67-72 Chevy Truck fuel gauge diagnostics 101

[TBONE1964]

Hello all,

It has come to my attention that there are a few people out there that are having trouble figuring out if it is there fuel sending unit, gas gauge, or something in between that is causing there fuel gauge to not read correctly.

My goal here is to show some basics and also show what happens when you loose ground or power to the sending unit or gauge. Please bare with me and ask questions. I am going to attempt to make this as clear as possible.

The first set of photos is showing what you have and what you need to diagnose a sending unit and gauge.

The two basic tools I use for my testing are a known good sending unit and a DVOM (digital volt ohm meter) set to the OHM 200 scale. For my testing I also use a battery carger set on 10 amp to simulate a 12V car battery.

[TBONE1964]

Next, I will show the ohm meter readings for the fuel gauge and the fuel tank sending unit. The sending unit is not from a 67-72 truck but it is a 0 - 90 OHM scale and will be the same as the truck except the float arm is shorter.

As you can see, I have hooked up the ohm meter to the fuel gauge in several different ways so you can see the meter reading differences. I always test with the resistor installed and tight on the gauge as it would be in a normal situation.

I will repeat this on the fuel sending unit so you can see the reading changes depending on the float level.

[TBONE1964]

More photos

[TBONE1964]

In this set of photos, I will show the proper wiring from the gauge to the sending unit. Sorry about the wire colors for my test. For this I will use a green wire with alligator clips to simulate the tan wire and the yellow wire will be the ground. I also will use the + side of the battery charger as the 12V pink wire which comes from the ignition switch on the truck.

I then will show what happens to the gauge when I disconnect ground and power at the sender and the gauge.

As you can see, if the ground or tan wire become disconnected from the sending unit, it will make the gauge read at 3:00 (past full).

If you loose power from the ignition switch, the gauge will stay in its last known position with power.

[TBONE1964]

More photos

[TBONE1964]

With no ground or tan wire on the sending unit

[TBONE1964]

Finally, with no power to the gauge from the ignition switch. The gauge will retain its memory from the last position it had with power. You may notice in your truck that the gauge will creep toward empty if you shut off the ignition for any length of time and come back to normal as soon as it gets 12V.

[TBONE1964]

I hope this helps some of you understand how the sytem works and what happens when you take power and ground away from the gauge and the sending unit.

Please ask any questions that you may have. I hope I did not confuse anyone here. There is a reason I am not a auto shop teacher

Take care and have a great weekend,
Tom

[VetteVet]

VERY VERY NICE!!!!!!!!

vv

[jjzepplin]

So my gauge reads at 1/4 when the tank is full. But on "e" when it is empty. Never lower and never higher.
I guess that would mean that my sender may be of a incorrect ohm scale? Possibly 0-50 or less rather than 0-90?

[TBONE1964]

Originally Posted by VetteVet

VERY VERY NICE!!!!!!!!

vv

Thank you,

Coming from you Vett, that is a nice compliment. I always look forward to your expertise on electrical issues.

Have a good weekend,
Tom

[TBONE1964]

Originally Posted by jjzepplin

So my gauge reads at 1/4 when the tank is full. But on "e" when it is empty. Never lower and never higher.
I guess that would mean that my sender may be of a incorrect ohm scale? Possibly 0-50 or less rather than 0-90?

Someone can correct me if wrong but I believe GM fuel sending units have always been 0- 90 ohm scale from the mid 60's all the way to the 80's

It is possible that it is only reading 0 - 22 (1/4 tank approx) although the float is moving all the way across. The sending unit works somewhat like a dimmer switch on your dash lights. There is a sweep that controls the amount of voltage going to the gauge.


The only real way to test your specific sender is to pull it from the tank and do an ohm sweep test. If it gets to 22 ohms and stops there but you can still move the float from 1/4 to full then I would suspect a bad sender.

If the sender checks out, then I would look at the gauge. It sound like your wiring is OK. The wiring on these are usually on or off and both ways have there own unique symptoms.

Hope this helps.
Tom

[VetteVet]

More often than not the cause of a 1/4 tank reading is a bad resistor on the back of the gauge. I would try the sender sweep first and if it checks out then look at a new resistor.

[TBONE1964]

Originally Posted by VetteVet

More often than not the cause of a 1/4 tank reading is a bad resistor on the back of the gauge. I would try the sender sweep first and if it checks out then look at a new resistor.

Thank you Vette again. I am thankful that you can/will add your own experience to this thread. My thought was to get some diagnostic procedures out there that someone can reference and help solve there issue.

Is this a common problem that you see on your end? I test each and every fuel gauge for a complete empty to full range and I have not had one to date that would not read past 1/4 on the gauge.

Interesting, I have a drawer full of these resistors, red, orange and blue and they range in ohms from 95 - 108 and the color does not have make much difference. I even have they early wrapped wire resistors and they all read around 100 ohms give or take 5.

I am wondering if you have a bad resistor that restricts a gauge from going past 1/4, what would the ohm reading be then?

Good stuff here.

Take care,
Tom

[VetteVet]

As you know the gauge reading is proportional to the resistance of the gauge combined with the sending unit resistance. That is, the higher the resistance, the higher the gauge reading and the lower the resistance the lower the reading.

This would suggest that either the sender resistance is not exceeding 1/4th of 90 ohms or approximately 22 ohms. this is give or take because the sender reading is variable and not linear.

OR

The resistor on the gauge is low enough not to allow the gauge needle to advance to full.
This would not seem to be the case when the gauge needle goes to the three o'clock position when the sending unit wire is disconnected. Have we established that in Mr. JJZEPPLIN's case?

There is also the possibility that there is a defect on the needle deflection coils inside the gauge itself. The thing to understand is that the amount of resistance on these coils allows them to pull the needle left or right depending on which coil has the least resistance. These coils are protected by the resistor on the gauge as well as affected by it.

Here is a favorite diagram of mine on the fuel indication circuit on many vehicles.




I don't think it shows the complete wiring on the gauge internals but is trying to show the relationship of the sending unit resistance to the gauge.

The variance on the gauge resistors is probably due to the gauge needle, coil windings on different models and the number and size of their windings.

I might suggest an experiment to test my theory. The resistance across the fuel gauge terminals is 44.4 ohms per the picture in the first set. Let's reduce the ohms of this resistor by an amount that only allows 1/4 tank reading on the gauge when it includes the full 90 ohm tank resistance. Keep in mind that this resistor protects the gauge coils, so a small nine volt battery might work for the power input without damaging the gauge.

I see that you have used a battery charger with probably a 2 amp setting so that should fit the bill.

So here's what we have.
Battery positive to the gauge positive terminal.
sender resistance of 90 ohms to sender side with the end of the resistor grounded to the ground terminal.

Battery negative connected to the same ground terminal as the sender resistor. This should give a reading of full on the gauge. I'm out of state at the present so I can't do this myself to prove my theory.

Next we will substitute a lessor value resistor for the one on the back of the gauge and determine if the reading drops below full. We know it will if we change the sender resistance because that's how the system works. Less resistance on the sender equals a lower gauge reading.

To sum all this up I'm asking the question "What happens if the gauge resistor goes bad by shorting across it's coils and dropping it's resistance." Does it prevent the gauge from going above 1/4 tank or is it something else?

What happens if we replace the gauge resistor with one of a higher value.

I'm not playing here, I really don't know.

[TBONE1964]

Very interesting. I will see if I can come up with some resistors for the gauge and do a test. Myself, I have to travel tomorrow for most of next week. Not sure when I will have another opportunity for a while but am looking forward to seeing what the result is.

I am also understanding that some of these new aftermarket fuel and temp gauges are not using external gauge resistors but have them built in. That will be another chapter to work on in the future.

I like the way you think Vette. Have safe travels yourself.

Take care,
Tom

[TBONE1964]

Another point I would like to make on the 1967-1972 Chevy?GMC truck fuel gauge diagnostics is this:

The fuel gauge and the temp gauge do get there power from the same source. 20 gauge pink wire from the ignition switch. On the cluster connector, that is going to be the pink wire in position # 3. Thought the printed circuit board it splits and routes power to each gauge.

It will be helpful if you think you are not getting 12V to your fuel gauge to see what is happening with your temperature gauge also.

[67 Burb]

The resistance across the gauge (as shown in the first picture in your post #2) must vary a lot. I checked that resistance on a gauge I have and found it to be 31 Ohms. And the gauge checks out OK.

[TBONE1964]

Originally Posted by 67 Burb

The resistance across the gauge (as shown in the first picture in your post #2) must vary a lot. I checked that resistance on a gauge I have and found it to be 31 Ohms. And the gauge checks out OK.

I cannot do it for a while yet but I have 20 + fuel gauges that all work. I can do this ohm test on them and see what kind of variation there is. I also have a NOS fuel gauge to test.

Good point you have made. I am curious to see what others might be.

Take care,
Tom

[VetteVet]

Here's a diagram of the gauge wiring you mention.













Lowering the voltage brings in the issue of Ohms law which shows the relationship between voltage resistance and current. It turns out that resistance and voltage are directly proportional to current.

I did some calculations and they show this.

with the resistance constant and the voltage drop we get.

V=12 R=45 C=.2666 where V is voltage, R is resistance, and C is current.

We drop the voltage by 1/4th to 9 and we get,

V=9 R=9 and C becomes .2

Dropping to 6 volts gives,

V=6 R=45 and C becomes .1333

So we go to resistance and we see that if we leave the voltage constant and increase the resistance by 1/4 we get,

V=12 R=60 and C becomes .2

If we increase by 1/2 we get,

V=12 R=90 and current becomes .1333.

If we drop the resistance we get a larger increase in current.

A 1/3 drop in resistance gives,

V=12 R=30 current doubles to .4

Another drop in resistance of 1/3 gives

V=12 R= 15 and C becomes .8.

Remember, when we ground the sending unit side of the gauge we get an empty reading on it, so this would seem to bear out the idea that lower resistance and more current will give a lower reading on the gauge.

The temperature gauge is the opposite, when we ground the sender we get a higher reading on the gauge.

My conclusion: Lowering the voltage will not have nearly the effect as lowering the resistance.

[jjzepplin]

I have a 35 gallon tank and it's a mother to drop. Couldn't I fill it up and check the reading on the sender?
I would of course check the gauge first and do this if it checks good.(30-45 ohm) LOL!

[VetteVet]

Originally Posted by jjzepplin

I have a 35 gallon tank and it's a mother to drop. Couldn't I fill it up and check the reading on the sender?
I would of course check the gauge first and do this if it checks good.(30-45 ohm) LOL!

Have you pulled the tan wire from the sender off the fuel gauge in the fuse panel. It is located on the end of the panel in the center. Pull it off and turn on the key. The gauge needle should go past full to about the three o'clock position.

Next you want to run a ground wire to this terminal from any good ground. Go to the battery negative if you want to be sure. With the key on, the gauge should read dead empty.

If you have a multimeter like the one shown in this thread above, you can check the sending unit. Just connect the red lead to the wire and the black lead to the tank. You won't need the key on because you're reading resistance. The reading should be somewhere between 0 and 90 ohms. zero being empty and 90 being full. If you can do this as you're adding fuel you should see the meter resistance climb from where it's at to 90 ohms.

A failure to get the readings means either the sending unit is defective or the sending unit to tank ground is poor or the tank to frame ground is poor.

Read through Toms excellent posts above and you'll get the visual of all this, and you'll understand how the indicator system works.

[TBONE1964]

Originally Posted by jjzepplin

I have a 35 gallon tank and it's a mother to drop. Couldn't I fill it up and check the reading on the sender?
I would of course check the gauge first and do this if it checks good.(30-45 ohm) LOL!

JJ,

To add to what Vette explained, here is how I would address your issue.

Get yourself an OHM meter and hook it up as I explained. The OHM meter should be set on the 200 scale and hook the positive (red) lead to the tan wire and the black lead to the sending unit ground. This will show you ohm resistance on the sending unit.

I would try and do this test starting from a empty tank or as near empty as you can get. Get your OHM meter hooked up and take a freind to the gas station.

For a 35 gallon tank, lets break it down into 1/8 increments. IF you are empty, you will need 4.375 gallons of fuel to get 1/8 tank. The ohm reading should be 11.25 ohms for an 1/8 tank.

35 gallons divided by 8 = 4.375
90 ohms divided by 8 = 11.25 ohm.

I would pump 4 .37 gallons at a time and watch the ohm meter.

1/4 tank would be 8.75 gallons and 22.5 ohms and so on and so on.

If you pump another 4.375 gallons and your ohm meter does not move, then I would suspect a bad sending unit. If it goes up to 33.75 ohms, then continue and keep watching the ohm meter compared to the amount of fuel added.

This may take some time but you are trying to isolate a problem.

Keep us posted on how it goes.

Take care,
Tom

[67swb72klb]

great write up thanks for taking the time to do it pics and all

[jjzepplin]

10-4. Will test the gauge as above.
As for the tank...I am not sure how much is in it so maybe I can test ohm reading after trips as I run the level down. I carry a Jerry can with fuel just for kicks and if I see stranded people.
I am almost a road ranger of sorts. I suspect the sender as it is most likely aftermarket.
I do have a known good unit that I have repaired in my blazer tank that is easily accessible.
Wasn't looking to get into this today but saw this thread and thought what the heck, everything else is working and this has been this way for 10 or so years. May as well do some checking.