Another fuel gauge issue

[87scotty]

On my bosses 87 we have had valve issues so I put one way checkvalves in and didn't use the electric valve for the fuel still left it plugged in for gauge! My problem now is it seems to read both tanks at the same time (if there path full it is completely pegged) if one is empty and one is at half it will read half tank doesn't seem to matter which is empty and which is full! And switching from tank to tank does not affect gauge!

 

[chengny]

I don't fully understand what you mean by the following:

so I put one way checkvalves in and didn't use the electric valve for the fuel still left it plugged in for gauge!

I'm assuming the fuel system is the TBI type (with in-tank fuel pumps). The only way I can imagine your plumbing arrangement is:

You installed a check valve in the discharge line of each pump, tee'd the outlet sides of the check valves together and then coupled that common line to the normal engine supply line. This would allow one tank's pump to operate (and pressurize the engine's TBI system), but without cross-feeding over into the other tank.

But if that is what you did, how are you switching the power supply to the fuel pumps from tank to tank?

Here is the dual tank wiring diagram for a 1987 with TBI:

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The 3 pink wires (pink, pink/black & pink/white) make up just the tank level indication system - and they are an entirely different circuit than the fuel pump/transfer valve power supply circuit.

The contacts connected to the pinks are switched to show the level in the active tank. But, these contacts are only mechanically interfaced within the tank switch actuator. They are somehow "piggy backed" on to the linkage that moves when the tank switch valves/pump power are shuttled.

When you change the position of the dash switch, you are only switching the main power supply (tan/white) to either the gray or tan leads. These leads are split off before the transfer valve and one leg goes to the pump while the other continues on to the valve where it shuttles the valve. When the valve changes position it also moves the contacts for the level sender.

This is not the best explanation/answer to your question, but I really don't get what was involved in your modification. Also, I am in the middle of doing a blower motor replacement on my daughter's Saab 9000 - that is a sucky job - so I'm a bit distracted.

 

[87scotty]

Yes

 

[87scotty]

Not motor valves sorry the fuel valve on frame rail

 

[chengny]

Not motor valves sorry the fuel valve on frame rail

Oh yeah, no problem there. I understood that you meant the dual tank transfer valve. But re-read my post. I edited it for clarity.

 

[87scotty]

Yes didn't see post! the switch from tank to tank switches the pimps

 

[chengny]

So the plumbing remains the same as stock - except that you added a check valve to each pump's discharge (to eliminate the possibility of cross feeding from tank to tank).

But what about this:

and didn't use the electric valve for the fuel still left it plugged in for gauge!


How did you not "use the electric valve"? Did you cut the gray and tan leads at the valve's harness connector?

If so (and after reading the post above), do you now see how that would effect the tank level operation?

Because the valve guts are mechanically linked to the sender switch, if the actual valve plug does not rotate/shuttle, the contacts that feed the level senders will not move either.