1973-1991 Dual fuel tank systems theory of operation

[hatzie]

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.

[rodstored-72]

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!

thanks!!!

[SunSoaked]

Main is driver side (from the seated POV).

[hatzie]

Originally Posted by rodstored-72

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!

thanks!!!

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.

[83GMCK2500]

Originally Posted by rodstored-72

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!

thanks!!!

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.

[rodstored-72]

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....

[rodstored-72]

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!

[hatzie]

Sounds right.

[rodstored-72]

Originally Posted by hatzie

Sounds right.

thanks!!

[69Tom]

Originally Posted by hatzie

There were three fuel valves used between 1973 & 1991.
None of the systems are rocket science to repair once you understand what makes em tick.

On all three systems...
-The fuel is routed from one of the tanks to a common OUT port. Only one tank at a time is valved open to the output port.
-If the truck is equipped with a fuel return line the common return port from the engine is switched from tank-to-tank as well.
-On systems with a return line... The return line should be routed to the tank that fuel is being drawn from. If you mix up the return lines you can overfill the tank not being used. Hosing down the guy in the next lane with raw fuel may not garner a friendly reaction.
-Both LH tank lines into the same side and both RH fuel lines into the opposite side.
-the COMMON side of the valve To/From-The-Engine only has 1 or 2 lines. Feed and, if equipped, Return.
NOTE: It's worth noting the 1981-1991 systems likely all have return lines. I haven't run into one without.

**1973 & 1974 & some 1975
Cable System... had a cable operated valve with a SPDT switch behind the dash knee panel that activated via the cable to run the fuel gauge. If you google it you'll find a thread on Chris's board about the valves with pictures of this setup. This is LONG obsolete. Folks replaced them with the 75/76-80 system when the parts ran out but there are a few survivors. NOTE: I'm not sure whether this system used the NL2 RPO or not... The 1975 wiring diagrams list the cable system and then NL2 in separate panes.

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.

[hatzie]

Originally Posted by 69Tom

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.

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.

^^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.

[hatzie]

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.

[Moose Drool]

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.

[hatzie]

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.

[Moose Drool]

Originally Posted by hatzie

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.

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?

[hatzie]

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.

[Hart_Rod]

By chance, does anyone have a part number for the oil pressure switch that allows the engine to start when above 10 psi? I've searched but can't find it. Thanks in advance!

[44th Miss Inf]

The one on my 88 R30 w/454 was GM 25036938 (standard PS221 is the cross) this is the one for the computer, not sure what the one for the gauge is, as I’ve never changed it

[Hart_Rod]

Originally Posted by 44th Miss Inf

The one on my 88 R30 w/454 was GM 25036938 (standard PS221 is the cross) this is the one for the computer, not sure what the one for the gauge is, as I’ve never changed it

[rodstored-72]

aotte1, this is the tank I purchased for my '86 2wd swb for a LS conversion. this is a 16 gal tank for shortbed purpose. you will need to get a 20 gal tank for a longbed...
link:https://www.amazon.com/Spectra-Premi...+Chevrolet+C10

hope this helps.. good luck

[aggie91]

Originally Posted by aotte1

Thanks for sharing your knowledge on this subject. 1986 K10 short bed, needs right side tank replaced, suggestion on good replacement tank, and/or key factors to look for.

Thanks,
Les

Originally Posted by rodstored-72

aotte1, this is the tank I purchased for my '86 2wd swb for a LS conversion. this is a 16 gal tank for shortbed purpose. you will need to get a 20 gal tank for a longbed...
link:https://www.amazon.com/Spectra-Premi...+Chevrolet+C10

hope this helps.. good luck

If you are using fuel injection with an in tank pump, the GM11C for short bed or the GM1C for a long bed is a preferred tank as it has a baffle around the pump to help keep fuel at the pump when the level is low.

Long bed version:
https://www.amazon.com/Spectra-Premi...+Chevrolet+R10


Otherwise for a regular carb set up, the GM1B tank is the one for the long bed.
https://www.amazon.com/Spectra-Premi...+Chevrolet+C10

[rodstored-72]

aggie (aotte1),
I believe I linked to a different tank than what I purchased, I believe I did purchase the GM11C tank, because I wanted the baffle for the pump. I apoligize for any confusion.

[SunSoaked]

I'm converting my 85 to 87 for my efi. Purchased D/S (Main) tank and sending unit. It appears the P/S sending unit remains impossible to find. I read in the description on one of the sites that the difference was which way the outlet hose barbs are pointing and that may be why they are not interchangeable?

So they question becomes, if the hose barbs are made (whatever means) to point inward (away from the frame) would it work on the P/S? I have no problem with fabrication skills.

[aggie91]

Originally Posted by SunSoaked

I'm converting my 85 to 87 for my efi. Purchased D/S (Main) tank and sending unit. It appears the P/S sending unit remains impossible to find. I read in the description on one of the sites that the difference was which way the outlet hose barbs are pointing and that may be why they are not interchangeable?

So they question becomes, if the hose barbs are made (whatever means) to point inward (away from the frame) would it work on the P/S? I have no problem with fabrication skills.

Short answer is yes. If you can rework the steel lines to point back toward the frame, then it will work. I am thinking that if you just add a 180 bent tube to the end of each existing tube, that will work.

[SunSoaked]

Originally Posted by aggie91

Short answer is yes. If you can rework the steel lines to point back toward the frame, then it will work. I am thinking that if you just add a 180 bent tube to the end of each existing tube, that will work.

Thank you.