Re: 1973-1991 Dual fuel tank systems theory of operation
 

This is the diagram I'm using to troubleshoot.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

That's the NL2 option wiring for the 87-91 TBI trucks.

There's a two position Weatherpak with the wire from the B terminal of the six position Weatherpak on the valve. This plugs into the standard wiring harness. That's where I'd start looking for Mickey Mouse wiring.

This all assumes that pouring five gallons in the LH tank will raise the needle on the gauge past 1/4 tank.
Does it do that?

Re: 1973-1991 Dual fuel tank systems theory of operation
 

LH tank showed 3/8s of a tank and I put approx 13 gals and the gauge went to full.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

So the gauge is working.

It really sounds like the LH sender is tied directly to the gauge bypassing the sender switch in the valve.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Well after reading this thread about 100 times, studying multiple diagrams and thinking about everything that Hatzie has told me to look at, I finally figured it out. Somebody had two of the connectors plugged in wrong! Both of these connectors are by the LH tank and the wire colors are very similar (see diagram below). It's crazy that everything still worked (other than the RH tank reading incorrectly). I plugged the connectors in correctly and everything works perfect now. Thanks everyone for all the help, especially Hatzie!:metal::metal::metal::metal:

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Thanks for posting back what you found. It helps people out that may be having the same or similar issue.
Enjoy the truck it looks like a nice machine.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

This may have been asked, but I didn't see it anywhere. How do you know what tanks sizes came in our trucks?

SWB - both tanks 16 gallon capacity?
LWB - both tanks 20 gallon capacity?

Thanks in advance!

Re: 1973-1991 Dual fuel tank systems theory of operation
 

LWB could be either a 16 or 20 single tank. I've owned both.
I doubt SWB has the room for a 20 gallon tank.

Not sure someone that went to the trouble of ordering NL2 and the 8' bed would order a pair of 16 gallon tanks. I doubt a dealer would order something odd like that for stock either.

Honestly you can fit the 20gallon tank with the 16 gallon straps so if you have a LWB truck just get the 20 gallon tanks and have done with it.

Something that occurred to me that needs to be said.

The NL2 dual tank harness is an add-on to the production fuel system wiring that plugs right into the existing single tank sender/pump connection on the standard harness. Probably for ease of assembly and reducing the number of electrical harnesses to select on the line. This also would allow a dealership or customer to add it if desired but I doubt that was part of the GM thought process.
Someone swapped the standard production fuel tank connector and the connector from the valve. Probably when doing service on the LH tank.

I have several colors of 3M Super 33+ Electrical tape that I wrap on harness wrap ends with plugs that can be interchanged... to avoid the mistake you discovered.

The LH fuel pump was running dead head to the valve when the RH tank was selected. That can't be safe. Not sure what mayhem you avoided by fixing this but it's all good now.

Re: 1981-1991 NL2 Dual fuel tank systems theory of operation
 

I have a stand alone harness with its own fuel pump relay and power wire. Any reason I can't just plug that fuel pump power wire from the LS harness into the 'test' wire on the factory TBI relay on the firewall? Trying not to butcher the factory harness, nor make more work for myself.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Terminal D, the hot coil terminal of the fuel pump relay, was driven by the TBI ECM on Connector A Terminal 1... if the wiring diagrams are correct. I posted a snippet of the fuel pump circuit in the tech writeup.

The most elegant way to do this would be to drive the relay coil exactly like the original setup.

I'd find the original TBI ECM end of the fuel pump relay drive wire and attach it to the fuel pump drive on the LS PCM. This will offload any high current draws that might make the PCM fuel pump driver transistor unhappy.

You should be able to buzz the original ECM Connector A Terminal 1 wire to the fuel pump relay terminal D to be sure you have the correct wire.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

I guess some of the info here is over this redneck's head. Lol

Dumb it down for me, which color wire do I need to splice into on the firewall relay harness to the LS fuel pump wire into to make it all work? The LS harness already has its own 40 amp relay with power just for the fuel pump. It's a single 12v power wire intended to wire directly to a single fuel pump.

Thanks.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

If you don't want to use the factory relay, then something like this coming from your EFI standalone pump relay...

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Awesome!!

Exactly what I needed. Thx!!

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Bypassing the relay is a bad idea.
It's a more reliable setup to use the 1987-91 fuel pump relay as it was originally setup by GM.

Why?
The relay isolates the PCM from the fuel pump power circuit. This keeps fuel pump wiring problems and fuel pump problems from smoking the fuel pump driver transistor in your PCM. When you let out the magic smoke you can't put it back.


http://johnnynightstick.s3.amazonaws...ematic_TBI.JPG

The original TBI ECM has two connectors.
A & B.
The individual terminals in each ECM connector are numbered.

The fuel pump relay connector terminals are lettered A-F. As you can see in the above wiring diagram.

Splice the LS fuel pump power wire to the TBI fuel pump power wire... according to the diagram above it's a Green with White stripe wire from the old ECM Connector A on Terminal position 1 to the relay plug terminal position D.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

I guess I am confused. I have a separate relay for the fuel pump in my stand alone harness. Why would I want to run through another relay? Not meaning any insult, just asking for clarification.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

The terminals in your relay are open to the weather for corrosion to get into the wires, terminals, and the relay itself. A setup like that will likely last a few years before the corrosion sets in. I've run that exact relay socket type in the engine compartment and it lasted about three years before the green puss ate it up.

The GM fuel pump relay is a sealed unit with a sealed Metripak plug. The terminals are not likely corroded after 30+ years in service.
And you can tie it in with one wire directly from the LS PCM.

Your choice.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

I am sure you are correct, especially with New Hampshire Winters. Sunny California Winters are much milder (the only benefit to this state, I might add). However your point is well taken.

So I guess there is no need for the relay on the stand alone harness, just the fuel pump signal wire connected to the green w/white wire going into the stock TBI fuel pump relay on the firewall. Correct?

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Correct, that is a better way of doing it. Just make sure the original relay is good.

Hatzie do you forsee any issues with the original tank wiring/setup handling the high amperage draw of the higher PSI pumps?

Re: 1973-1991 Dual fuel tank systems theory of operation
 

That Metripak relay was used by GM up through at least 2005 ish (probably later) for lots of different things on lots of their cars, vans, and trucks.
O'Riley stocks them for cheap.

Electric fuel pumps draw about 5-8amps at 14.5vdc and a tiny bit more at 13.5vdc. The newer more efficient LS pumps aren't going to draw significantly more than the original old inefficient TBI pumps from the late 1980's.

The wiring diagrams state the GM wiring is 0.8mm² (18AWG)

The PCM controlled AD series alternators on the LS motors don't typically idle state charge at much more than 13vdc. They can climb to 14.5v if the PCM detects higher current draw but typically you'll see them run 13vdc out.

18ga stranded copper automotive wire will carry 18amps at 13vdc and a little more at 14.5vdc. That's a pretty healthy safety margin.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

That's good info to know, if I decide to do a future LS swap...:D

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Almost done with my LS swap into my dually...

Will putting 12v accidentally on the wire going to the tank sender kill it? We weren't getting the fuel pump to kick on so ran a hot lead straight to the 2 wire connection, but my buddy touched the sender wire for a second instead of the pump wire. The pump works find, but the driver side tank isn't reading. Needle pegged on empty. The passenger side tank sender is working fine.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Speaking of LS swaps, does an 85-86 C10 with factory dual tanks have the correct wiring to install an updated 65 psi valve or is it still required to get the whole harness/switch/valve kit?

Re: 1973-1991 Dual fuel tank systems theory of operation
 

ditto for me! ...:lol:
I believe that the answer is yes, but I enjoy hatzies terrific knowledge!:metal: so hopefully he will guide us straight...;)

Re: 1973-1991 Dual fuel tank systems theory of operation
 

I recently had my OEM Pollack valve completely fail to the point of requiring replacement. For decades it worked to the point of being passable. It really never had trouble switching to the left tank, switching to the right was finicky. When it did not switch completely, the gauge would over sweep clockwise and truck would starve for fuel. I would switch back and forth and to the point it finally read properly on the RH tank, the switch-over was full and complete. That process recently failed and neither tank would read properly but I could pull from the left, I just didn't know how much fuel in LH tank. Needless to say, time to change valve.

I went through and tested 9 or 10 valves I had with test on truck, wife inside, me underneath. One flat did nothing. All others you could clearly hear and feel the switching action. Most read properly on LH tank, RH tank gauge would over sweep clockwise. Only two passed the test 100%.

It seems a comprehensive test requires fuel gauge feedback. Most valves fail for RH tank selection. All but one valve felt and sounded operable on simple bench test, even the one just removed, however, such valve will not uncover ports for RH tank.

FYI

Re: 1973-1991 Dual fuel tank systems theory of operation
 

The wiring is the same. Pollack mad a change to the shuttle and body to withstand 65psi. Or they actually tested it at that level and discovered it works...
Either way I wouldn't put an old valve together with a higher than TBI pressure fuel pump.

Sounds like they are gummed up or worn out. You might be able to soak the valve section in hot PineSol and water to free them up. How long they'll live after that treatment is questionable at best.

I would probably exercise the valve to keep it happy.
Fill up both tanks. Run one down to 1/2 and switch over. Fill the low tank before the runnig tank is down to 1/2 and don't switch back til the one you're operating out of is at 1/2.
Then flip back and two that way so the valve doesn't have a chance to get gummy and stuck.
On long trips you can run down to 3/8 switch and grab a fill when both are down. That way you have 1/2 tank of fuel between them so it's not urgent. I usually have to urgently go to the Mens room or get some caffeine before I need fuel.