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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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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Re: 1973-1991 Dual fuel tank systems theory of operation
Great thread!
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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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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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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! :metal: |
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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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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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;
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. http://johnnynightstick.s3.amazonaws...AUGE_CURVE.jpg |
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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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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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. |
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, :metal:. |
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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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.
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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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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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.....:lol:. |
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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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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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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