Dual tank fuel gauge makes no sense.

[JM in KY]

Posted before with no replies. Someone take pity on me.

My dual fuel tanks switch appropriately and both pumps work well. Switch in the cab and selector valve are new (replaced by me). However, fuel level reads only from left tank regardless of which tank is selected. How is this possible? I’ve looked over wiring diagram and can’t see a way for this to happen. Wiring on my truck appears to be original and untouched everywhere I’ve looked.

 

[wanderinthru]

Something is wired wrong. Duh, huh. That being said, new parts don't mean good parts. good chance the valve has a screwed wire in it. Did it work before you changed the valve?

 

[85K304SPD]

I wish I could help you. How do you know that it only reads the left tank? You have to push the switch real hard to make it change. I have never had a Square with fuel injection, so Im not too sure about the pumps in the tank.

 

[JM in KY]

Before changing the selector valve, the gas gauge did exactly the same thing. It would only read the left tank, regardless of which tank was selected. I replaced the switch and valve because switching to the right tank immediately made the truck die (not sputtering out of fuel - instant shut off).

Now I can hear the pumps change when I change the switch and I know the right tank is only about half full (left is full).

The newer squares (or at least the TBI models) use a switch that just rocks up and down and does not require you to “depress fully” like earlier switches. Just changing the switch position makes it change tanks.

 

[WP29P4A]

So everything else is working correctly, it just won't flip the gauge to the right tank? Can you post the diagram your using?

 

[SquareRoot]

Your right side sending unit is bad or the wiring/ plug are bad. Trust me. I just converted my 85 to EFI and replaced the whole system. I know it well. The switching valve has two input (left or right) and ONE output. If the left side works, the issue is on the input side. The switching valve may also be defective, despite it being new. Swap the input wire and see if the problem follows it.

 

[wanderinthru]

Before changing the selector valve, the gas gauge did exactly the same thing. It would only read the left tank, regardless of which tank was selected. I replaced the switch and valve because switching to the right tank immediately made the truck die (not sputtering out of fuel - instant shut off).

Now I can hear the pumps change when I change the switch and I know the right tank is only about half full (left is full).

The newer squares (or at least the TBI models) use a switch that just rocks up and down and does not require you to “depress fully” like earlier switches. Just changing the switch position makes it change tanks.

I'm with @SquareRoot, I'm wagering on sending unit in the tank. It ain't reading the full tank it has failed to full, not sure about the tbi's but the old one's generally show full when they are broke, or, that's my experience. It runs on both tanks now?

 

[JM in KY]

It doesn’t read “over full” like what happens when you disconnect the ground wire. It’s constantly showing me the volume of fuel in the left tank, including the small fluctuations that happen when you come to a stop, accelerate, etc. it runs on both tanks just fine, but never switches to show the right tank fuel level.

I can hear each pump running when the appropriate tank is selected with the switch. This is true whether the truck is running or if I use the testing wire on the relay at the top of the firewall.

It just doesn’t make sense to me that the pumps run correctly but the fuel level always reflects the left side.

 

[Kasey 1977 K10]

I did an EFI conversion on my '77 and don't have any issues with the fuel level based on the selected tank. Take a look at https://www.gmsquarebody.com/threads/tank-replacement-for-efi.24131/page-2#post-472294 which shows the parts list I used. I agree with others that it is most likely incorrect wiring or a bad selector valve or sending unit.

 

[Dave Arends]

Check your wires real good. Because they have a few factory splices in them. May have to remove factory tape and see if corroded or damaged. Mine wouldn’t switch on left tank pump because of corroded splice. Could also be bad sender in tank or selector valve like others have said.

 

[WP29P4A]

What happens if you use a piece of wire to bypass the polick valve, and directly connect the sender to the gauge, comparing both tanks?

 

[SquareRoot]

Your switching valve is bad. Probably just the internal part that switches the sending unit wire from left to right.

 

[mtbadbob]

Before changing the selector valve, the gas gauge did exactly the same thing. It would only read the left tank, regardless of which tank was selected. I replaced the switch and valve because switching to the right tank immediately made the truck die (not sputtering out of fuel - instant shut off).

Now I can hear the pumps change when I change the switch and I know the right tank is only about half full (left is full).

The newer squares (or at least the TBI models) use a switch that just rocks up and down and does not require you to “depress fully” like earlier switches. Just changing the switch position makes it change tanks.

My '87 TBI has the tank switch that stays on the selected tank, NOT the rocker style. I had to splice a factory plug onto my harness as the PO had it rigged with a toggle & about 8 wires. Anyway, the earlier 80's harnesses had a different plug that would only fit the rocker type fuel tank selector. I found that out by trial & error, and that's how I realized the TBI's need the switch that stays on the selected tank as the fuel pumps need the constant 12v to keep them running. The rocker type just send an initial 12v signal to tank selector valve. My '87 factory wiring manual confirms this with the wire color combo's for the fuel tank & valve switches. I had also originally purchased the rocker-type tank selector, and it would not fit the correct 1987 harness plug. My fuel gauge reads both tanks correctly when switched that way also.

 

[SirRobyn0]

My '87 TBI has the tank switch that stays on the selected tank, NOT the rocker style. I had to splice a factory plug onto my harness as the PO had it rigged with a toggle & about 8 wires. Anyway, the earlier 80's harnesses had a different plug that would only fit the rocker type fuel tank selector. I found that out by trial & error, and that's how I realized the TBI's need the switch that stays on the selected tank as the fuel pumps need the constant 12v to keep them running. The rocker type just send an initial 12v signal to tank selector valve. My '87 factory wiring manual confirms this with the wire color combo's for the fuel tank & valve switches. I had also originally purchased the rocker-type tank selector, and it would not fit the correct 1987 harness plug. My fuel gauge reads both tanks correctly when switched that way also.

Hey, Bob sorry I didn't see your previous thread or this one until now.

As far as the systems GM used over the years they are all pretty similar except the 70's models that ran only a single wire to the valve. In those systems the switch on the dash was terrible and notorious for failure. But the later systems like yours where the valve handles the gauge change over those were much more reliable.

With that said I just can't see it being anything on than the valve. If it's new, it's got a warranty swap it out and see if that fixes it. If your sending unit was open or shorted the fuel gauge would either end up on E - past full for that tank. I can't see it being a sending unit issue. I can't see how it would be a wiring issue at the dash switch.

But I do have some questions:
- I assume or may have missed this but does the pump change, what I mean is when you switch to the right tank does the left pump turn off and the right pump turn on?
- And what do you mean by "My fuel gauge reads both tanks correctly when switched that way also." With the rocker switch or toggle switch that was in there?

 

[mtbadbob]

Hey, Bob sorry I didn't see your previous thread or this one until now.

As far as the systems GM used over the years they are all pretty similar except the 70's models that ran only a single wire to the valve. In those systems the switch on the dash was terrible and notorious for failure. But the later systems like yours where the valve handles the gauge change over those were much more reliable.

With that said I just can't see it being anything on than the valve. If it's new, it's got a warranty swap it out and see if that fixes it. If your sending unit was open or shorted the fuel gauge would either end up on E - past full for that tank. I can't see it being a sending unit issue. I can't see how it would be a wiring issue at the dash switch.

But I do have some questions:
- I assume or may have missed this but does the pump change, what I mean is when you switch to the right tank does the left pump turn off and the right pump turn on?
- And what do you mean by "My fuel gauge reads both tanks correctly when switched that way also." With the rocker switch or toggle switch that was in there?

My truck does not have the rocker switch, and my fuel gauge reads correctly on each tank. I was merely informing JM of KY on the status of my '87 TBI set-up as I thought he suggested that the newer switches were the rocker-type, which mine isn't. My switch is the correct one that stays depressed on the selected tank. As you know, the switches will not interchange with the different factory harness plugs as there is an odd pin on the back of the switch to prevent that from happening. If you look them up on NapaOnline, you can view both types of switches & see the difference in the switch pins/terminals.