Fuel Gauge Drama

[DuallyGuy]

My fuel gauge stopped working. It was pegged past full. So I crawled underneath and discovered that the sending unit plug came detached from the tank. So now I have to figure out how to plug it back in without dropping the whole tank. In the process of elimination, I decided to eliminate the whole tank selector switch since there's only one tank underneath anyhow. So, long story short, I would like to know if anyone can tell me how many wires must be connected for the passenger side tank to work and where must they go? I already spliced the sending unit wire which was split up for the sake of the selector switch to the gauge. (Previous owner installed the switch. This was not a factory dual tank truck) So unfortunately, no wires are color coded. ALL black. I see a ground wire from the sending unit, mounted to the frame which I verified has a good ground. Then there's the other wire which I'm assuming is the sending wire which came detached. Other than that, what other wire should there be and where do I connect it? Sorry for being so long. I searched and searched but unfortunately all other threads although helpful, didn't quite pinpoint what I need.

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[chengny]

Well, the problem is - and I'm not trying to be be a wiseguy - if you don't know how the tank valve should be wired, we can't know either.

Is the valve a 5 wire rotary type? It might say Pollak on it and have a 6 pin connector? Or is it single wire solenoid type valve? I'd handle this in a different way - see below.

This one first:

So I crawled underneath and discovered that the sending unit plug came detached from the tank. So now I have to figure out how to plug it back in without dropping the whole tank.

Look behind the cab back and down in the gap in front of the bed. I am not sure about a 1973, but I think you should be able to see where the lead from the gas gauge to the sender goes.

You must be registered for see images attach

You have already confirmed the ground leg of the sender. So now, to have a working gauge, it's only a matter of connecting the sensing leg of the gauge to the positive side of the sender. The terminal is just a brass pin - no threads, the factory connector just pushes onto it. If you can see that pin, you should be able to somehow get either your hand down in there - or some kind of tool - and press the connector onto the pin.

The original wire from the gas gauge to the tank should have been tan. You might have to fish around under the dash a bit, but you should be able to locate it- at least the section that ran from the cluster to the dash switch. I can't imagine why anyone would disturb the wiring leading into the switch.

Even if you have to run a whole new sensing leg from the dash back to the tank, it won't be a big deal. The wiring diagram shows the tan 30 lead coming through the firewall connection block in the upper inboard corner. That may have been changed when the second tank was added but the remnant might still be there. Run the wire along with the rear lighting harness inside the LH frame and then take a left hand turn at the cab mount cross member and run it over to the RH tank.

Anyway once you have a working gas gauge, connecting the plumbing should be easy as well. Can I make a suggestion? Rather than trying to rewire the tank transfer valve to always be lined up to the RH tank, why don't you consider simply eliminating the valve and all the tubing/hoses used to do the modification all together?

If the dual tank system was a modification, the original tubing to the production (LH) tank was most likely cut and piped to the transfer valve location. I'll bet the original tubing is still intact all the way up to the RH side of the cab back. It would be a simple matter to remove the valve and all the rubber splice hoses and repipe the fuel pump supply line directly to the RH tank. It is perfectly okay to use rubber sections in the gas lines as long as they are not near any exhaust piping.

Let us know - and send pictures if possible.

 

[DuallyGuy]

*update*

In the pictures I attached you can see that I successfully reattached the sending wire. You can also see the ground wire which seems to be properly grounded. I also attached a photo of the transfer valve which had no hoses attached to it only one black wire which ran to the fuel selector switch. I removed the wire because I didn't see a need for it. Was that a mistake? The fuel gauge still does not work.

 

[chengny]

Yep, that is a single wire solenoid type tank transfer valve. After leaving the dash mounted select switch, the two gauge sensing legs and the power feed to the solenoid valve go their separate ways. The gauge sensing lines go nowhere near the valve.

Here is the quick litmus test for whether the gauge is working (provided the sensing leg to the sender is properly connected to the gauge):

1. If the sensing leg (from the gauge to the sender) is broken/disconnected the control leg exerts the greater force and the needle on the gauge should peg to Full.

2. If the sensing lead is allowed to contact ground (before passing through the sender), the sensing leg has a greater force and the needle on the gauge will peg low.

Try grounding the lead you've been working with and (with the key in RUN) note whether the gas gauge pegs low.

Are all your other gauges okay? Were you able to locate any part of the original tan gauge-to-sender lead?

That certainly looks like a GM factory sender connector. There's no way around this. You are going to have to determine if the wire that you currently have connected to the sender is actually connected to the gauge a the other end. It doesn't have to be all the way to the gauge but at least into the cab were you can visually verify that it does lead to the instrument cluster. You can do this a few ways; by a continuity check from the sender end to the gauge end, confirm voltage at the connector with the fuse in and no voltage with the fuse pulled, try the test described above and note if the gauge reacts as designed, pull on the wire at one end and see if it moves on the other, etc.

I wish we knew a few things before you go on a mission that is doomed to fail:

1. Does the gauge even work

2. Does the sender resistance change according to tank level

3. Like above - is the wire shown attached to the sender connected to the gas gauge at the dash

Do you have a multimeter and a pretty good idea how to use the VDC & Audible Continuity functions?

Were able to verify that the tan wire passes through the firewall connector at the very top and inboard position? That's all we are dealing with here - making sure that one wire goes from the gauge to the sender. If we can verify that and it still doesn't work...at least we can begin to troubleshoot confident in the knowledge that the issue isn't wiring related.

From the 1973 service manual. It's my earliest one and I have trouble reading it - most of it is written in hieroglyphics:

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[DuallyGuy]

I did my best to verify that the tan wire going from the sender to the gauge was indeed intact and connected. I have yet to test it with a multimeter because of schedule. According to the chart you attached, it does look like the correct wires are connected and tan in color. What I did want to mention is that I did pull the fuel gauge out and inspected it. It seems to be connected correctly and did work a month ago until I had some engine work done. Now the gauge needle sits at quarter tank. When I turn the key on, it pegs past full. I will pull out my multimeter this evening. But what a pain in the neck it has been. (literally)

 

[Camar068]

While troubleshooting mine a 6' jumper helped half split troubleshoot it back to the gauge.

 

[chengny]

If you're confident that the sensing lead from the gauge to the sender is intact, it might be that the sender is bad.

But given this:

It seems to be connected correctly and did work a month ago until I had some engine work done. Now the gauge needle sits at quarter tank. When I turn the key on, it pegs past full.

I would look for damage to the tan lead (sensing leg) in the engine compartment. I don't know how representative it is, but the wiring diagram shows the tan lead as transiting the firewall at the main connector and then running over to the RH side - before turning and heading back to the sender. Maybe it was damaged during the engine work?

 

[Rusty Nail]

This title of this thread is funny.

 

[DuallyGuy]

>>update<<

Sorry it's taken so long to report my findings. As the title says, it's just been a bunch of drama. So basically, even though I was able to plug the sending wire back on to the sending unit, the fuel sending unit still didn't work. So first, I pulled an old sending unit out of the trash at work and used that to verify that the fuel gauge sure enough does work. It went from empty to full just as it's supposed to. When I verified that, I knew I had to drop the tank and replace the fuel sending unit. I even connected it directly to the fuel gauge to re-verify that it didn't work before spending $70 on a new one. And just to be on the safe side, I even ran a fresh wire from the fuel gauge all the way to the new sending unit through the firewall and along the frame. But the drama didn't end there...NOW the fuel gauge works, but only reads 3/4 tank on a full tank of gas. I verified from several different sources the part numbers on various websites and various parts stores. (Shrug) Well at least I know I have about a quarter of a tank when the needle hits E. So I guess that can't be too bad....? If anyone can suggest what may have gone wrong, I'd like to know. Thanks for all of your help.

 

[87scotty]

Id pull er out give the float a tweak

 

[Rusty Nail]

Yep.

Peace through strength.

Drama!

My drama gauge acts in a similar fashion, I'm anxiously awaiting the outcome.... Gonna fix mine pritty soon! It bounces around FULL, all the time.

 

[Rusty Nail]

Nah. It was bouncing around 3/4-1/2 today and went to fill up, it took 25 gallons.

SO!

1/2 tank = 7-8 gallons. Is 33 gallon tank?
Whatever.