafe position behind the rear axle. Mercedes-Benz 300SL

For safety considerations, in modern cars the fuel tank is usually located ahead of the rear axle, out of the Crumple zones of the car. This location has been used by Volvo since 1974.

Cars like the Ford Pinto or the models that still use the Ford Panther platform (Ford Crown Victoria, Lincoln Town Car and Mercury Grand Marquis) are notorious for having the gas tank behind the rear axle. Since 1980 new Ford models corrected this problem and had the fuel tank in front of the rear axle ^[1].

General Motors 1973-1987 Pick Up trucks have the fuel tank located outside the frame. According to the Center for Auto Safety this creates a fire hazard ^[2].

The above comments can be viewed in full at this link under placement & safety:-

http://en.wikipedia.org/wiki/Fuel_tank

Yes it is further to the rear than the axle. It is well protected by a very strong cross member that is even further to the rear of the tank that runs from one frame rail to the other. Thanks for the concern on the safety aspect though. The truck also runs with lap and shoulder belts for safety.

Stop immediately. This truck is unsafe to drive. I can not allow you to continue to risk your live behind the wheel of this truck. I am sending a flat bed truck to remove this dangerous vehicle from your life forever. Please sign the title over to me so I can legally drive the s#$t out of it recycle it.

PS. If you here from Lynlynch, do not let him take the truck. He will only joy ride it till he ruins it.

Does it still work correctly at 6V? If so, you might consider using a Zener diode to regulate the voltage. If not, then there may be no easy solution for the gauge to read properly across the whole volume of the tank.

The whole truck has been converted to 12 volts. The gage and sender are both 12 volt units.

Are there any markings on the tank sender? Any idea of company? (GM,Ford,Chry)

no markings at all. I even took the sending unit around to different parts stores and dealership parts depts. with no luck. But the sending unit does function as it should, just at the wrong ohm's.

OK, Here goes.

All GM products from 66-67 till 94 had an ohm reading of 1 at low reading, 44 at mid point, and 88 at full reading.

GM products from before 64 had 0 low, 15 mid, and 30 at high.

The only exceptions to this are the Nova, Spectrum, and Sprint imports, and the Saturns.

Ford Aerostar and Rangers 91-94 , and magnetic designs used in 62-72 cars are 15 low and 160 high.

Electronic senders from Ford cars from 90-94 are 11 low, and 168 high.

I do not have any specs for bigger Ford trucks.

ALL other US built vehicles use high ohm readings for low fuel, and lower ohm readings for full fuel tank readings.

If your fuel gauge reads in the correct direction, it must be from one of the GM products, or the few Ford products, or from a foreign vehicle.

If you can identify the colors of the wiring used on the tank sending unit wires (assuming the wires are still factory wires, not replacements from the gauge-12 volt) I can probably determine weather Ford, or GM.

Another way to go is to access the wire going from sender to dash gauge. With the fuel tank full, remove the connection going to the gauge, and measure resistance between the wire going to the tank sender, and ground. From that we will know what the full reading design for the sender is.

But, unless you are willing to try to replace the resistance part of the sender, you should really think about just replacing the dash gauge. Do you know who the manufacturer of them is. What about the model of them?

Decisions, decisions, decisions.

Did I understand correctly that your units are "12 volt Ready"?

If the answer is yes, you only need to put a half watt 50 Ohm Resistor ACROSS THE SENSER UNIT from the wiper to ground.

That will drop the maximum resistance to just over 39 Ohms, which means it will now match your Gauge almost exactly. Now the resistance will vary from 0 Ohms (Tank full), to 39 Ohms (Tank Empty).

This should (provided the float has full movement in the tank), give you as linear a full to empty reading as possible, at least as good as the original setup.

You could of course use a Potentiometer and adjust it more finely AND even set up some reserve by setting the total value to slightly less than 36 Ohms (that means that the pot itself will need to be set to slightly less than 50 ohms).....

About 3.6 Ohms less than 36 Ohms, should give you a 10% of tank contents as a reserve when it shows empty.....if you so wish......

I would (in your place) bench test this all (well away from flammable stuff like gas for example), to get the adjustment perfect.....the Pot should not be mounted in the tank area, but in the cab, in a sealed box.