Newbie with Fuel Pump and Gauge issue 1987 R10 dual tanks

[shannon.posey.56]

Hi there.

I'm having an issue with the Right-Hand tank on my '87 R10.  When I flip the switch to the Right-Hand tank, the engine stalls and the fuel gauge doesn't move at all (still reads what's in Left tank).

I've already replaced the selector switch, so I'm hoping it's just a bad ground.  Am I on the right path as far as narrowing down the problem?

Any suggestions or ideas would be greatly appreciated.

Thanks,
Pose

[frotosride]

That would definitely be the first and cheapest place to start. The plastic 'throw' behind the switch that actually moves the contacts inside the switch wears on most occasions and can leave I stranded. You can take the actual toggle part of the switch off and move the plastic throw by hand to see if it will fully engage the circuit. I remember mine was a pain in the but until I discovered this was the actual problem.

[shannon.posey.56]

I cleaned the old selector switch to see if that worked prior to putting the new switch in, and it didn't make a difference.  The new switch didn't help either.

[bd]

A 5-wire, 6-port fuel selector valve is mounted inside the right frame rail, hidden above a sheetmetal splash shield, beneath the front of the bed.  The valve is motorized and typically will make a brief ratcheting sound as it switches between tanks.  The valve not only switches the fuel tank lines, but simultaneously switches the fuel gauge between the tanks.  Power to energize the tank selector valve and the selected fuel tank pump is controlled directly by the dash switch.

Do you hear the selector valve momentarily "z-i-p" when you flip the dash switch?  Do you hear the in-tank electric fuel pump run for 1-2 seconds after flipping the selector switch and then turning on the ignition?

[hatzie]

Grab the wiring diagrams I scanned and Chris hosted.  Right click and Save as so you can read it offline --->http://www.73-87chevytrucks.com/techinfo/7387CKMans//Wiring/ST_350_87_1987_Chevrolet_GMC_Light_Truck_Wiring_Manual_Complete_11x17.pdf
The schematic for R/V series NL2 Dual Tanks is on PDF page 203.

You can test the ground pretty quick using a multi-meter or test lamp at the switch plug inside the cab.  There are 4 terminals on the switch.  The Gray and Tan wires go to the tank valve.  Tan/Black goes to a plug on the frame and back to the rear light harness plug on the bulkhead... See Grid K-7 & W-7 on PDF page 212. 
The Black wire should stay in the cab running to the Buss Bar under the dash.  Black should be Ground and the Tan/Black wire should be Hot.
You should get full ignition voltage when you switch the ignition on and probe those terminals with a meter.  Since she runs in the LH tank position you will have volts on the hot wire... getting no volts across the Tan/Black and Black wires means the ground is likely unplugged from the buss bar.

The other possibility is the valve has died or the gray wire has a break between the switch and the valve plug.
Both in-tank fuel pumps will run if the dash switch is good even if the valve has given up the ghost.
Both in-tank fuel pumps will run and the valve will not... if the switch is good and the valve is not receiving a ground through the switch.
The gauge will not switch over to your other tank till the valve has moved.

[shannon.posey.56]

Lots of useful info, there.  This will help narrow it down a bunch.

Thanks in advance!

[shannon.posey.56]

I checked the tan/white and black connections at the switch.  They show 12 volts for 2 seconds as it should.  Ended up having to reassemble the switch toggle to the switch itself.

The gray wire to the valve looks good as well, but the only sound I heard was the relay.

After I reassembled and reinstalled the switch, when I flipped to the right tank the fuel gauge went to the 3:00 position.

Any thoughts?

[bd]

I assume you can hear the left fuel pump sing since the engine runs off the left tank.  Does the right fuel pump ever sing when flipping the dash switch - for example, after selecting the right tank from the dash switch and the engine dies, if you shut the ignition off for 5-10 seconds and then switch the ignition back on, does the right fuel pump energize for 2 seconds?

If the right pump energizes, either the selector valve is faulty or there is an intermittent open/excessive resistance in the tan and/or gray wires connecting to the valve, between the valve and the harness splices (see the linked schematic).  If the right pump never sings, look for an open ground from the tank sending unit.

[shannon.posey.56]

Right pump never sings.

I also fixed the ground connection to the buss on the firewall, so the gauge responds all the time.  Under the truck I go......

[bd]

Check the sender ground and check for power on the gray wire running to the right fuel pump.  If both test good, you probably have a faulty pump or pump connections.