Fuel gauge

[MoJo]

Before posting, I always search the forums for suggestions to help resolve any issues on my 1966 300. I followed all the posted suggestion and still cannot figure out why the fuel gauge is not working. In any event, I trying to troubleshoot why my fuel gauge reads empty all the time. Below are the steps and results I've done so far in trying to resolve the problem:

1. Checked an verified all fuses are good
2. Put temp ground jumper on fuel sending unit body and tested ground with test light all the way back to the battery to verify good ground - Result, no change in the fuel gauge
3. Put test light on sending unit wire at the tank and got a pulsating light on the tester
4. Put ohm meter across the fuel sending unit connector stud and unit ground and read 31 ohms
5. Grounded out the fuel sending wire and gauge swings over to full when i turn on the key.

So, I've got a good ground at the sending unit, pulsating voltage on the wire at the tank which indicates the voltage limiter is working, 31 ohms through the sending unit and I verified the gauge works when grounding the wire at the tank. It should be reading 1/3 to 1/2 of a tank, but only shows empty. Of course, this sending unit was new when I installed it last year and hasn't worked from day one. Any thoughts?

 

[Mudeblue]

Before posting, I always search the forums for suggestions to help resolve any issues on my 1966 300. I followed all the posted suggestion and still cannot figure out why the fuel gauge is not working. In any event, I trying to troubleshoot why my fuel gauge reads empty all the time. Below are the steps and results I've done so far in trying to resolve the problem:

1. Checked an verified all fuses are good
2. Put temp ground jumper on fuel sending unit body and tested ground with test light all the way back to the battery to verify good ground - Result, no change in the fuel gauge
3. Put test light on sending unit wire at the tank and got a pulsating light on the tester
4. Put ohm meter across the fuel sending unit connector stud and unit ground and read 31 ohms
5. Grounded out the fuel sending wire and gauge swings over to full when i turn on the key.

So, I've got a good ground at the sending unit, pulsating voltage on the wire at the tank which indicates the voltage limiter is working, 31 ohms through the sending unit and I verified the gauge works when grounding the wire at the tank. It should be reading 1/3 to 1/2 of a tank, but only shows empty. Of course, this sending unit was new when I installed it last year and hasn't worked from day one. Any thoughts?

View attachment 321877

If it is the same gauge as the 65 Plymouth; new field gauge. Fought that problem for years and gauge was problem. Resistor in fuel guage!

 

[rkrochen]

Sounds like a bad tank sending unit. Could be the float is leaking and saturated.

 

[Big_John]

There's a lot of extra nonsense in this thread, but here is what I did to diagnose and correct my gauge issues.
Gas Gauge Fix

 

[Davea Lux]

As noted, you probably have a bad sending unit float. Pull the sending unit and rattle the float, if you hear liquid sloshing inside, it is a defective float. This must be fairly common because I have run across 3 like that in the last year. Floats are made overseas, so probably just more of the usual Chi-Com crap.

Dave

 

[MoJo]

Thanks for the suggestions. My confusion is the ohm reading vs the gauge indication. I think i'll try another sending unit or possibly a 100 ohm potentiometer to test the gauge function.

 

[traintech55]

The fact that grounding the sending unit wire makes the gauge move tells you it is in the float. Did you replace the ground strap with the new float assembly? If you did then it is the float.

 

[Big_John]

Thanks for the suggestions. My confusion is the ohm reading vs the gauge indication. I think i'll try another sending unit or possibly a 100 ohm potentiometer to test the gauge function.

The magic numbers for that are 10 ohms for full, 23 ohms for half and 74 ohms for empty.

 

[73Coupe]

The magic numbers for that are 10 ohms for full, 23 ohms for half and 74 ohms for empty.

I tried asking for the resistance readings from a few vendors on ebay before purchasing their overpriced new or NOS senders. Especially since a lot of them are listed "no returns".

The responses I received were either crickets or an "FU".... so I didn't bite.

So for now I am still relying on my "new" but barely functional sender. Fook.

 

[Big_John]

I tried asking for the resistance readings from a few vendors on ebay before purchasing their overpriced new or NOS senders. Especially since a lot of them are listed "no returns".

As far as I've been able to figure out, there is only one offshore manufacturer building the pickup/sender. Spectra seems to be the largest importer/distributor but I don't think they are actually manufacturing them. I you'll notice, the part number is pretty much the same across all the vendors.

I really think the answer is a "work around" with the Meter Match adding the corrections or fudge factors to make the gauge accurate. See the link I listed above for my experiences and a link to the Meter Match. IMHO, that would correct for both the sender characteristics and an aging dash gauge. It does require a more constant power, so an electronic voltage limiter is needed, but I think the old mechanical limiter is often part of the issue too.

 

[Polara_500]

I tried asking for the resistance readings from a few vendors on ebay before purchasing their overpriced new or NOS senders. Especially since a lot of them are listed "no returns".

The responses I received were either crickets or an "FU".... so I didn't bite.

So for now I am still relying on my "new" but barely functional sender. Fook.

Hey, I have three new senders with good ohm readings, the only problem is none of them fit my tank. So there's other ways to mess it up, as all the vendors sold me units for my specific vehicle.

 

[MoJo]

Big John, I saw those ohm reading you posted in another thread and thinking is with a reading of 31 ohms, the gauge should show some movement. If the ohm meter showed 100 or more ohms, then I could understand the gauge reading. Remember, when I ground the wire, the gauge swings to full so I know it's working. Perhaps it is a combination of failing voltage limiter and crappy sending unit. The Meter Match device looks promising and I will definitely look to install one to get the gauge to read accurately, however, it's not reading at all, it's pegged all the way past empty. The sending unit is a Spectra product purchased last year when I installed a new tank. I should have tested it before installing. I'll keep digging and see what i can find out. Thanks for all the input.

 

[Big_John]

Big John, I saw those ohm reading you posted in another thread and thinking is with a reading of 31 ohms, the gauge should show some movement.

31 ohms should give you something around 1/4-1/3 on the gauge.

when I ground the wire, the gauge swings to full so I know it's working.

That tells you the wire is intact, and the gauge moves. Not necessarily working right though.

Perhaps it is a combination of failing voltage limiter and crappy sending unit.

The sender gets blamed for everything, but I don't think it's the case here. It may be off, but if you are getting a reading of 31 ohms, that tells me it's working. Any idea how much gas is actually in the tank?

The 100 ohm pot idea is pretty good. It would work like my decade box and you can check the gauge. If it's like my '70, there's a plug under the driver side kick panel/door threshold trim that you can unplug to connect your pot and not have to crawl under the car. I really think that is what you need to check.

 

[MoJo]

Big John,

The gauge reads past empty and never moves regardless of the amount of gas in the tank. The 31 ohms reading was with about 1/2 full tank. I just refilled the tank and will get a new reading to see if it changes and I'm picking up a potentiometer today to test the gauge function and accuracy. I'll post the results.

 

[Gerald Morris]

I have a funky sending unit too, w a cheap sino-sewage gauge which reads from 5/8 full to full when the tank is bone dry to full. I might try a simple resistive interpolator to give me a wider range of response or even the Meter Match, but I also am still looking at a deal on another sending unit in fools hope it may do better. At least I CAN look and get an idea of how much petrol I have.

ONE THING which made meaningful results happen for me was DITCHING THAT DAMNED OLD ANTIQUE ELECTRO-THERMAL VOLTAGE SUPPLY! Get a nice little solid state USB supply set at 5 V and at least 2.5 amps and see what you get then.

(you probably will want more like 7-10 amps if you still use the entire nominal 6 volt dc instrument panel. Solid State is Great!)

You SHOULD be able to feed your fuel gauge circuit with such a power source NOW to do a sanity check. Give that a try.....

 

[John Kirby]

Not sure if you have tried this but, is the gas tank grounded? There is a strap that goes across the little piece of rubber hose at the sending unit. If it is not there your gauge will read empty all the time. If you lost it you can run a wire from the tank to the frame. Don't drill a hole anywhere gas could leak. I used the metal flange where the tank is welded together. Or you can hunt the strap down on ebay.

The reason the voltage supply to the fuel gauge is current limited is you don't want a spark (or heat source) inside the tank. It could easily cause an explosion. I don't recommend replacing the original regulator with a high current version for this reason. Doing the math, 5V/10 Ohms = 500mA. That's 1/2 Amp. Way more than the gauge needs. Think of it as dissipating 2.5Watts (5V x 0.5 Amp) inside a full tank of gas. The sending unit is not designed to dissipate that much heat and will get rather hot. And it will destroy the gauge. A series resistance is needed to limit the current to a safe value. Without knowing the current rating of the gas gauge I can't calculate what that would be. (basically it's a milliammeter and they have a current rating for full scale)
The only reason a solid state version makes sense it it eliminates the noise your radio picks up when the mechanical unit operates. Normally this is not an issue, there is so much noise your radio is picking up naturally you don't ever hear it.

 

[MoJo]

John,
Thanks for contributing, I haven't been able to get back to this issue for the last 3 weeks, but am hoping to look at it soon. To clarify your statements, are you recommending against a solid state voltage limiter for the fuel gauge? I understand not wanting high current flowing through the fuel tank sending unit, however, isn't this accomplished by limiting the voltage as opposed to having a high current power supply? Shouldn't the circuit only pull the current needed based on the resistance and the voltage supplied? If the power supply is capable of more current, it won't push that current through the circuit given the same 5 volts? I'm trying to reconcile your recommendations with the previous post from Gerald who does recommend a solid state voltage limiter for the fuel gauge.

 

[Gerald Morris]

John,
Thanks for contributing, I haven't been able to get back to this issue for the last 3 weeks, but am hoping to look at it soon. To clarify your statements, are you recommending against a solid state voltage limiter for the fuel gauge? I understand not wanting high current flowing through the fuel tank sending unit, however, isn't this accomplished by limiting the voltage as opposed to having a high current power supply? Shouldn't the circuit only pull the current needed based on the resistance and the voltage supplied? If the power supply is capable of more current, it won't push that current through the circuit given the same 5 volts? I'm trying to reconcile your recommendations with the previous post from Gerald who does recommend a solid state voltage limiter for the fuel gauge.

The power dissipated by any impedance will be the square of the voltage divided by the impedance. To wit: P = (V^2)/Z with Z being a quadratic sum of both Real, simple resistance which varies little with frequency, and REACTANCE, encompassing both capacitive and inductive POTENTIAL impedance, which depend on frequency (f) with w = 2pi*f as a linear product being either wC for capacitance or wL for inductance. So our expression for power above becomes

P = (V^2)/[(R^2)+(wC^2)+(wL^2)]^(1/2)

Since there IS a frequency component in automotive circuitry when the engine is turning, and since the wiper in a fuel sending unit contacts a wound coil with a palpable impedance, power dissipated by a fuel sending unit will actually DECREASE with increased rotational velocity (frequency) of the engine, as the impedance will increase. Therefore, it behooves folks to have a CONSTANT VOLTAGE, WHICH CRUDE THERMO-ELECTRIC CONTACTOR REGULATORS NEVER HAVE SUPPLIED! They approximate a voltage halving by heating a bimetallic spring contactor which opens when heated, and closes when cooled. Thus these have an inherent impedance and frequency of their own to further complicate matters with the fuel sending unit. Perhaps a Kindly Providence guards us fools from all going up in blazing, speeding fireballs when driving vehicles so equipped! OR, maybe 2.5-3.6 Watts maximum in a tank of alkane liquids isn't apt to explode.

As a man of Faith, AND a Skeptic, I know which hypothesis I lean toward.....