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Old 01-05-2020, 04:01 PM   #51
hatzie
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Re: 1973-1991 Dual fuel tank systems theory of operation

As long as you aren't exceeding the pressure capacity of the valve I don't see a problem with it.
Use high volume low pressure lift pumps in the tanks to feed one frame mounted high pressure pump.
The lift pumps will keep the high pressure pump from starving and the return pressure should be low enough that the 65PSI valve will handle it.
Several Japanese and European cars are designed that way.
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Old 01-05-2020, 05:47 PM   #52
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Re: 1973-1991 Dual fuel tank systems theory of operation

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As long as you aren't exceeding the pressure capacity of the valve I don't see a problem with it.
Use high volume low pressure lift pumps in the tanks to feed one frame mounted high pressure pump.
The lift pumps will keep the high pressure pump from starving and the return pressure should be low enough that the 65PSI valve will handle it.
Several Japanese and European cars are designed that way.
I appreciate the timely response.

The inlet pressure for the valve will be stock TBI pressure, no worries there. The return pressure might be higher, but unlikely since the return should be free flow returning to the selected tank.

I figured this would work, many diesels operate this way, low pressure, high volume lift pump supply to the injector pump.
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Old 01-05-2020, 08:58 PM   #53
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Re: 1973-1991 Dual fuel tank systems theory of operation

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Popeye...I messaged you that I offer ready to run dual tank harnesses for your exact application-let me know if you need one! Pic is for reference regarding one if many versions of the harnesses I have on hand for this setup.
Where can I find a simple plug and play set up like this?
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Old 01-10-2020, 04:08 PM   #54
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Re: 1973-1991 Dual fuel tank systems theory of operation

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Where can I find a simple plug and play set up like this?
Send a PM to gmachinz on here or look up Jabin Wood (Harnessworx) on FB. I have gotten window harnesses direct from him before and he does exceptional work.
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Old 02-26-2020, 12:28 PM   #55
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Re: 1973-1991 Dual fuel tank systems theory of operation

Great thread!
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Old 02-26-2020, 06:46 PM   #56
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Re: 1973-1991 Dual fuel tank systems theory of operation

I think I have a bad pump relay. Where's the best place to purchase one?
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Old 02-27-2020, 05:39 AM   #57
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Re: 1973-1991 Dual fuel tank systems theory of operation

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I think I have a bad pump relay. Where's the best place to purchase one?
Disregard the relay is good. Either my switch is bad OR the valve is not moving.
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Old 02-27-2020, 09:27 AM   #58
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Re: 1973-1991 Dual fuel tank systems theory of operation

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Disregard the relay is good. Either my switch is bad OR the valve is not moving.
For those that need one it's a very common GM 14089936 sealed relay.
The AC Delco part is cheaply made in China now so I'd check over the other usual suspects like NAPA Echlin AR279 or Standard Motor RY109... one of these may still be made somewhere outside of China or India.
GM used this for sealed control of a bunch of circuits on the 86 and later squarebodies and the 1st design GMT400 trucks.
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Old 02-27-2020, 09:33 AM   #59
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Re: 1973-1991 Dual fuel tank systems theory of operation

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For those that need one it's a very common GM 14089936 sealed relay.
The AC Delco part is cheaply made in China now so I'd check over the other usual suspects like NAPA Echlin AR279 or Standard Motor RY109... one of these may still be made somewhere outside of China or India.
GM used this for sealed control of a bunch of circuits on the 86 and later squarebodies and the 1st design GMT400 trucks.
Good advice! I got one at AZ and it worked, but you could see the difference in quality between it and the original one.

Hatzie I have some more troubleshoot questions. Would you like me to PM you as not to clog up this thread or start a new thread of my own? Thanks again for all of the great information!
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Old 02-27-2020, 10:12 AM   #60
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Re: 1973-1991 Dual fuel tank systems theory of operation

Ask here if it's about the dual tank fuel systems.
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Old 02-27-2020, 10:32 AM   #61
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Re: 1973-1991 Dual fuel tank systems theory of operation

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Ask here if it's about the dual tank fuel systems.
Sweet! Here's what I'm seeing: Switch in LHS, truck starts normally and gauge reads correctly. When you switch to RHS, truck keeps running, gauge reads the same. I haven't pulled the switch out to check the connections because I don't want to mess up the dash (it's a very low mileage truck). I'm trying to figure that part out without damaging anything.
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Old 02-27-2020, 11:54 AM   #62
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Re: 1973-1991 Dual fuel tank systems theory of operation

Anybody have a dimensional picture of the hole for the dash switch. If I remember correctly it is not just a square hole. It has a notch on the bottom.
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Old 02-27-2020, 01:24 PM   #63
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Re: 1973-1991 Dual fuel tank systems theory of operation

Each fuel pump gets power from the polarity reversing switch and ground from the frame.

The fuel gauge sender select switch is in the valve. There's no reason the gauge switch couldn't fail in one position or the other and the valve itself still route fuel correctly.

Try this;
  • Get some 1/2W resistors in 100Ω, 47Ω, and 22Ω. or close to those values.
  • Get some alligator clip electronics test leads.
  • Get a handful of MALE Weatherpak terminals. Wire gauge rating is not important... you're using these to make sure you don't damage the FEMALE valve plug terminals by jamming stuff in them that isn't supposed to go there.
I keep male and female Weatherpak and Metripak 150, 280, 480, & 630 terminals in my meter bag along with several other connector families so I can do test hookups without damaging sensors and wiring harnesses. U-Line sells small plastic Ziploc baggies. They're darn handy to keep these kinda test terminals separate in the meter bag. I use blue tape and a sharpie to label the oddball terminals.
  1. Unplug the six position Weatherpak tower plug from the valve.
  2. The fuel gauge should go to the 3:00 position with the ignition on.
  3. Attach the jumpers from a known good ground and a MALE Weatherpak terminal in the B terminal of the valve plug.
  4. One at a time insert the 100Ω, 47Ω, and 22Ω resistors in between two jumper leads and look at the gauge with each resistor.
    100Ω should drive the needle to just past F, 47 should drive it to somewhere around 1/2 and 22 should drive the needle below the 1/4 mark nearer 1/8.
  5. Insert two Male Weatherpak terminals with two test leads jumpering from B to A and watch the gauge with the ignition switched on. This is one of the actual senders.
  6. Insert two Male Weatherpak terminals with two test leads jumpering from B to C and watch the gauge with the ignition switched on. This is the other sender.

If the senders are both working and the tanks are at different fill levels the gauge should read different levels in 5 & 6.
If your tanks are filled to the same level then the gauge will read the same in 5 & 6.

Steps 1-4 are a sanity check to make sure the fuel gauge and the associated wiring is in proper calibration. This is a bitmap of a curve I plotted in Excel based on squarebody fuel gauge needle position vs a calibrated decade resistance box in place of a GM sender on my bench.
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Old 02-27-2020, 11:00 PM   #64
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Re: 1973-1991 Dual fuel tank systems theory of operation

I'm pretty sure it is the valve. I guess it would make sense to upgrade to the higher pressure valve. Where is the best and quickest place to purchase one?
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Old 02-28-2020, 08:39 AM   #65
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Re: 1973-1991 Dual fuel tank systems theory of operation

Hatzie - if I'm reading your post #63 correctly, if the valve isn't moving to the other tank, THEN the gauge won't read the other tank. Or said another way, the gauge doesn't start reading the "new" tank, until the valve has switched to the "new" tank. I went ahead and ordered and new 65 psi valve and it should be next week. Hopefully swapping that out will not be that difficult and will fix the problem. CHEERS!
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Old 02-28-2020, 01:59 PM   #66
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Re: 1973-1991 Dual fuel tank systems theory of operation

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Hatzie - if I'm reading your post #63 correctly, if the valve isn't moving to the other tank, THEN the gauge won't read the other tank. Or said another way, the gauge doesn't start reading the "new" tank, until the valve has switched to the "new" tank. I went ahead and ordered and new 65 psi valve and it should be next week. Hopefully swapping that out will not be that difficult and will fix the problem. CHEERS!
That is correct.

The TBI trucks also use the power wire from the dash switch to energize the selected fuel pump. One of the output wires is power and the other is ground and the polarity changes based on the switch position. If you select the LH tank and the valve doesn't move the RH fuel pump still shuts off and the LH fuel pump still turns on... as long as the wiring hasn't been tampered with.
The engine keeps running so I have to assume the switch and valve are changing tanks and fuel pumps. This leaves the little microswitch inside the valve that handles the gauge. If the gauge microswitch inside the valve on terminals A B & C has failed but the rest of the valve is working it would exhibit your list of symptoms.

Get a valve made by Pollack and be done with it. I believe the latest and greatest good to 65psi is Pollack 42-159P. They aren't cheap but they work for a long long long time.
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Last edited by hatzie; 02-28-2020 at 02:16 PM.
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Old 03-01-2020, 10:42 AM   #67
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Re: 1973-1991 Dual fuel tank systems theory of operation

Before I forget, 1987 R10 1/2 Ton, 305/700R4. OK, So I replaced the switch on the dash and I replaced the switching valve with a new 65 PSI Pollack unit. I'm going to fill up the RH tank so that I know how much fuel it should show on the gauge and then I'm going to start the troubleshooting over again.

One quick question, what is the procedure to test the pumps using the relay pigtail under the hood?

Also, when I unplug the transfer valve underneath, it was my understanding that it should park the fuel gauge at 3:00, however, it doesn't seem to affect the gauge. Thoughts?

Thanks again Hatzie for all your guidance, .

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Old 03-01-2020, 11:58 AM   #68
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Re: 1973-1991 Dual fuel tank systems theory of operation

I made sure the RH tank is full, LH tank is approximately half. Gauge shows approximately half in both LH & RH positions. I hear the pump cycle and the valve move when I turn the key on in either positions. Truck runs in both positions. Thoughts? Bad ground on the RH tank?

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Old 03-01-2020, 06:40 PM   #69
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Re: 1973-1991 Dual fuel tank systems theory of operation

Probably one of three things not in order of probability. Test first then replace or repair.
  1. The main fuel gauge sender wire from terminal B to the gauge sender terminals in the instrument panel has a resistive short to ground. It's not a dead short to ground which would drive the fuel gauge needle to E.
  2. Either power or ground connection, or both, through the instrument panel to the fuel gauge has failed.
  3. The fuel gauge itself is malfunctioning.
    3a. Connections to the bias resistor are loose.
    3b. The bias resistor has failed from vibration wearing through the power connections or a cracked ceramic substrate.
    3c. The gauge itself has failed.
You can pull the main bezel, the cluster lens, and the fuel gauge without disturbing the instrument cluster bucket. This will give you access to the power, ground, and signal connections for the fuel gauge.
Power is hot with the ignition switched ON.
Ground is Ground all the time.
The B terminal of the valve plug gives you access to the other end of the common gauge wire.

Don't overlook the plastic printed circuit. If you have higher than one ohm resistance from the sender clips to the B terminal at the valve pull the cluster bucket, being careful not to break off the tabs, and unplug the cluster connector. Then you can test resistance from the fuel gauge wire on the cluster connector to valve terminal B and test the resistance of the printed circuit sender trace with a meter as well.

Removing the nuts from the bias resistor on the back of the gauge and re-installing with star washers can fix dodgy connections to the resistor. Don't be a ham handed gorilla when you tighten them back down... you can and will crack the ceramic substrate of the resistor. Snug with a nut driver is good enough. If it looks like the resistor is cocking to one side loosen it up and get it level with the back of the gauge can.
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Last edited by hatzie; 03-02-2020 at 12:00 PM.
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Old 03-03-2020, 01:07 PM   #70
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Re: 1973-1991 Dual fuel tank systems theory of operation

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Probably one of three things not in order of probability. Test first then replace or repair.
  1. The main fuel gauge sender wire from terminal B to the gauge sender terminals in the instrument panel has a resistive short to ground. It's not a dead short to ground which would drive the fuel gauge needle to E.
  2. Either power or ground connection, or both, through the instrument panel to the fuel gauge has failed.
  3. The fuel gauge itself is malfunctioning.
    3a. Connections to the bias resistor are loose.
    3b. The bias resistor has failed from vibration wearing through the power connections or a cracked ceramic substrate.
    3c. The gauge itself has failed.
You can pull the main bezel, the cluster lens, and the fuel gauge without disturbing the instrument cluster bucket. This will give you access to the power, ground, and signal connections for the fuel gauge.
Power is hot with the ignition switched ON.
Ground is Ground all the time.
The B terminal of the valve plug gives you access to the other end of the common gauge wire.

Don't overlook the plastic printed circuit. If you have higher than one ohm resistance from the sender clips to the B terminal at the valve pull the cluster bucket, being careful not to break off the tabs, and unplug the cluster connector. Then you can test resistance from the fuel gauge wire on the cluster connector to valve terminal B and test the resistance of the printed circuit sender trace with a meter as well.

Removing the nuts from the bias resistor on the back of the gauge and re-installing with star washers can fix dodgy connections to the resistor. Don't be a ham handed gorilla when you tighten them back down... you can and will crack the ceramic substrate of the resistor. Snug with a nut driver is good enough. If it looks like the resistor is cocking to one side loosen it up and get it level with the back of the gauge can.
Rechecked the selector switch, it is working correctly. Fuel relay check OK. Transfer valve is switching when the selector switch is cycled from one side to the other. Both pumps are priming and the truck is running correctly. The gauge works for the LHS tank (selector switch in LHS position). So, I believe it is just a sending unit/grounding issue on the RHS.

If the ground on the RHS sending unit is bad, could that cause the gauge to remain in the same position as it was reading for the LHS tank?

With the selector switch on RHS, if I ground the pink/black wire on the RHS (sending unit wire), then the gauge should read E correct?

Shouldn't the gauge park at the 3:00 position if I disconnect the transfer valve 6 pin plug? It doesn't. It remains in position it was at when I disconnect it.
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Old 03-03-2020, 06:36 PM   #71
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Re: 1973-1991 Dual fuel tank systems theory of operation

I got home and did some more troubleshooting.

Things I know for sure: Selector switch checks good (New). Pump relay checks good. Transfer valve checks good (New Pollack 65 psi).
LH Tank is known 1/4 full, RH Tank is known FULL. The gauge shows 1/4 full when the tank selector switch is in LH or RH position (makes sense for the LHS, but not for the RHS).

Troubleshooting: I disconnected the plug to the RH fuel pump/sender. The gauge stayed at 1/4 full with the tank selector in LH or RH position. The truck would start and run in the LH position (pump/gauge checks good), but would not start or run in the RH position (makes sense, pump not connected).

Then, I disconnected the plug to the LH pump/sender. The gauge went to the 3:00 position with the tank selector in LH or RH position (makes sense for the LHS, but not for the RHS). The truck would start and run in the RH position (pump is working), but would not start or run in the LH position (makes sense, pump not connected).

Possible problem: Sensor ground on RH tank? Hatzie, over to you......

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Old 03-04-2020, 12:31 PM   #72
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Re: 1973-1991 Dual fuel tank systems theory of operation

I'll reply later today when I have time to write it down.
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Old 03-05-2020, 08:25 PM   #73
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Re: 1973-1991 Dual fuel tank systems theory of operation

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I'll reply later today when I have time to write it down.
Were you able to figure anything out? Thanks! Rob
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Old 03-06-2020, 09:35 AM   #74
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Re: 1973-1991 Dual fuel tank systems theory of operation

It sounds like someone wired past the valve directly to the LH sender.

The valve and switch are working as they should.

Does the SPID (Service Parts Identification) label in the glovebox call out Option NL2?

The six position Weatherpak on the valve is lettered A-F. F should not have a wire so A is on the opposite end of the plug... easy to find.

With your Digital Volt Ohmmeter. What ohmage do you get when checking resistance between Valve plug A to ground & Valve plug C to ground. One of them should be 90Ω ish and the other should be somewhere around 29Ω ish... give or take a few Ω. The graph in my post above shows Ohms VS gauge position.
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2009 Impala SS LS4 V8


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Last edited by hatzie; 03-06-2020 at 09:44 AM.
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Old 03-06-2020, 10:02 PM   #75
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Re: 1973-1991 Dual fuel tank systems theory of operation

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It sounds like someone wired past the valve directly to the LH sender.

The valve and switch are working as they should.

Does the SPID (Service Parts Identification) label in the glovebox call out Option NL2?

The six position Weatherpak on the valve is lettered A-F. F should not have a wire so A is on the opposite end of the plug... easy to find.

With your Digital Volt Ohmmeter. What ohmage do you get when checking resistance between Valve plug A to ground & Valve plug C to ground. One of them should be 90Ω ish and the other should be somewhere around 29Ω ish... give or take a few Ω. The graph in my post above shows Ohms VS gauge position.

You may be correct about the rewiring, but this is the cleanest most unmolested old truck I have ever owned. It is a NL2 according to the window sticker. I'll ohm out the wires some time this weekend. Thanks again for all of your suggestions.
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Last edited by Hart_Rod; 03-06-2020 at 10:17 PM.
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