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.

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

Hatzie,
THANK YOU AGAIN for your time and sharing of your knowledge. I am currently looking to install my new '87 tbi tanks with installed AC DELCO EP 381 fuel pumps I also have a replacement AC Delco fuel valve (U7000) and a replacement WVE pigtail connector. I am wondering if you might share what/how I should connect the pump wires? the pumps have the "factory" weather pack connectors on them?
again thank you for your knowledge and time!!
the truck is 1986 2wd swb silverado with factory duel tanks doing a 4.8LS swap.

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

I have a 1987 Chevy pickup with dual tanks. The plug on the dual tank switch has been cut off and I have 4 wires hanging out of the dash. tan, tan w/white stripe black w/white stripe and gray.Does anyone know how they go onto the new switch i bought?

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

There is a diagram on Post #76. It has the info you are looking for on the switch.

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

I have a similar question. On my 85 with factory dual tanks, am I splicing into the power wires to the senders to run the fuel pumps? Where does the "relay" come into play mentioned in earlier posts? Was that just a safety feature for the factory TBI systems from 87 up?

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

http://johnnynightstick.s3.amazonaws...ematic_TBI.JPG
The stock TBI relay on the firewall is wired inline with the incoming power to the NL2 option rocker switch in the dash.
The relay GM used is a Metripak sealed 5 terminal relay. Electrically it's the same as your garden variety Bosch mini ISO relay but it's more compact and the terminals are not open to the elements.

You have an electromagnet on terminals D & F that pulls a Common terminal E switch reed from the Normally Closed C terminal to the Normally Open A terminal.
The electromagnet receives power from the ECM and ground from the chassis.
The relay switches power to the fuel pump to protect the ECM driver circuit from the fuel pump motor startup inrush current, motor running current, and shorts in the fuel pump wiring.
I believe terminals D & F are Metripak 150 and A C & E are higher current Metripak 280 terminals.

The relay, oil pressure switch, and the ECM perform an important safety function. When the engine is running the ECM gets pulses from the ignition module and switches on the fuel pump relay along with enabling injector pulses. As you can see in the diagram the oil pressure switch has open contacts to power the fuel pump once the oil pressure rises and cut power to the fuel pump(s) when the engine stops rotating. If you have an accident, and the engine shuts off, the fuel pumps will stop running as well rather than feeding a fire. Not all early fuel injected vehicles had this safety feature. The accident scenes were extremely unpleasant.

The RED wire test terminal allows you power the fuel pump(s) without actuating the relay.

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

hatzie, I tried to skim through and find out, but can you tell "us" again which tank (side) is the main tank? and which is the aux. tank? I am trying to do this on my '86 LS swap now... still!:lol:

thanks!!!:metal:

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

Main is driver side (from the seated POV).

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

1981-1991 tank switches are marked RH & LH. The tank you use as your "main" tank is up to you.
Why? The motorized valve stays where it comes to rest and doesn't change again til you select the opposite tank. Select the RH tank with the valve plugged in and hook the RH tank hoses to the side that's open through the valve. Or select the LH tank and do it that way.

AUX and MAIN are really only meaningful with the 1980 and prior solenoid type valves. AUX is the tank selected in the powered valve position MAIN is the unpowered position. AUX is entirely dependent on the way the valve was hooked up by the last guy that was working on it. I've seen LH AUX and RH AUX tanks.
I believe the 1973-1980 primary tank was the LH tank but don't quote me on that. My single tank 1974 had the fuel door on the LH side of the bed. Yes the fuel door bed came off a later truck. No I don't know whether one of the previous five owners moved the tank from the RH to the LH side of the truck.

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

hatzie, thanks ... that does make sense. I have the '87 switch (for my LS swap) I decided to make the driver tank the main and will run feed and return lines accordingly.

in general I just want to publicly thank you for your time and knowledge that you share on this board. I am sure that I am not alone when I tell you thank you....:wave:

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

hatzie, I just want to make sure that I am thinking correctly. on the EP381 pump (and sender unit) the purple wire(at gas tank) connects to the sending unit wire for the appropraite tank sending unit wire on the switch valve 5 wire harness? and the grey wire connects to the appropriate power wire on the switch valve harness?
thanks!

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

Sounds right.

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

thanks!!:metal: ;)

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

So my truck has the cable system. It's a 74. Interestingly enough, I'm looking at the 73-74 parts book and part 329854 is a rocker-type selector. I thought that wasn't available until 75, but maybe I'm wrong.

In any event, I'd like to learn a bit more about the system on my truck, as I'll be shortening the bed and replacing the 20 gal tanks with 16 gal tanks. I want to may sure my system works correctly, as I'm going to need both thanks with a 454. To what site is being referred above? Does anyone have the link or more info?

Thanks.

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

There are a bunch of inconsistencies around the dual tank systems in the 73-75 parts and wiring manuals.
http://forum.73-87chevytrucks.com/sm...0970#msg270970
^^ This is the thread I was referencing. Not a lot of information there but it's the most I've seen anywhere..

My understanding, never having owned one myself, is the 73-75 cable system is a cable operated valve under the truck with some kind of switch built into the dash assembly to select which sender was connected to the fuel gauge.
Attachment 2106287
^^This is the picture VileZamboni posted in the thread on Chris's board.

If someone has a better picture of the dash switch/bezel and the valve showing the hose connections and the switch on the bezel it would be an appreciated addition to post it here.

1973-1975 are not likely to have return lines so you likely only have three fuel lines at the original valve.

As long as you mark the valves' hoses and hook them up to the correct tanks' hardline you shouldn't have any issues. Painters tape and a sharpie...

The 16 & 20 gallon saddle tanks use the exact same senders. If yours are working I'd keep them.

Most 3/4 & 1 tons of that vintage likely used a transmission/axle vent on the end of a chunk of SAE J30R7 fuel line as the vapor system. Adding a three hose 1970's Corvette or F-body vapor can is an easy way to make sure the garage will not smell like raw fuel. I'm not a pushy obnoxious environmentalist. If the smell of raw fuel in the garage doesn't bother you just carry on without an evap can.

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

When ordering/building the truck the single tank option defaulted to passenger side from 73-80, and driver's side from 81+. Hatzie has the perfect response to how the switch/valve treat the main/aux distinction. I just wanted to mention the above oddity in the two generations.

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

I had a 78 & 79 that I could swear had the tank on the LH "driver" side.
I bought em 3rd & 4th hand so the prior owners could've installed 1980's beds on em.
It's also been over 30 years so I reserve the right to remember things incorrectly.

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

I have an '84 GMC K1500 that has a carburetor, mechanical fuel pump and dual tanks. Aux. tank has been disconnected (reason unknown).The PO removed all items related to EFI and emissions. It has the Pollak 6 port selector valve (which will be replaced with a new one) and Standard DS-1807 switch. All wiring and connectors are there.

Would there be any reason not to reconnect the auxiliary tank and use the new valve and switch? I also plan on installing an adjustable fuel pressure regulator with a return line back to the valve.

Any help is greatly appreciated.

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

84 wouldn't have Fuel Injection GM started that on the 1987 trucks.

The mechanical fuel pump will deliver fuel at the proper pressure for your carburetor from whichever tank you select.

Change the switch, check for volts and ground at the plug, and then check the old valve operation before you do anything with the new valve. A fair percentage of the time the old valve will work fine.

From 1981 on GM stopped using the AUX and Main designations and got right to the point LH & RH. You should make sure the LH tank is selected when the switch is set to LH and RH when RH is selected.

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

Thanks for the info. The carb manufacturer recommends 5-5.5 psi, the pump is over that. I changed the switch to the one posted. Should I change to another one?

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

The switch posted should run the valve if the wiring is good and the valve is in working order.

6PSI maximum pressure doesn't mean it's running that pressure all the time. It means that 6PSI is all you're going to get.