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01-04-2018, 09:55 AM | #1 |
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1973-1991 Dual fuel tank systems theory of operation
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. Here's a thread on rebuilding the manual valve pak. https://67-72chevytrucks.com/vboard/...d.php?t=829266 **1975/76-1980 Solenoid system... used a single wire solenoid valve with three or six ports. --The valves returned to a rest position as soon as power was removed. In 1980 GM added a retained power relay to keep the valve energized in the AUX position while cranking. <-- Click to see the post about this timer relay. --The AC Delco 467513 Six port valve or a similar function unit was used on trucks that came with return lines. These are not being produced by Delco or any other manufacturer. If you feel like spending big bucks search that part number they occasionally show up on evilbay. --Three port valves were for trucks without return lines. NOTE: Standard Motor Products sells a three port version FV1 but there are no six port solenoid valves available. Folks have used the 1981-1991 Pollack valves but generally they butcher the wiring. -Four wires run through the firewall. --Two tank sender wires. --One Dash Gauge feeder wire. --One Solenoid Valve power wire. -The dash switch is a five terminal DPDT (Double Pole Double Throw) ON-ON rocker switch. ----Each switch is called a pole and each switch position is called a throw... ------Double Pole means there are two switches actuated by one rocker. ------Double Throw means there are two different switch positions with a common input or output terminal. ------ON-ON means there are two ON positions with no centre OFF position. --The reason there are five and not six terminals is to key the switch to the connector such that it can only be plugged in one way. You can use a standard 6 terminal DPDT ON-ON rocker or toggle switch by clipping off one of the switch terminals to match the original switch. --On the three terminal side of the switch there's one wire direct to the dash gauge in the common position and the fuel level sender wires to the switched terminals. This selects which sender is routed to the dash gauge. --The two terminal side of the switch is still a double throw switch but it's missing one of the switch terminals to key the connector. This is so it can't be plugged in backwards. It also assures that the unwashed masses will buy switches from GM and not McMaster Carr or Grainger or... It has one power wire from the fuse panel, or a relay in 1980, on the centre common terminal and a green wire that routes power to the solenoid valve when the switch is set in the AUX position. This is a depiction of the internal switch action. Contrasting color diagram to show function... Not GM wire harness colors... The wire colors vary by year and can be found in the wiring diagrams for your year truck.
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And please let us know if and how your repairs were successful. Last edited by hatzie; 04-01-2022 at 05:35 PM. |
01-04-2018, 09:55 AM | #2 |
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1981-1991 NL2 Dual fuel tank systems theory of operation
**1981-1991
Motorized Valve system... used a six port motorized valve made by Pollack. These are still being produced by Pollack on Jan 1st 2018.
Electrical;
Motorized tank switch details...
NOTE: Stock 1981-1986 & Diesels don't have a 2 position Weatherpack disconnect at the sender with Fuel Pump Power (just a single sender wire from A or D on the valve), and obviously no in-tank fuel pumps, they are otherwise identical in function to the TBI setup. TBI TANK SWITCH SET TO RH TBI TANK SWITCH SET TO LH 1981-1986 & 1981-1991 Diesel TANK SWITCH SET TO LH NOTE ON SWITCHES:
1987-1991 RV series TBI Fuel Pump Relay for TBI tank switch and fuel pumps. Year specific information can be found in the 1987-1991 Emissions and Driveability service publications. You can find them in my manuals post. The TBI fuel pump relay (AC Delco 158240 Multi-Purpose SPDT Relay) can be found on the firewall between the transmission tunnel and the Heater/AC box. Fuel Pump relay without the transmission kickdown relay. You can see the pump test terminal dangling beside the split loom. A Hot Start Fuel Pump Controller is mounted behind the dash on some 5.7L and all 7.4L TBI trucks. More pictures of the harness plug behind the instrument panel are in this thread. -->http://67-72chevytrucks.com/vboard/s...d.php?t=757299 RV series TBI Fuel Pump Relay test terminal. The Fuel Pump Test Terminal that's present on all RV series TBI trucks is shown pulled away from the harness. This picture also includes a transmission kickdown relay mounted next to the fuel pump relay. The kickdown relay is supposed to be mounted nearer the RH side of the engine bay (US passengers' side) on the tabs. The two relays are the same part and it's been 30 years, give or take, since it rolled off the assembly line... You can also find this AC Delco 158240 multi-purpose SPDT sealed Metripak 280/630 relay and mounting tab used elsewhere in the engine bay for DRL on some Canadian models, Aux cooling fan, Glow Plug Inhibit, and probably other things. The replacement plug is a Delphi Metripak hybrid 280/630 # 12052287 or AC Delco PT1115 is the plug with a repair pigtail. Fuel Pump Relay Control
Converting TBI to Carburetor senders without butchering the truck harness Converting the senders from TBI to single wire on an NL2 dual tank truck is Extremely simple. As long as the orignal NL2 sub harness is in one piece... There's no need to pull new wiring into the truck and there's no need to butcher the existing sub harness so it's irreversible. If the TBI senders are no longer serviceable you can liberate the wiring needed for the changeover from the senders. Unplug the two position Weatherpak sender plug from the NL2 harness in the truck. Leave the truck harness alone til you plug in your new sender pigtails. Cut the sender ground wire off at the top of the TBI sender and crimp a 1/4" female disconnect to the end of the wire for the carburetor sender tank ground. Cut the 2 wires routed to the 2 position Weatherpaks off the at the top of the TBI senders with at least 6" of wire. Purchase or otherwise source a molded GM fuel sender pin connector with at least 12" of wire. Connect the molded pin plug pigtail to the sender wire on the Weatherpak plug you liberated in step 3. Loop the end of the fuel pump wire over @ 1" and cover it with adhesive heatshrink to seal the end. Tape the fuel pump wire to the sender wire and cover the resulting sender pigtail with split poly loom. Plug the two position Weatherpak plugs into the truck harness and the tank. Plug the tank ground you made in step 1 onto the tank sender ring ground tang. If the senders and fuel pumps are serviceable... don't chop em up. You can sell them or give em to someone that needs em. You still need to make up the pigtails but it's not difficult. If your senders are working and you aren't comfortable crimping Weatherpak terminals and seals onto a wire you can buy Weatherpak repair pigtails and make them up just as you would've with cutoffs from the original senders. I'd just roll the connectors from scratch but I have the tools and the parts onhand. Parts: two 2 position Weatherpak tower shells, two female Weatherpak terminals, two Weatherpak cable seals, two Weatherpak cavity seals, several feet of 12ga SXL black wire, several feet of expanding mesh wire sleeve, molded pin sender connector pigtails, two ring terminals, two 1/4" female disconnect terminals. Slip some expanding mesh sleeve over the molded pin connector pigtail leaving @ 2" uncovered wire then strip and crimp the Weatherpak seals and terminals on the ends. Make up two black sender to frame ground wires that will reach... Strip and crimp the ring terminals on one end, slide some expanding mesh over the wire leaving @ 2" uncovered then strip and crimp on the Female disconnect terminal. Insert the cavity seal in the fuel pump power wire position of the Weatherpak shell. This safely terminates the connection without chopping it off the truck harness. Insert the Weatherpak terminal in the sender position of the Weatherpak shell. Plug the parts into the truck and test. All years NL2 system NOTES:
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And please let us know if and how your repairs were successful. Last edited by hatzie; 04-04-2019 at 12:08 PM. Reason: Add TBI to Carburetor sender conversion. |
01-04-2018, 12:50 PM | #3 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
Awesome info. Thanks for sharing. You got any info like that regarding the vent lines/canister,etc?
Specifically, I just want to know if using a vented cap will prevent any issues if I remove my charcoal canister and use the vent line for my EFI return line. |
01-04-2018, 02:23 PM | #4 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
The vapor lines are tiny.
I'd keep the charcoal canister. It'll keep the garage from smelling like gasoline in the summer. You can simplify to a 1970's three hose can. Vapor, Ported Vacuum to run the valve, and SAE J30R7 purge hose teed into the PCV valve hose. If you're installing EFI you'll want to have the baffled R/V 10-30 series 1987-1991 tanks. Get the TBI senders for the later tank that already have return lines... That's what I did with my 76. TBI had the in-tank fuel pump hanging from the sender.
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1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
And please let us know if and how your repairs were successful. Last edited by hatzie; 04-16-2018 at 02:56 PM. |
01-04-2018, 03:01 PM | #5 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
Thats great info
Last edited by wjjeeper; 01-04-2018 at 07:22 PM. |
01-05-2018, 02:29 AM | #6 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
Nice writeup. I bought a switch and valve for my 81 because the right tank will fill when using the left tank as the draw even though my fuel pump only has 1 inlet and one outlet to the Holley. It baffles me. But now I can check the wiring and hoses when I replace them.
If there are return lines at the senders i was thinking of running a hose up high to a fuel filter as a breather. I might have a charcoal can on one of my parts trucks that I can use instead.
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01-05-2018, 08:44 AM | #7 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
It's real easy to swap the returns such that you're filling the un-selected tank from the engine fuel return.
I always double check the hoses on the LH side and RH side of the valve are coming from the same sender hoses. Green and Blue painters tape makes good assembly identification. I'm not saying it's impossible to leak past the return o-rings on the valve shuttle but it would be very unusual. Swap the return hoses on the tank side of the valve so the supply and return from each tank are on the same side LH supply & return in the LH side of the valve and RH supply and return in the RH side of the valve instead of opposite sides. This will likely fix the problem. Then you can take the replacement parts back to the store.
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1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
And please let us know if and how your repairs were successful. Last edited by hatzie; 01-05-2018 at 05:49 PM. |
01-05-2018, 07:03 PM | #8 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
This is excellent. I needed to know this stuff.
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01-06-2018, 11:28 AM | #9 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
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01-07-2018, 11:18 AM | #10 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
Thanks a lot for this great info!
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01-08-2018, 01:33 PM | #11 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
Added 1987-1991 TBI fuel pump relay information.
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1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
And please let us know if and how your repairs were successful. |
01-29-2018, 09:20 PM | #12 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
Hatzie,
Thank you for the diagrams and info as I've swapped a 5.3 into my 84' SWB and I've purchased the FiTech Ultimate LS management system. I purchased the 87' baffled tanks along with 87' sending units and Walbro 255 pumps all already installed. My original harness obviously lacks power for the pumps as it ran off of a mechanical. I visited Painless today and one of their engineers drew a diagram for wiring up to the OEM switch as long as the FiTech system supplies the 12v power. My question is simple, is there a vendor that I can source an 87' harness so that I can "plug and play" the wires on the 87' sending units thereby powering my in-tank pumps and sending units when switching tanks or will I be relegated to fabricating my own harness???? Any help would be greatly appreciated. FYI I'm new to all of this ! Father passed away and I felt compelled to attempt a frame off on the only "NEW" truck he ever owned........... Thanks in advance! Popeye |
01-30-2018, 07:20 AM | #13 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
Great post, thanks for the info.
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01-30-2018, 09:32 AM | #14 | |
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Re: 1973-1991 Dual fuel tank systems theory of operation
Quote:
It's not as bad as it sounds. Lay the original out on a sheet of plywood and place nails at the bends and end points so you can lay out the new wires to match. Leave around 4-5" extra at the termination points and put wire ties at the breakouts to keep the wires organized til you wrap them with split poly or wire harness wrapping tape (NOT Electrical tape). The factory connector part numbers are in the 88-91 schematics. Mouser Electronics and NAPA sell Weatherpak, Pak Con, and Delphi Packard 56 connectors and terminals. The Pak Con ignition power connector for the system is obsolete. The Pak Con terminals are not. You can probably carefully liberate one off your factory motorized valve NL2 harness. You will need to liberate the obsolete Packard 56 switch connector from your 81-91 factory motorized valve harness along with the firewall grommet and wire boot. If your truck was not equipped with the NL2 dual tanks a 1/2" round hole, for a toggle switch, is much easier to drill than a rectangular hole for a GM rocker switch... you just use a 20A Polarity Reversing toggle switch and a Packard 56 keyed toggle switch plug 12052978 connector.
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1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
And please let us know if and how your repairs were successful. Last edited by hatzie; 01-30-2018 at 09:59 AM. |
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01-30-2018, 10:35 AM | #15 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
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.
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01-30-2018, 10:01 PM | #16 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
Awesome Awesome Awesome, this should be a sticky... thank you
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02-12-2018, 06:27 PM | #17 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
ok I am still having trouble figuring out how to get my dual tanks and fuel gauge to work in my 77 k3500 with a 6.0 ls engine swaped in ..they are 5 wires on my tank switch blue green tan/white pink/black and tan,,,,, I have bought a new tank switch and Pollack switching valve there are only 4 wires that can go to the new dash switch what wire needs to go where ?? and the one wire that is left there where does it go???? also I tried to put the new switch in the old factory square hole but it acts like the square hole is not big enough any help will be thankful.
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02-12-2018, 07:42 PM | #18 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
Read the first post for how the gauge circuit works on the pre 1981 system.
The existing solenoid valve will not handle the pressure for the LS fuel injection. You can't easily use the existing early dual tank wiring. The easiest solution is to find or fab a 1987-1991 NL2 sub harness. The newer switch is probably larger than the hole. You're going to have to file it larger
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And please let us know if and how your repairs were successful. |
02-14-2018, 01:51 AM | #19 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
ok will just fab up a new harness and get rid of the old I have been trying it with the old harness with no luck
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02-18-2018, 07:26 PM | #20 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
Added more information on anti-vaporlock hot start module.
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1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
And please let us know if and how your repairs were successful. |
02-23-2018, 09:58 AM | #21 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
Added more information on Fuel Pump relay.
Corrected info on hot start module.
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1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
And please let us know if and how your repairs were successful. |
07-27-2018, 07:53 PM | #22 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
I knew I saved this thread for a reason. I am trying to figure a couple things out to troubleshoot my system. I have an 85 2wd C10 w/ saddle tanks. First, it was hard to switch tanks, sometimes not work at all and the fuel gauge isn't reading accurately. It isnt going past 3/4 on either tank. I noticed the dash switch was kinda sloppy so I replaced it. It helped the switching back a little but, the fuel gauge still isn't right. Filled up on the right tank yesterday and it went to 3/4 then, filled the driver's tank today and it went up to 3/4.... could this be the Pollack Valve or a connection issue?
I think its odd both tanks are reading the same error - what is the common denominator that would cause that to happen? Also, it isnt the original motor... it has a 350 - no canisters or emissions hooked up to it. - if that helps.
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Frank (Hilo, Hawaii) Trucks, Tacos, and Beer go together like Peanut Butter and Ladies! 85 C/10 SOLD http://67-72chevytrucks.com/vboard/s...d.php?t=760383 67 C/10 SOLD http://67-72chevytrucks.com/vboard/s...d.php?t=489298 79 K5 Blazer SOLD http://67-72chevytrucks.com/vboard/s...d.php?t=657461 Last edited by 808_67C10; 07-27-2018 at 08:09 PM. |
07-28-2018, 03:10 PM | #23 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
If it was just one tank I'd look at the sender or wiring but this is the same on both tanks. More to the point it's exactly the same on both tanks...
What's the same no matter which tank is selected??? The fuel gauge and the sender wire from terminal B of the valve to the gauge. Typical Fuel Sender Resistances were within ±3Ω or so on my test gauges from several different years:
¾ is less resistance than F. The only way to get less resistance in this circuit is for the wire insulation to be rubbed through and make an intermittent ground connection. That's unlikely to be repeatable over time. I'd say it's probably the gauge itself.
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1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
And please let us know if and how your repairs were successful. Last edited by hatzie; 04-28-2023 at 06:27 PM. |
07-30-2018, 11:46 AM | #24 | |
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Re: 1973-1991 Dual fuel tank systems theory of operation
Quote:
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Frank (Hilo, Hawaii) Trucks, Tacos, and Beer go together like Peanut Butter and Ladies! 85 C/10 SOLD http://67-72chevytrucks.com/vboard/s...d.php?t=760383 67 C/10 SOLD http://67-72chevytrucks.com/vboard/s...d.php?t=489298 79 K5 Blazer SOLD http://67-72chevytrucks.com/vboard/s...d.php?t=657461 |
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07-30-2018, 01:37 PM | #25 |
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Re: 1973-1991 Dual fuel tank systems theory of operation
HA! So, I started the truck this morning and the gas gauge read right on full.... then, after driving about 2-3 miles, she topped out about 1/4" past Full - I swear, these square bodies and their electrical issues. I miss my 67 c10 so bad right now! LOL
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Frank (Hilo, Hawaii) Trucks, Tacos, and Beer go together like Peanut Butter and Ladies! 85 C/10 SOLD http://67-72chevytrucks.com/vboard/s...d.php?t=760383 67 C/10 SOLD http://67-72chevytrucks.com/vboard/s...d.php?t=489298 79 K5 Blazer SOLD http://67-72chevytrucks.com/vboard/s...d.php?t=657461 |
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