73-80 Fuel Gauge Problem!!!!!

[54Caddy]

Well i have a thread in the 73-87 side but it has not moved over here so ill start a new one i need an answer quickly. Okay im working on a sand drag truck that originally had dual tanks. Now it has a fuel cell in the bed. Here is where i need help the fuel gauge was pegged was past full so i am tracing wiring from where original tanks use to be on up. I found that the switch is still in the cab for both tanks and i am wanting to eliminate it. It has four wires coming out. 1 Blue 1 Green & 2 Tan. I know the Tan ones are for the Sending Units. I have the cluster out and see that pin 7 & 18 are for the gauge. Now here is what I am thinking of doing, there is a wire jumping 7 & 18 (factory wire) then pin 7 has one going out to harness. Now if I get the one going out from the main harness behind cluster and directly connect it to one of the Tan wires after the Fuel Tank Switch to totally elminate switch would it work correctly? Trying to eliminate any unneeded wiring on this truck.

[motornut]

http://67-72chevytrucks.com/vboard/s...d.php?t=535659
sometimes the mods are busy and don't see a request and need a PM to move a thread

[ray_mcavoy]

The dual tank wiring on these trucks uses the stock single-tank sending unit wire along with a separate wiring harness for the tank switch & dual sending units.

In other words, if you were to trace that tan wire from pin 7 on the cluster, you should find that it runs out into the engine harness, down near the starter, and then back along the passenger side frame rail where it terminates in a plug that would go directly onto the sending unit.

But instead of going directly to the sending unit, it plugs into the dual tank wiring harness. Which routes it back up to the dash switch and then back (through two more wires) to the sending units on each tank.

So to convert back to a single tank, all you need to do is locate that plug along the passenger side frame rail where the stock single tank sending unit wire plugs into the dual tank sub-harness. Unplug that connection and hook it directly onto the single tank (or fuel cell in your case). That will allow you to completely remove the dual tank sub-harness and switch.

As a sidenote, the reason why the sending unit wire is jumpered to both 7 & 18 on the cluster plug is so that the same plug will work with both the regular gauge cluster and the gauge cluster with a tach that re-locates the gas gauge to the lower left corner.

[54Caddy]

Understood but this one does not have a harness like you are talking about. I believe that is on the 81 on up models. This truck is a 74, Like i said there are only 4 wires. One Dark Green Wire, One Tan wire on passanger side, One Tan wire in the center with the green wire, and a Dark Blue wire on the driver side. The harness comes out from the firewall in the center behind the driver side valve cover and runs down the driver side frame rail, I will go snap some pictures to possibly aid in help.

[54Caddy]

Heres some pictures

These are the wires running down the Driver Side Frame Rail. Dont mind the messed up connections those will be fixed in the morning


Blue wire that was already cut by where the original driver side saddle tank would be


The 2 Tan wires one was by the original passanger side saddle tank and still had the wierd round push connector


Showing where the harness comes out the firewall


Harness Leading from inside of the cab

[54Caddy]

Fuel Tank Selector Switch stuffed in the dash. Previous owner shaved the hole for it


Dash Cluster Harness showing tan wire jumping from pin 7 to 18


The Cluster, Just need to finish painting fuel gauge

[ray_mcavoy]

Well, I don't know for sure about 73-76, but I do know 77-up used the separate add-on harness for dual tanks like I described.

I just took a look at the factory 77 wiring diagrams and it actually shows 3 different dual tank wiring setups. However, two of them are labeled "unused" so I'm guessing those are probably carry overs from the older years that for some reason didn't get deleted from the service manual.

Anyway, one of those "unused" diagrams looks similar to what you're describing. It has the tan wire from the gauge going out through the bulkhead connector on the firewall, splicing directly to an orange wire, running back in under the dash through a grommet, and connecting to the selector switch. From the selector switch, 3 more wires go out through the grommet (a dark blue wire that splices to a tan wire goes to the main tank sending unit, a light blue wire goes to the aux tank sending unit, and a light green wire goes to the tank switching valve). There's also a pink/black wire going from the fusebox to the switch.

With this setup, it looks like the factory basically cut the tan wire somewhere under the hood (probably near that grommet) and spliced one end to the orange wire and the other end to the dark blue wire leading in to the switch. Changing back to a single tank setup should be as simple as locating the point where they cut the tan wire and spliced it into the orange & dk blue wires, cutting those splices back apart, and re-joining the two ends of the tan wire.

[54Caddy]

Yeah no orange wires in this set up, what im going to do is trace back wiring from the selector switch, If i find the same tan wire that leads to the cluster harness then i will just join the two and delete everything else out. I need to invest in some manuals for these trucks for sure

[ray_mcavoy]

Here's a couple of scans from the 1977 factory service manual that probably do a better job of showing the wiring than what I'm describing.

First is the setup with a sub-harness that plugs into the standard single tank wiring along the frame rail near the RH tank. This is what I was trying to describe in my first post and is identical to the factory dual tank setup on Dad's 77 GMC.




And here are the two diagrams that are labeled "not used". The top one is the setup that looks as though the factory cut/spliced into the tan wire to join it to the selector switch. And the bottom one looks as though it would just switch the gauge and be used with a manual selector valve or something.

[54Caddy]

Okay thanks for those. But why on all of them does one tan wire go the the passanger side and the other some how loops back to gauge thats throwing me in left field ahaha. Well looks like I will be taking off most of the tape to revill all wiring and see whats going on

[54Caddy]

You think i could join Tan30 A to Tan 30c and it should work?

[ray_mcavoy]

Yeah, connecting tan30A to tan30C at the selector switch would do the job. Although it's going to leave a bit more wiring zig zagging back & forth than necessary.

All you really need to keep is the "tan 30" wire represented by the dashed line. The wiring diagram doesn't do a good job showing the physical layout, but assuming the 74 layout is similar to 77, that wire should run from the gauge cluster, out through the bulkhead connector, down near the starter, and back along the passenger side frame rail. Just connect that end of the wire to the sending unit.

[FLEET72]

Originally Posted by ray_mcavoy

Yeah, connecting tan30A to tan30C at the selector switch would do the job. Although it's going to leave a bit more wiring zig zagging back & forth than necessary.

All you really need to keep is the "tan 30" wire represented by the dashed line. The wiring diagram doesn't do a good job showing the physical layout, but assuming the 74 layout is similar to 77, that wire should run from the gauge cluster, out through the bulkhead connector, down near the starter, and back along the passenger side frame rail. Just connect that end of the wire to the sending unit.

I just checked my '74 wiring diagram and it is the exact same as the '77 that you posted.

[54Caddy]

So this is confusing. Heres what i think will work. I i run a wire from pin 7 back to the sending unit it should read correct?

[54Caddy]

So I just removed the complete harness from the selector switch inside the cab to all the wires that i have pictured. It is not in any way spliced into another tan wire. On my personal truck i know the tan wire runs down by the started on onto the passanger frame rail, So i uncovered some wiring and found a tan wire that is not being used pulled it all the way up and have it resting on inner fender. Does any one have a wiring diagram of the bulk head connectors coming out from the firewall. I believe this is the wire but want to make sure first.

[FLEET72]

This doesn't show actual placement of wires but it does confirm what you are seeing at the cluster connector.






This shows the bulkhead fitting and the Tan (Circuit 30) wire and which cavity it occupies. Passenger compartment on the right engine compartment on the left.




On the diagram it appears that the tan wire goes from the firewall and straight back to the tank but it does not show routing.

[54Caddy]

Thanks man im going to check that wire for anywhere from 3 to 5v reference once i reinstall the gauges here in a bit. If it checks fine then i will route it to the sending unit and see what happens. What book are you guys using for this diagrams i really need to order one.

[FLEET72]

Here is one on ebay.

Here is a set, service and wiring diagrams.

[motornut]

http://forum.73-87chevytrucks.com/sm...?topic=11766.0
may help for now

[54Caddy]

Well i got the wiring all figured out reinstalled gauges and ran wires back to sending unit. The fuel gauge still goes past full. I checked for a reference voltage back at sending unit and im getting 5 volts at the posts of the sending unit. So with seeing voltage there the sending unit is most likely the problem correct?

[ray_mcavoy]

I'm glad to hear you got the wiring figured out. I agree, it is kind of confusing the way they ran the wires back & forth for the dual tank selector switch.

As for your gauge still reading past full, I'd recommend checking your sending unit ground. Also check the resistance range on your sending unit ... should be 0 to 90 Ohms to work with the stock GM gauge.

[54Caddy]

Sending unit is bad going to order one up monday. Should be complete after that, then onto the temp and oil pressure gauge ahahah!

[77c10sb]

ttt. i know its old but im doing something similar and wondering which wire u ended up using