1973-1991 Dual fuel tank systems theory of operation

[aotte1]

Rodstored-72 and Karl,
Thanks for the help on the auxiliary fuel tank replacement. Which one to buy. Our new to us 1986, K10 short bed has a bad aux fuel tank. This info will let me get a good replacement. For now going with a carb set-up.
Will remove bad tank this week and power wash underside of cab/bed, to remove last of mud.

Les

[Sabaka454]

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.

https://www.ebay.com/itm/23405053783...oAAOSwpz1gxgi0

[52napco]

Pretty sure the dash switches are not interchangeable between the depress fully to the later style..I think the power and grounds are opposite and the plug won't interchangeable.
My truck is 1991 and the older style wouldn't work.

[Moose Drool]

I have an '84 GMC K1500 with dual tanks. I am replacing the tank selector switch. I purchased a Standard DS-1807, that was mentioned on page 1. It has 4 terminals. Is there a way to use it or do I buy a different switch?

[hatzie]

This has been discussed earlier in this thread across several posts but I'll rehash it in one post.

You can't use the 81-86 switch on a TBI truck because the 81-86 switch is momentary. When you remove your finger from the switch the selected fuel pump will shut off.

You can and probably should use the TBI tank switch on all of them.

There's a bubble in one of the male switch terminals that mates with a keying notch in the plugs. This prevents swapping the TBI switch with the earlier 81-86 switch like you tried to do without modifying the connector to add an additional notch.
The converse is also true. You have to notch the 81-86 and diesel plug to use the TBI switch.

Open up the 1991 wiring diagrams and go to pages A80 & A81

The plug terminals in the Diesel and 81-86 gasoline models are on the centre switch terminals with the ground on the LEFT and Hot on the RIGHT.
The TBI plug terminals are on the outer switch terminals diagonally positioned with the ground on the RIGHT and Hot on the LEFT.

The valve motor power wires on the dash switch are rotated right to left and up & down in the same manner so the polarity across the switch to the valve motor in the RH & LH switch positions ends up exactly the same.

[52napco]

You are the man Hatzie ...thanks again

[52napco]

OK ...now I don't have power to the switch...I can run a wire to the positive side of switch and it works..so if I am understanding this correctly the switch power comes from the tan and white wire out of pump relay ?

[hatzie]

Originally Posted by 52napco

OK ...now I don't have power to the switch...I can run a wire to the positive side of switch and it works..so if I am understanding this correctly the switch power comes from the tan and white wire out of pump relay ?

On TBI yes
On carbureted and diesel power comes out of the fuse panel

[52napco]

Okay I started to send this via private message but I'll post a here in case it helps somebody in the future .. The new switching valve has to be bad with ? unplugged it runs fine off the left fuel pump I plugged the weather pack plug back into the brand new Pollock valve, it clicked really loud and shut all fuel pressure off from either tank.. a test light reveals that the polarity is switching on the plug itself via the dash switch.. pretty sure I've got a defective brand new Pollack valve thoughts on this

[hatzie]

It could be. Never rule out brand new parts.

[52napco]

This just keeps getting better I took my old original valve before I plugged it in it was flowing from the left tank you can blow on it in one side and out the other no problem plugged it in same thing shut both sides down you couldn't blow through either side so something in this harness is causing these valves to shut down tight

[hatzie]

You have something very very strange happening.
The valve is literally a pair of sealed pellets on threaded shafts
The motor inside the valve turns the threaded shafts forward or backward moving the pellets to block off the left or the right inlets

The valve motor on TBI trucks should only stop when the overtravel switch on the motor routes the electrons from terminal E to a diode biased against to the polarity presently driving the motor.

It shouldn't stop mid travel.

Is your dash switch marked "DEPRESS FULLY"?

[52napco]

No it's the correct dash switch..
This thing is kicking my ass lol

[52napco]

The truck is a 1991 V3500 with 5.7,.could it be the hot start module or does it even have one?? How can you tell if it has a hot start module other than looking for it

[hatzie]

That would only shut off your fuel pump power feeding into the NL2 subharness. The hot start doesn't have anything else to do with the NL2 wiring.

Step back and look over the basics. Page A80 or A81 of the 1991 wiring diagrams.

If the valve is malfunctioning it's probably in the control wiring or the switch itself is fouled up somehow.

[52napco]

Good advice Hatzie...
I'm riding home a while ago from Lowe's racking my brain and it dawned on me I had put the hoses back like they were on the old switching valve I thought well you know maybe they're on the wrong port the left fuel pump might be running but it's Dead Heading against the right side port of that valve... worth a look makes sense as to why I'm not getting any fuel pressure... well low and behold I should have slowed down to speed up.....somebody I guess the previous owner, had the hoses switched from right to left so when the right fuel pump would switch on it was dead heading against the left port on the switching valve so I switched the hoses around and walla it runs like it's supposed to...

So at the end of the day the best advice I can give anybody is take about 30 minutes and really just look at everything to see what's been messed with over the years.. a lot of stuff happens on a 31-year-old truck over the years.

[hatzie]

Originally Posted by 52napco

Good advice Hatzie...
I'm riding home a while ago from Lowe's racking my brain and it dawned on me I had put the hoses back like they were on the old switching valve I thought well you know maybe they're on the wrong port the left fuel pump might be running but it's Dead Heading against the right side port of that valve... worth a look makes sense as to why I'm not getting any fuel pressure... well low and behold I should have slowed down to speed up.....somebody I guess the previous owner, had the hoses switched from right to left so when the right fuel pump would switch on it was dead heading against the left port on the switching valve so I switched the hoses around and walla it runs like it's supposed to...

So at the end of the day the best advice I can give anybody is take about 30 minutes and really just look at everything to see what's been messed with over the years.. a lot of stuff happens on a 31-year-old truck over the years.

Taking a step back seems to be the best medicine when you're stuck sometimes.

When I mess with the hoses on fuel valves I tape the supply and return tank hoses with blue and green painters tape and write S on the supply and R on the return. I use regular tan masking tape on the engine compartment hoses again marked S & R. All so I don't hook up the Left return hose to the Right hand barb on the valve and-or deadhead the supply hoses or screw it up some other way.
And never trust what the last guy did
I started the 72 F250 with two full tanks and a new valve and lines. Shortly the RH tank was overflowing out the vent cap cause I hooked up the return hoses bass ackwards. That was the last time i wanted to make that mistake.

[52napco]

Thanks for all the time you have put in on this thread...my right sender is bad but otherwise I'm good now and thanks again

[squarebodyaddict]

Hello Friends, I tried to start My 91 V3500 thats been sitting in the garage and not started for 18months. Crank no start, no fuel to the injectors but will start with ether shot. I started digging into the cause, turns out i only have 9.2 volts to the tank switch and valve which will not activate the valve. Thoughts on where to start will be appreciated! Battery is fully charged and brand new. I took the fuel valve out and it will switch with full 12volts applied. Fuel pump in tank runs but only with 9.2 volts its slow.

[hatzie]

The fuel pump relay may not be passing full voltage through the relay switch contacts anymore. They can get dirty just like ignition breaker points.

Back probe relay Terminal A and Terminal E to a good ground.
Terminal A is the input voltage from the ECM.
Terminal E is the output voltage to the tank switch and selected fuel pump.

[squarebodyaddict]

Originally Posted by hatzie

The fuel pump relay may not be passing full voltage through the relay switch contacts anymore. They can get dirty just like ignition breaker points.

Back probe relay Terminal A and Terminal E to a good ground.
Terminal A is the input voltage from the ECM.
Terminal E is the output voltage to the tank switch and selected fuel pump.

Thanks for the Info Hatzie.
I tried 4 used relays and 1 brand new relay, all had same result... 12.4V back probing Terminal A and 8.9-9.1V on Terminal E, grounded direct to battery.

I'm not great at finding info in these forums, I feel like I've searched every possible combination of word search... would like to know any suggestions for a next step.

P.S. Some Clarification:
91 V3500 454: While testing the A terminal (key on) 12.3+/- volts. Testing the E terminal I get 12.3V+/- at initial key on and while cranking, but falls back to 9Volts +/- when not cranking/not triggering fuel pump....
Interestingly enough I did the same test on my 91 V2500 Suburban 350 it gives similar readings except when testing E terminal key on when not cranking/not triggering fuel pump it then gives a 9 MILI VOLT reading.
Any Idea why the V3500 gives 9Volt reading while the same test on the V2500 gives different reading of 9 Mili Volt???

Also I checked firewall and Engine Ground both seem perfect.

[hatzie]

Either the relay is not passing volts through the switch contacts or the terminal in the E position of the plug is corroded or loose or both.

[rodstored-72]

not sure if my question is good for in here... but here it is... I am looking for info on how the tank vents were/are setup on dual tanks related to the venting lines & the charcoal canister? i am guessing there is a "diagram" somewhere or a hose routing for this?
thank you for any info provided....
Hatzie, i have mentioned it before, but feel it needs to be mentioned again that your time & effort on this thread is VERY appreciated

[hatzie]

There are three crossover lines from the LH tank to the RH frame rail. Supply, Return, and Vent.

On my 76 I ran a single vent line from the stock vapor can location under the LH battery across the cross-member under the radiator and up the RH frame rail to a Tee. The stock vapor line should be the same for the single and dual tanks.

[bigdaddy4x4]

i have 77 gmc crewcab with dual tanks did a ls swap and used the 87 efi tanks with 96 vortec fuel pumps and pollack switch over valve my ? is does the dual tank small fuel gauge i got to replace the orginial oem gas gauge need to be wired different for the small fuel gauge to work like it should ?