Fuel gauge problem

[53Rick]

I'll try this again. Didn't post last time. I have a '72 GMC 1/2ton. The fuel gauge reads empty at all times. I have replaced the fuel sending unit 3 times in the course of the past 10 years. Nothing changes. I've done the following tests: (1) pulled the sending unit wire at the tank. The gauge swings to Full with the key on. (2) Grounded the sending unit wire to the tank. The gauge goes to Empty. (3) The unit has only the one wire leading to the fuse block. No ground wire. So I have run a ground wire from the sending unit to the frame and also from the fuel tank to the frame. No change and the gauge stays on empty. (4) I've followed the wire from the tank to the fuse block and inspected it carefully for breaks, bare wire, etc. All is fine. Anyone have any suggestions on what the problem is and where I should go next with this? Everything I've read seems to indicate with the above tests, the gauge is good, but I can't believe 3 sending units could be bad. If memory serves correctly, I believe one replaced sending unit registered 1/4 tank when the tank was full for a little while and then quit registering. Any advice would be appreciated. Thanks.

[oldiron]

Some how you ended up in the suburban forum. I'm sending you over to the pickup forum.

[JimKshortstep4x4]

Welcome to the 67-72 Truck Board from West Michigan!

My suggestion would be to pull the sending unit. Make sure the float is good and that it moves up and down freely.

Using an ohm meter check the resistance of the sending unit from the terminal to the metal tube. With the float in the empty position the reading should be 0 ohms. Moving the float to the full position should give you a reading of 90 ohms.

If the ohm readings are not between 0 and 90 there is a problem with your sending unit. Everything that you mentioned in your post points to the sending unit not working.

Good luck!

Jim

[53Rick]

I'm back after a year or so and working again on the '72 GMC fuel gauge/sending unit problem. I ran the ohms test on the sending unit and it did fine 0-90 ohms in the test. The sending unit wire runs to the fuse box where it connects to the wiring presumably down from the dash harness. A voltage check at the fuse box, though, shows 6 volts. Same at the end of the wire attaching to the sending unit. It is properly grounded and I even attached extra ground wires. When disconnecting the sending unit wire at the sending unit, the gauge goes from empty to full with the ignition switch on. Doesn't sound like a gauge problem, right? But what about the 6 volts at the fuse box? Several people told me it should read 12 volts there and they suggested running a jumper wire from a 12-volt location to the connector for the fuel gauge/sending unit. I did that and fried the sending unit and the fuel gauge went from the empty to 3/4 past full and stayed there. Back when this started, the problem was the gauge would never read full. It would go only to 1/2 tank when the tank was full. Then it dropped to empty and stayed there. I replaced the sending unit twice and also the float. Today I discovered the new brass float had a pin hole in it as did the new one I had previously installed, so that probably was the cause of the gauge not staying at 1/2-tank and dropping back to empty after a day after installation. I may have resolved that problem, but since the sending unit is/was (until fried) good on the ohms reading, what's up with the gauge not reading past 1/2 tank? I'm guessing a weak gauge, though I am still confused about the 6-volt reading. I've ordered a new sending unit, a new gauge, a new sending unit wire and a new printed circuit. Is there anything else anyone can think of that might be causing this problem? Thanks. Rick

[mclairmo]

Gauge sounds fine to me. Check wiring continuity. Also, try to follow wiring from fuse box to the gauge, if possible. Check connectors for corrosion. Check to see that the printed circuit is ok. Also, be sure the actual gauge is grounded properly. Also, make sure that ground wire you added to the sender is correct. How did you add this ground wire? Did you solder it on to the sender? If all 3 brand new senders did not have a ground wire, then maybe it is not supposed to have a ground wire but is gounded in some other manner?

[ll____b0t____ll]

behind the gauge under the printed circut board is a resister , those are known to go out and could be your problem .
Ide check into replacing that resistor.
My 100th post whooooooooohooooooo!!!!!!!!!!!!

[67chevemall]

I have the same problem.
I think mine is the sensor is stuck from sitting dry
But it could be the resistor too....thanks

[Legolas894]

OK-Now I've got this problem too. Had the sender re-built (guy in St. Catharines, ON does it).

Fuel gauge would sit at about 4 o'clock (past full) if it was a clock. When you start it, it moved about 3 o'clock. When you touch the wire from the fuseblock to ground, the gauge reads Empty.

The sender I took out had the resistor wire fried in side. What is the resistor on the gauge supposed to be?

Frustrated!!

[bimaldo]

hmmm, I am experiencing the same thing.

[Legolas894]

I'm going to try buying just the gauge and see if that works.

My gauge used to sit around 1/2 tank when the key was off. I noticed a lot on ebay that sit there too (when they're right out of the truck). Mine is sitting at the 4-o'clock position with no power applied.

[Longhorn Man]

in your original post, you say you grounded the sending unit.
Try removing the added ground.
There is no added ground needed. the sending unit is grounded to the tank, which is bolted to the cab(and grounded through the 8 or so bolts) which is grounded to the frame and engine.
Everything from the end of the tan wire to the guage works fine, and you said the sending unit tested right with the 0 - 90 ohm...
I would remove the added grounds and retest. If it doesn't work, i would then remove the sending unti, and then check to see if the float isn't full of fuel, causing it to sink.

[Legolas894]

One check I did today. I put an ohm-meter on the sending unit in the tank with the gauge wire off. It read about 57 ohms. I added some fuel to the tank, the ohms went up to about 60. Therefore, sending unit is OK I assume since when empty it should go to 0 OHMs which is basically the same as grounding the gauge wire.

Higher ohms reading = more fuel.

I also happen to have a 0-100 ohm variable resistor from another hobby. I put this on in place of the sending unit and the gauge would only move if I moved the resistor to 0 ohms. 90ohms should have made the gauge read full but it stayed at the way-past-full the whole time until 0 ohms.

There's a few gauges on ebay and a new one is only around 40 bucks.


I must have a gauge problem.

[peleni]

The fuel guage resistor should read 90 ohms (not installed in the cluster)
When I installed mine the guage went to 4 oclock (way past full) on advice from an old friend I then reversed the resistor and all started working. Something tells me there is a diode in with that resistor.

[Legolas894]

Does your resistor just look like a flattened coil of wire on the back of the gauge? I can't see what reversing would do (unless there's a diode in there).

This is what my gauge looks like now.

I think it's odd that this gauge (off ebay) is sitting in the same position as mine. There are others on ebay that are sitting at 1/2 full.

What's up?

[peleni]

I've got a 72, the resistor is about 1 inch long by 1/2 inch wide and about1/8th of an inch thick...piece of plastic with a small hole on each end that goes on the posts on the cluster...you can see the resistor and ohm it out....funkiest resistor I ever seen. My gauge was in the exact position you show in the picture...till I reversed it.

[garold99]

How do you change the resistor? Do you have to pull the cluster off to get to the back, or just reach up behind while lying on your back on the floor? Also I have this link to checking for problems, but you've probably already gone over everything this covers. http://www.6066gmcguy.org/Electrical-02.htm

[bimaldo]

Mine seems to have had a miraculous recovery. After reading all the information about the hidden resistor behind the gauge this morning, I thought I'd check mine out. Got in to pull it in the garage & wamo !! - it was reading again ( almost empty of course ) - but not the 4 o'clock position.

Not complaining, but I don't get it ?

[Mondo]

Here's my thread when I was having problems... mine was the resistor

http://67-72chevytrucks.com/vboard/s....php?p=3249410

[53Rick]

Update on my gauge problem. I bought a new gauge, printed circuit, sending unit and sending unit wire from Classic Parts. The gauge came with the needle resting in the 4:00 position. I installed all and the gauge read accurately at about 1/2 tank. Filled the tank and it reads full. However, when I turn off the ignition key, the gauge needle goes back to the 4:00 position. When I turn it on, it goes to the full mark. Guess I'll know in a few days when the tank level goes down a little, but I'm pretty sure this isn't a normal resting position for the gauge. There is really nothing else to replace since I replaced everything but the dash harness. The new gauge came with a built-in resistor and according to Classic Parts, it doesn't need an external resistor. Does anyone have a solution? It's working fine (or appears to be), but it isn't normal to have the gauge reading 4:00 when the key if off. By the way, I have given up totally on brass floats for the sending units. I have now replaced the sending unit 3 times and I dad had it replaced twice when he had the truck before he passed away. Every time the float was replaced and every time it leaked. NAPA sells a float with a brass end (where it attaches to the sending unit) and the rest made of another type of material that is fuel resistant. Costs about $10, but well worth it. It is solid material and can't develop a pin hole and fill with gas. Obviously parts aren't made like they used to be. Must be from China or something instead of the good old U.S of A.

[Legolas894]

Glad you are ok now. My gauge used to rest where ever it was (1/2 tank etc). Since it's been at 4'oclock, it hasn't worked. I've got a new gauge on the way so I'm hoping that works.

[Longhorn Man]

i had one that would drop to that position when turned off, but only if it had more than 3/4 tank. If it had less than 1/4 tank, it would drop to the 7 o'clock position.

[toolboxchev]

Unreal, 45 years and this is what the inside of my tank looks like. The yellow area is the bottom and the fuel.

I pulled my sending unit out to test and inspect I thought it would be toast, Unreal condition.

Having the floating needle problem and ohms test shows its working yet going past 100. HMmm?

[toolboxchev]

So what would make my fuel gauge read yet bounce around dramatically when driving?

Its moving over 1/4 of a tank in range. I am stumped at this point without tearing out the dash. I am thinking its the gauge or silly little resistor on the back of it.

Help pointing out the way is appreciated, I have tried testing it with an ohm test and the old unit works ok, plugged in a new one and moved the float around and the guage seems the same. Moves rather suddenly when slowly moving the float manually by hand.

I have a decent ground all the dash functions are working ok.

Still stumped????