Fuel tank selector switch HELP!!

firebane

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I can confirm that both wires from the sending units do not go through the plug at the fire wall. They come through a separate opening just above the plug.

Man this sucks.

Yeah so it sounds like someone just wanted to make it work and just fudged it. In some ways that makes things easier... Although you do have a bit of work ahead of you hah.

I would still do wire tracing so you can make sure all wires are in their proper places for the sending units and valve.

Trying to locate a picture is pretty tough but essentially the way it worked was the wires for the tanks would feed through a grommet into the engine bay and then go into a plug. From that plug it would feed down to the tanks and valve.

There was 3 different setups for dual tanks including the elusive and VERY rare push/pull setup for the 73/74 era and then changing out in 75.

Most times when converted you end up with some kind of hokey mess of a 4 or 5 prong plug before it comes into the cab.

When I redo my wiring I am going to use a plug like this (http://i00.i.aliimg.com/img/pb/905/325/124/1124325905_200.jpg) and do it clean and make it function properly.
 

chengny

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From a previous post:

Pull the appropriate wires off the switch (the ones that run from the switch back to the tanks). Take a jumper lead and connect it to whichever stab on the switch that corresponds to the tank you want to test first.

First, just let it dangle in the air and turn the key to RUN. The gas gauge should go to the hard over full position.

Then, with the key still in RUN and one end of the jumper still attached to the switch terminal, ground the other end of the jumper to a convenient good/clean/paint free spot under the dash. Look at the gas gauge - it should at that point be reading all the way empty.

Turn the key off and shift the jumper to the switch terminal that the other tank is connected to. Do the same 2 tests.

See what happens. These two tests will help you zero in on where the problem exists.


Okay I did this for both sides and it did exactly as you said it should. Haha, now what do I do?


As far as tank level indication, you have proven good all the way through the dash switch. Now you have to check the leads from the switch to the individual tanks and also the sender ground connection. Unfortunately, this will require that you gain access to the connections at the sender head. The images below show an 80's era sender, but your's should be similar:

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With the leads connected to the dash switch, pop the feed from the switch off the sender (s) and conduct the same tests as described above - the ones that you did at the dash switch.

But first, have you made sure that the ground path from the sender (s) to the frame has a good, clean, tight connection at the frame? If not, do that first - if that should happen to be the problem, it will save a ton of time diagnosing this issue.
 
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BigPoser

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From a previous post:

Pull the appropriate wires off the switch (the ones that run from the switch back to the tanks). Take a jumper lead and connect it to whichever stab on the switch that corresponds to the tank you want to test first.

First, just let it dangle in the air and turn the key to RUN. The gas gauge should go to the hard over full position.

Then, with the key still in RUN and one end of the jumper still attached to the switch terminal, ground the other end of the jumper to a convenient good/clean/paint free spot under the dash. Look at the gas gauge - it should at that point be reading all the way empty.

Turn the key off and shift the jumper to the switch terminal that the other tank is connected to. Do the same 2 tests.

See what happens. These two tests will help you zero in on where the problem exists.


Okay I did this for both sides and it did exactly as you said it should. Haha, now what do I do?


As far as tank level indication, you have proven good all the way through the dash switch. Now you have to check the leads from the switch to the individual tanks and also the sender ground connection. Unfortunately, this will require that you gain access to the connections at the sender head. The images below show an 80's era sender, but your's should be similar:

With the leads connected to the dash switch, pop the feed from the switch off the sender (s) and conduct the same tests as described above - the ones that you did at the dash switch.

But first, have you made sure that the ground path from the sender (s) to the frame has a good, clean, tight connection at the frame? If not, do that first - if that should happen to be the problem, it will save a ton of time diagnosing this issue.


Thank you for all the help. I know the grounds are good because that was the one of the first things I did.

On a side note, I was changing the valve cover gaskets tonight and trying to make sense of the sloppy engine compartment and I noticed several wires that didn't go anywhere. Two of them were in a connector mounted on the firewall on the upper left area of the brake booster. One of the 3 (tan in color) went back into the truck somewhere. I'll try to get a picture in the morning.

I also found a grounding strap behind the motor on the left side of the firewall that didn't go anywhere either. Assuming that it goes to the motor, but I have no idea where. Could this be part of my problem with my gauges?
 

chengny

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Oh yeah, for sure. That ground strap can be a big help as far as getting your gauges to work right. While there are other grounds that the gauges can utilize, the firewall to engine block (via the RH cylinder head) is the clearest/most direct path back to the battery negative. Not saying that this will solve your problem, but it will eliminate another possible issue.

Clean both ends of the strap and firmly connect it to the RH cylinder head and to the firewall (clean those areas too).


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chengny

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I can confirm that both wires from the sending units do not go through the plug at the fire wall. They come through a separate opening just above the plug.

Man this sucks.


Does this look more like what you are dealing with?

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BigPoser

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I can confirm that both wires from the sending units do not go through the plug at the fire wall. They come through a separate opening just above the plug.

Man this sucks.


Does this look more like what you are dealing with?

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Could be. When I started working on this issue, there were 5 wires going to the switch. The wire that went to the fuse panel, wasn't connected, and the wire from the passenger sending unit was cut and coiled up in the frame rail near the motor. So I connected the wire from the panel on a new swtich, finished off the run from the pass sender and hooked them all up to a new switch. It is currently set up like the previous diagram you posted as did Firebane. Tested the gauge and it works per your tests on both senders. Bed is on and might be a while before I can get that taken off to test the senders themselves, but I do get a dim light on my light tester from each of the senders at the switch. I'm no expert, but that seems to be a grounding issue?

As Firebane knows, I do have an electric fuel pump in the system (but am currently looking for a regulator), and will be ordering all new gauges today. I will also be getting a new complete wiring harness soon, as well as seriously considering going to a single tank in the back.

This truck is more or less a wiring nightmare.
 

BigPoser

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Oh yeah, for sure. That ground strap can be a big help as far as getting your gauges to work right. While there are other grounds that the gauges can utilize, the firewall to engine block (via the RH cylinder head) is the clearest/most direct path back to the battery negative. Not saying that this will solve your problem, but it will eliminate another possible issue.

Clean both ends of the strap and firmly connect it to the RH cylinder head and to the firewall (clean those areas too).


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Yup that's the one. It's just hanging from the firewall. Might be a bitch to get it back on with the motor and tranny in though I suspect.
 

firebane

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I put the ground from left head to brake booster frame.
 

Rebar1968

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So I was hoping to get some info bringing up a old post lol. I just picked up a truck papers say 73 but not it is a C20 I'm guessing a 77 ish , has duel tanks has the old bolt in the dash 5 wire switch, and a single wire valve,, wired have been played with back near tanks the wire on valve powers off and on with the switch looks like one wire to each sending units how do I test this set up another off thing is the single wire valve I find is 3 ports this one is a 6 port any one seen this before when I use test light at switch 3 terminals do nothing
 

AuroraGirl

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So I was hoping to get some info bringing up a old post lol. I just picked up a truck papers say 73 but not it is a C20 I'm guessing a 77 ish , has duel tanks has the old bolt in the dash 5 wire switch, and a single wire valve,, wired have been played with back near tanks the wire on valve powers off and on with the switch looks like one wire to each sending units how do I test this set up another off thing is the single wire valve I find is 3 ports this one is a 6 port any one seen this before when I use test light at switch 3 terminals do nothing
if you need to replace your 6 port you can use 2 3 port 1 wire ones
 

bucket

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if you need to replace your 6 port you can use 2 3 port 1 wire ones

The problem with that is what happens if for some reason one valve fails. Then there's the potential to overflow one tank as you drive.

There seems to be one heck of a demand for these single wire, 6-port valves lately. I've had a good spare for 10 years now and have never needed it. But I know darn well that if I let it go, the valve on my truck will soon fail, lol.
 

AuroraGirl

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The problem with that is what happens if for some reason one valve fails. Then there's the potential to overflow one tank as you drive.

There seems to be one heck of a demand for these single wire, 6-port valves lately. I've had a good spare for 10 years now and have never needed it. But I know darn well that if I let it go, the valve on my truck will soon fail, lol.
I would love if mine wasnt looking like this because id give it a 50/50 shot of still working otherwise because my gpa would have been the one to bypass that all because the sock in the one tank got plugged, like, he would do that, so If it wasnt looking like this I would so offer it to someone lol
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I couldnt find any 6 ports for sale new, and you are right they became famous lately.

I would do the 2 3 port one IF I had a 1980 "delay" wiring that bex actually taught me about the one day, the failures of the solenoids was everytime you cranked, turned the key, it would actuate. the delay wiring would make it not actuate everytime and have a timed wait before it would allow it to do it again, so if you are cranking or whatever its not wearing faster than a newer cars door lock solenoid

however, they also wore out because of gasoline and debris.. since the fuel filter system is before the carb, not the solenoid, they were attacked by that too.

I would, if possible, put a small screen filter or something before the "send" one and the return would benefit from one.... before.... the other direction tho lol. But the fuel pump may not like restriction..
 

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