Fuel gauge “bouncing”

Hi,

I haven't seen this problem for a long time. First, it could be the rheostat on the sending unit is going bad and not making a good connection.

I remember the gauge would fluctuate when turning or accelerating, but your issue looks like it is a connection issue. Have you checked the grounds at the tank for the sender? Also, at the firewall, check to make sure the bulkhead connector is in good condition, and nothing is loose or corroded.

I tried to find wiring schematics for you, but because of the model year, there aren't any available in my manual. However, check for what I mentioned to see if anything is loose, corroded, damaged, or anything that could cause a weak connection.

I have a 78, so I can go check things for you if needed. Different year, but still very similar.

Take care,

Joe

Thanks Joe! The wiring harness up front is brand new, and all other gauges seem to be working fine. I re-grounded the engine harness, and the sending unit ground as well, to clean bare metal.

Hi,

I can't guarantee it, but I suspect the rheostat is bad on the sensor. Remove the level sensor and inspect it. You could try running a continuity test to see if the resistance changes when you move the switch on it and if it fluctuates or glitches like the gauge was doing.

Let me know what you find or if you have other questions.

Take care,

Joe

I dropped the tank over the weekend and pulled the existing sending unit. Tested it and it's working perfectly. I reinstalled everything, but still no gauge function.

I replaced the dual tank selector switch on the dash, but still have the dual tank selector valve down by the tanks. It's not connected, other than electrical (fuel lines are bypassed directly to the engine). The fuel gauge is reading at 3 o'clock, so it's getting a signal, but no resistance.

This is how my setup is currently connected: pink wire from engine harness -> down driver side frame -> connects to pink wire going across frame -> aux/pass tank sending unit. (I only have one tank currently.) I can't figure out what function the selector valve is serving, if any, in the system, but I suspect it's something. Is there a way to bypass that and just get a straight signal from the sending unit to the gauge, or is that a lot of wiring work? Any thoughts on getting the gauge to work again?

Hi,

If it's at 3 o'clock, it's seeing too much power. Have you checked the wire for a short to power? Also, I was unaware this had dual tanks.

As far as how it works, this is a variable voltage design. The variable voltage type dash gauge consists of two magnetic coils to which battery voltage is applied. The coils act on the gauge pointer and pull in opposite directions. One coil is grounded directly to the chassis, while the other coil is grounded through a variable resistor within the sending unit. Resistance through the sending unit determines the current flow through its coil, and therefore pointer position. Based on that, it sounds like the wire going to the sender is shorted before the sending unit. It could be at the selector switch.

Let me know your thoughts and how the sender was tested.

Joe

That makes sense. I pulled the sending unit and the gauge out. Hooked them up to a 12V battery and tested that way. Used this video as a guide: https://www.youtube.com/watch?v=Bq3YfCLq4So&t=275s. I also bought a new sending unit and hooked it up to directly to the engine harness pink wire, but no reading on the gauge.

The engine harness and pink wire that runs from it down the driver side frame is brand new. I haven't checked for shorts on the wire that runs from that new wire across the frame to the sending unit, but it is an older wire; may be worth replacing?

Should I replace the dual tank selector valve/switch, or is it easy to bypass that and just go to a single tank?

That makes sense. I pulled the sending unit and the gauge out. Hooked them up to a 12V battery and tested that way. Used a YouTube video from LMC Truck as a guide. I also bought a new sending unit and hooked it up to directly to the engine harness pink wire, but no reading on the gauge.

The engine harness and pink wire that runs from it down the driver side frame is brand new. I haven't checked for shorts on the wire that runs from that new wire across the frame to the sending unit, but it is an older wire; may be worth replacing?

Should I replace the dual tank selector valve/switch, or is it easy to bypass that and just go to a single tank?

Since you are only using one tank, you can bypass the selector completely. Also, the wire you referred to may be fine. I wouldn't replace things unless you are sure there is a problem.

Let me know.

Joe

I've tried that, with no luck. I've attached a crude drawing of the current setup/connections.

Hi,

Take a look at pic 1. Is that how it is set up? Looking at your pic, it should work. Are you getting a voltage signal to the sending unit? Is there 12v direct to the gauge separate from the 12v from the sender? You have an X after the switch. What does that mean?

Joe

Yes, the sending unit has a ground cable that is connected to bare metal on the frame. The gauge is grounded through the printed circuit board/wiring harness. The X in my drawing is where the new wire connects to the old "pigtail" wire that goes across the frame to the sending unit.

Please bear with me. I'm trying to decipher what they are saying in the above pic.

Do me a favor. Take a look below. I was able to find an old schematic. It is for the dual tank set up. I don't see why you wouldn't be able to bypass the tank switch (pink wire) between the two, sender/gauge.

Let me know your thoughts.

Joe

See pics.

I agree; it should be simple, but I’ve tried everything I can think of in regards to wiring to the sending unit. If I bypass the selector valve, wouldn’t I need to bypass the selector switch on the dash as well?

Hi,

If you bypass it, the switch shouldn't change anything. You really don't need it anymore.

Try that and let me know what happens.

Joe

No go. I ran a brand-new wire from the sending unit in the tank, to the brand-new pink wire from the engine harness, and now instead of reading 3 o’clock, we are back to reading at E. If I unhook it, it goes back to 3 o’clock. I verified that I have 12 V at the instrument cluster where the gauge plugs in, and I unplugged and re-seated the instrument cluster harness. No luck.

At this point the selector valve is completely bypassed. I checked all my grounds as well, and they are all in clean bare metal. I’m at a complete loss.

Hang in there. I'm going to discuss this with the site owner and get his input. It should be working based on what I am seeing in the schematics and descriptions.

Joe

Thanks Joe!

Hi,

He indicated the wiring should work. He feels something is wrong with the sender (rheostat). I know you tried a new gauge. Did you try the sender or is it the original one?

Actually, if I recall, you tested the one you have. Please explain how it was checked. Did you use a multimeter to see if the resistance changed when you move the float?

Joe

I have tried both my existing sending unit, the one that is currently hooked up in the tank, and a brand-new one. I pulled The sending unit out of the tank, and pulled the gauge out of the cluster, and used some alligator clips electrical test leads along with the 12 V battery to confirm function. I have not used a voltmeter to measure the resistance, but the gauge moved when moving the sending unit float arm up and down. I tested this with both the existing sending unit and the brand-new sending you, both with the same results. The problem is when it gets back into the tank and back into the truck.

Hi,

If you have power to the sender and ground, it should work the same. Is there a ground issue?

Joe

It’s got to be. Can the tank itself cause a bad ground? Attached are pics of the current ground connection.

Some further information. I did some more testing today. I tested a brand-new Sending unit connected to the same wires and grounding location is the current in-tank unit, and it worked just fine. Gauge was reading perfectly. However, when I hooked up some brand-new wires to the current in-tank unit, we went back to 3 o’clock or empty. We had pulled the current in-tank unit out and tested it a couple weeks ago, and it was working fine. Something about being in the tank is causing issues.

Interesting. There has to be something getting pinched or a wire that is broken internally and the insulation looks good. If you connect up the one in the tank and move the wires around a little at a time while someone watches the gauge, does it change anything?

Joe

No sir, nothing. I think I’m looking at another tank drop and new sending unit. Yippee for me :)

Hi,

Ugh!!! No fun. However, I believe you are correct. If you have a chance, could you let me know the results? I'm interested in knowing how this turns out for you.

Take care,

Joe

Well, I'm here to publicly admit my stupidity and eat some humble pie. A big slice... Short story: the gauge is now working perfectly. Long story: we dropped the tank again and did some more testing with it off of the truck. During that testing, my wife noticed that the float of the sending unit was barely touching the fuel when it was set down in the tank. Basically, it was reading E because it was E! I was thinking it was a 16-gallon tank, so when I had five gallons in there, I thought I should have at least a quarter tank on the gauge. Turns out it is a 20-gallon tank, and the 4-5 gallons in there is the 'reserve' amount of fuel when it gets to E. I bought 5 more gallons of gas, dumped it, and we had almost a half tank on the gauge. Re-mounted the tank and it's reading the same. So, all in all, the gauge and wiring were all correct and working great, I just didn't have enough gas in the tank for the gauge to read it. Ugh. >face palm

I do have a final question, related to my original question post: when I shake the truck, the gauge does bounce a bit. Is that fixable, or just the nature of the beast?

Hi,

You made me laugh. But you would be surprised how often those types of things happen. LOL It's not just you, I promise. Regardless, I'm glad to know it's working. Also, your last question about the gauge moving around is normal on these vehicles. The tanks aren't baffled in a way to prevent sloshing. So, when the truck bounces, you make a turn, start or stop, the level changes. If you figure out a way to prevent it, let me know. My 1978 does the same thing. Also, thanks for the update. It will likely help others in the future.

Please feel free to come back anytime in the future if you have questions. You are always welcome here. Plus, give your wife a pat on the back for noticing the fuel level. That's an easy one to miss.

Take care,

Joe

I will :) haha Thanks for all of your help!!!

You are very welcome. Stop back anytime.

Take care,

Joe