Dual tanks not switching

[Ol' Red]

My truck is not wanting to switch tanks. It is stuck on the left side. Won't switch to the right. I took the switch out in the dash and confirmed that it is working correctly. I checked to make sure i am getting power to my selector valve like i should. I am. I have the 6 prong WeatherPak connector. I bought a new selector valve and it is showing no results proving that my valve is still good. I cleaned the ground from the sending unit to my frame on the right side. It is still not working. Does anyone have any ideas on what could be going on or what else i could check?

[JAMESCHURCH13]

I have the same problem on my Dually. I gave up and just use the right tank but I would like any hints this thread gets also.

[Duster]

Track your wires.

[wilkin250r]

Did GM change the configuration when they changed the selector valve? My default tank is my passenger-side (right side). If my switch breaks, or a wire breaks, or I somehow lose power to my selector valve, it defaults to the right side tank.

That probably doesn't help much, except I would be curious to know why you're stuck on the opposite side. I agree with Duster, start tracing your wires.

I'm not 100% familiar with the 6-prong selector valve, mine is the old single-wire 3-port valve, but I just replaced mine and I know the basics and theory.

First, I would find out if you're getting power to your selector valve.

If that works, I would verify that your valve is indeed switching, that it's routing the ports correctly to the correct inputs and outputs, and also that it is routing the proper electrical connections. This might be easier with your OLD valve on a workbench.

[Ol' Red]

What I'm not understanding is I seem to be getting power to the valve. When i press the switch both ways the switch seems to be working correctly too. I bought a brand new selector valve and still isn't working. I find it strange that it is stuck on the left rather than the default right. I am wondering if maybe something could be going on with the wires going to my sending unit or the wires from the sending unit to the frame ground. Also when I unplug it from the valve it doesn't seem to default to the right tank.

[Ville K]

Might be a stupid question, but have you pressed the swithc button long enough?
I thought I had a faulty switch also, but then lerned to push it few seconds and it works fine.

[Ol' Red]

Yes. I was working on it by myself at the time so i had the switch c clamped each way then crawled under the truck to check my voltage.

[Duster]

Don't rule out a bad replacement part either. Half that crap is made in china now.

[hatzie]

Is the polarity reversing dash switch momentary (M/ON-OFF-M/ON) or continuous (ON-ON)?

The switch should not spring back to a center off and it should continue to conduct in the selected position without holding the switch down. I've seen momentary switches listed as a proper replacement for the 81-91 dual tank system. Momentary switches are the wrong part.

The valve has two SPDT microswitches.

1. A limit microswitch and two diodes to cut power to the valve shuttle motor to prevent burning up the motor.
2. A SPDT microswitch to select the sender that feeds the fuel gauge.

[wilkin250r]

Again, I haven't brushed up on the 6-terminal switches, but I know my electronics. You only have three possible pieces to this puzzle, it has to be one of them.

1.) The Switch and/or Wiring. You're not getting proper power, or time, or polarity to the valve.

2.) The Valve. Don't rule out bad replacement parts, like Duster said.

3.) The tank connections and fuel lines are connected wrong.


I would start with scenario #2, because it seems like the most likely. If you suspect your old valve is still good, then you should VERIFY it is good. Put it on a workbench and test everything. Hook up a small tube and blow through each fuel passage and make sure they route correctly. Also, use the continuity setting on your multimeter (or even just the resistance setting) to verify it route the proper electrical connections for the fuel sending units. Hatzie can point you to the wiring diagrams.

Then apply proper power to activate the "switch" on the valve, and verify the valve switches everything, and re-routes everything correctly. Maybe even switch it back and verify again.

Since the valve is the most likely culprit, I would start with getting 100% undeniable proof that it's working correctly. Electrical problems are a matter of elimination, and I would suggest eliminating the valve as a possible cause, even though it's "new".

[Ol' Red]

I definitely like your thinking on the valve. I'm not all that experience with electricity, which terminals on the valve do I need to give power to for it to switch each way?

[hatzie]

Originally Posted by Ol' Red

I definitely like your thinking on the valve. I'm not all that experience with electricity, which terminals on the valve do I need to give power to for it to switch each way?

D & E

I'd put + on E and - on D.
If the motor doesn't wake up it may be all the way to that stop.
reverse the polarity to - on E and + on D.

Get a couple mating weatherpak terminals or a fuel valve pigtail to keep from mangling the valve connection. I think NAPA stocks them.
12089188, Female Weatherpak Terminals
12089040, Male Weatherpak Terminals

[Ol' Red]

I tested what you said to do. I didn't take it off the truck but I did unplug the connector and went under there with a 12 volt battery. I had a test light hook up in the circuit. I touched D and E with both polarities and the valve didn't do anything.
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[Ol' Red]

I should also add that even though the valve didn't do anything when i touched the terminals my test light lit up proving I have continuity through the valve but the motor isn't working still.

[jbristo67]

I had this problem in my 83 short wide. There is a sender that the switch works off of. It's been prob ten years since I had that truck but I want to say it was on the frame.
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[pontgta]

I would drop the tank and make sure that all the hoses in the tank are good. If you have voltage at the selector valve, and you have replcaced it ......My money would be on the hoses....Of course you could have a bad valve.....Test Bench it by adding 12v and then switch polarity on it, and you should hear or feel it. again check your hoses.

[Skunksmash]

On my truck it was the actual mechanism that makes the tanks switch. The part itself wasn't expensive, but it was a pain to change out cause you had to drop one of the tanks. They sell it on rockauto.com

Other than that, it could actually be the switch itself, if no one has mentioned that already. You can get a brand spankin new dash switch at autozone for $12. Replace that old grey crusty thing.

[hatzie]

Originally Posted by Ol' Red

I tested what you said to do. I didn't take it off the truck but I did unplug the connector and went under there with a 12 volt battery. I had a test light hook up in the circuit. I touched D and E with both polarities and the valve didn't do anything.
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Test light in the connector on the valve I assume.
If you tested the valve and get no joy replace it.

BTW if the valve is at the end of its' travel it should only conduct one way.

[chevy1ton]

I have messed with the dual tank circuits in these trucks several times and they can definitely get you spun up with some of the problems I've seen.
Mine is also stuck on one side at the moment but was working fine before I did some 'remodeling'.

Lets start out with the basics if we could. Start with the switch on the dash, you need power at the pink wire with ign on, (if none, check for bad fuse) and a good ground with your black wire. If you have both you should have power coming out of your switch on the dk green (left) and lte green (right), if so your switch is ok.
Next go to your 6 pin connector on your selector valve on the frame and confirm power at the dk green and lte green when you throw your switch there also. You can hear the switch when it is working properly.
If you have no power to either green wires when switching follow the green wires to the 2 pin connector near the firewall before they enter the cab, you may have a bad connection there or damage along the way.
If you do have power to both green wires when switching but valve still not working then you either have a bad switching valve or you may have a bad ground to the sending unit or a bad sending unit itself. The switching valve will not work without a good ground at the senders.
If you think you have a bad ground or bad sending unit, ground the pink/ wht (left) or the pink/blk (right) and that will bypass the sending unit and complete the circuit as though you have a full tank, now see if the switching valve reacts. If it finally works repair the ground or replace the sender.
If you follow all these instructions and it still does not work you missed something or have a burned or chaffed wire in the circuit somewhere.
Hope this helps.
Check out Hatzie's wiring diagrams that he was so nice to post for everyone.

[Ol' Red]

This is all very helpful. I double checked today that I am getting power to my light and dark green wires. So that leaves bad grounds off the tanks, sending units, or valve. I don't think it's the valve since I tried a new one and that wasn't it. I cleaned the grounds off of both tanks so that shouldn't be an issue. I guess I'll have to by pass the sending unit and see what happens. My question is should I by pass both at the same time or one at a time or just right since it won't go to the right?
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[chevy1ton]

Do one side at a time, go ahead and focus on the side that is not working, chances are that is where your problem lies.
I have to diagnose mine today as well, I will share what I find.

[hatzie]

Originally Posted by Ol' Red

This is all very helpful. I double checked today that I am getting power to my light and dark green wires. So that leaves bad grounds off the tanks, sending units, or valve. I don't think it's the valve since I tried a new one and that wasn't it. I cleaned the grounds off of both tanks so that shouldn't be an issue. I guess I'll have to by pass the sending unit and see what happens. My question is should I by pass both at the same time or one at a time or just right since it won't go to the right?
Posted via Mobile Device

Grounds on the tanks are for the senders and eventually the gauge not the valve. If they both are good then they will not keep the gauge from working. However they don't provide ground to the valve.

Let's go back to the beginning.

From the 1985 NL2 Dual Tank wiring diagram below.

• The Fuel Tank Selector Valves' ground comes from a buss bar under the dash. Usually near the E-Brake pedal.
• The Fuel Tank Selector Valves' power comes directly from a fused IGN socket on the fuse panel.
• The Fuel Tank Selector Switch reverses the polarity of the green wires to reverse the DC motor in the Fuel Tank Selector Valve.
• When you switch to the LH tank the LT GRN should be hot and DK GRN should be ground.
• When you switch to the RH tank the LT GRN wire becomes ground and the DK GRN wire becomes hot.

You tested for power with a test light, not a meter, and you have a positive result for power. Ground??

Set the tank switch to LH, Turn on the ignition, and connect your test light to both green wires on plug posts D & E at the Fuel Tank Selector Valve plug.

If the light doesn't glow flip the switch to RH.
If the light still doesn't glow check from a frame ground to DK GRN.
If the light glows from the frame your tank switch ground under the dash is the problem.
If the light only lights in one switch position replace the switch.
If the light comes on in LH & RH across the green wires and the valve doesn't move, when plugged in, replace the valve.

Colorized Switch LH & RH positions. Should help visualize how the polarity reversing tank switch works.


1985 NL2 wiring diagram

[wilkin250r]

Originally Posted by Ol' Red

I definitely like your thinking on the valve. I'm not all that experience with electricity, which terminals on the valve do I need to give power to for it to switch each way?

Don't worry about the electrical knowledge, I can help with that.

You should be able to hear your valve switching when you reverse polarity. You held the voltage there for several seconds, yes? The valve doesn't change instantly, it's a motor, it's not a solenoid (solenoids change much faster). You should definitely hear it.

Did you try it with your old valve also?

[Ol' Red]

Ok this is where I currently am. I have the original valve on the truck. I took the new one back to advance because I didn't think that was the problem at the time. So I am getting the proper power to both green wires. 12v from lt green to dk green just as hatzie said. I cleaned and sanded both grounds off the senders. Today I bypassed the sender and went straight to ground. It still didn't work. I hooked. 12v battery to the motor terminals d and e. the way I did it was I had 1 regular wire going from positive to d. Then I had a wire with a test light on it going to e from the negative terminal. The motor didn't go however the light lit up showing I have continuity through the valve motor. I am wondering if just maybe the reason the new valve didn't work when I tried it was because my ground to the sending unit was bad. The last time it was on the right side the gauge was at 3 o'clock which I've heard it lack of ground. I have since cleaned that ground but have not tried a new valve since the . One other thing, for some reason I can hear a small click when I switch left but not right. I hope this is all making sense what I said.
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[chevy1ton]

Thank you hatzie for correcting me on that, I confirmed that today myself as I pocked around with my test light and apologize for the misdirection.
I found my problem to be just a bad connection at the dash switch.