Testing fuel gauge

[DirtDonk]

When I disconnect the front tank it stays pegged.

Well that means that, at least as far as the current problem is concerned, it's not the sending unit. This is actually good news since that tank is a pain to pull out!

Do you know how much gas is in each tank? Pegged when it's disconnected is still a problem, but wondering if when things are connected you're just getting the correct reading?
Most older setups tend to read low even when full, but some read too high. Have you run both tanks dry yet to be sure the needles don't go down at all?
I assume you have, but figured I'd check anyway.

Btw it stays the same when I switch back forth between tanks.

So you tried the switch in both positions with both tanks to be sure?

The rear tank it goes to empty.

Well that's good news. That means that the switch and the wiring, at least for the rear tank position is working as you would expect. So that one sounds like the sending unit is bad.
Or the tank is full.

It's original wiring, or i can say it's not modern aftermarket new stuff.

How old do you think the dash switch is? These things are notorious for having a short lifespan. My original one went out when the truck was about 6 years old and under a 100k miles. The second one lasted about 10 years.
We sell a pretty good number of them at WH, and since we're not the only ones selling them, I bet that a ton get sold every year from all the sources.

Telling me this is where you could concentrate your search for now.

This is an inherited issue so I can't tell it's correct or not.

Did you just get the truck recently? Or you've had it awhile and it's just been doing this all along? Either way, it'll be good to have it fixed so you can drive with confidence!

What I would do for now are a couple of things to test.
Either pull the switch out, or stand on your head to get under the dash(!) and see if you can mess with things.
Initially, if you find the Orange wires nearby, each with that funky 90° connector, you can disconnect them from the switch side and connect them both together. The one is the rear tank sender and the other is the gauge, so this is how they would be connected from the factory with only one tank.
If the gauge now works as expected, then your switch is bad.
If it still pegs when the sender is connected and goes to empty when disconnected, then the sending unit in the main tank is toast.

To test the front, the second thing you can try is to swap the wires around on the switch to see if you can see a difference.
The center wire is the power from gauges. The outer wires are the two tanks. You can see which one is the main tank by where the plug was that you changed earlier. But that's not as important as just seeing what happens when you swap the two tanks over on the back of the switch. Or just connect them together temporarily.
If the front tank works as you would expect, then that's double-proof that the switch is bad.

The switch is a good guess, but it's not the only thing that can go bad with this system, so it's worth testing before buying a new one and finding out it wasn't the issue to begin with.

Good luck.

Paul

 

[Hotfeet]

Here's a photo of how the switch was wired.
I changed it to the way my 69 is but the main is full all the time and the aux is empty.
This is a 66 and I am not sure if this came with 2 tanks. It has not worked since I bought it. It is my 17 year olds daily driver and he runs out of gas a lot.

 

[Hotfeet]

Photo of switch

Here's how it was wired at the switch.

 

[DirtDonk]

Could have come with two tanks, but that is certainly not factory wiring by any stretch.
Not even close.

First, the original wiring is only on three of the six contacts. Not sure why they did that, but probably just using an existing switch on different vehicles.
Do the two tanks switch with the mechanical valve under the seat still? Or is it done electrically? if so, that would explain the other wires.

But for now, I would disconnect the switch to see if the pegged needle goes down.

And trace those wires to see what they do.

paul

 

[Hotfeet]

It has a manuel fuel tank valve.
Someone just connected it all weird.
When switch is disconnect gauge goes to empty.

 

[Wyflyer]

How to test the 10-72 sweep?

 

[DirtDonk]

When switch is disconnect gauge goes to empty.

Well then the gauge itself is working. But either the switch or that funky wiring is doing something.
First, you should only have 3 wires total at the switch. Not six.
One is to main tank sender, one is to aux tank sender, and the other is to the gauge. That's it.
Only reason to have six that I can think of is if it's controlling an electric selector valve and/or electric pumps at each tank.
Do you have an electric pump for each tank? If so, and they're not powered by individual relays, then the switch is likely overloaded and needs to be replaces.

If none of that applies, find out which wires go to the tanks and the gauge, then connect those only. If one of the other wires is grounded then that would explain why the gauge is pegged in one switch position.
As mentioned before too though I think, these switches only have a finite life expectancy. Figure on five to ten years per switch as a normal lifespan. My original died after about six years. The second one lasted almost ten.
Lots of other members here with similar stories.

Paul