Fuel tank selector switch HELP!!

firebane

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He's got something going on - probably with either the gas gauge or the common sensing leg between the gauge and the switch. His test probe should light right up.

Something to note though, is that when I use a test light and touch each of the tank wires or the gauge wire, I get a pretty faint light, which would mean that each of them are getting power, but not enough?

Well checking the wires at the switch just for the senders will let him know if those are working. Then he can voltage the 12v leg to ensure he gets 12v then he can trace the wire from switch to cluster.

Wiring sucks :\
 

BigPoser

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X 2 -

A DPDT (2 position) switch has all the switching capability you'll need to wire this circuit.
Plus it will be cheaper, less complicated to make connections to and easier to find.

If I were you guys I would source a decent heavy duty switch - and I would spring for the extra couple of bucks and get one with two color illumination:

Seadog 4202531 Red/Yellow Illuminated Rocker Switch - DPDT - On/Off/On $7.95

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http://www.boatersland.com/sdg4202531.html

But whatever switch you decide on, remember that it has to be a rocker style switch. It can't be a momentary switch. If it was momentary the instant you let your finger off of it, the tank valve would shift back to the production tank.

The later transfer valves were motor operated and the switch had to be held for a second or so to power the valve over to the other tank. But after that, it would stay in whatever position the operator left it in - until it was powered back in the other direction.

I would rather have the switch that you point out, but I need to have old school metal toggle switch so that it will fill the hole that I have from the old one.

Wiring totally sucks. I think that I'll be ordering a full wiring kit soon.
 

BigPoser

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Pull the sensing lead off the sender and try these two tests;

1. If the tan/white (or light blue) wire to the sender is broken/disconnected the control leg exerts the greater force and pegs the gauge needle to Full.

2. If the tan/white (or light blue) wire is allowed to contact ground (before passing through the sender), the sensing leg has a greater force and the gauge pegs low.

If those two things happen, it will at least confirm proper operation of the gas gauge.

I have the dumb right now. FYI the colors of my wires are different, but there are two wires coming out of a hole that go to the sending unit. One of which goes to the switch in the dash and the other grounds directly to the frame. My tanks are both mounted in the truck and the only place that I'd be able to disconnect from the sender is at the switch.

So if I understand what you are suggesting, then I need to disconnect each wire from the switch for the corresponding tank and turn the key to see what the gauge does? Or can I ground it out with the test light and see what the gauge does?

Thank you for all of your help guys. GM is buying back my other truck next week and I need to get this dialed soon as this truck will be my daily.
 

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I need to disconnect each wire from the switch for the corresponding tank and turn the key to see what the gauge does? Or can I ground it out with the test light and see what the gauge does?

Yeah, if that's the best you can do. At least that will let you observe proper operation of the gas gauge and also test the common wire that runs from the gauge to the switch.

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.
 

crazy4offroad

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I think the switch is bad on mine. When set to left tank I can get under the truck and the valve sounds like it is making Morse code. The gas gauge pegs past full but the tank has good ground and the sending wire has good ohm readings. Selecting the right tank the gauge works fine and the valve is silent, more than likely unpowered. Is the switch a common failure part?
 

BigPoser

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I need to disconnect each wire from the switch for the corresponding tank and turn the key to see what the gauge does? Or can I ground it out with the test light and see what the gauge does?

Yeah, if that's the best you can do. At least that will let you observe proper operation of the gas gauge and also test the common wire that runs from the gauge to the switch.

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.


Thank you. I'll try this tonight. So what would be the potential problem be if the gauge works for both senders?
 

firebane

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Thank you. I'll try this tonight. So what would be the potential problem be if the gauge works for both senders?

I would then be looking at the valve and making sure you have 12v going to it when the switch is applying 12v to it.

Have to remember how old that sucker is and fuel today does not comply with 40 year old rubber.
 

BigPoser

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I would then be looking at the valve and making sure you have 12v going to it when the switch is applying 12v to it.

Have to remember how old that sucker is and fuel today does not comply with 40 year old rubber.


I think I'm good there. I say this because when using my test light when the switch is on one side, that wire lights up, but not when I switch the switch. I have also replaced the switching valve with a new one.

Truck is currently at the shop having the alignment done and rear end looked at. I hope I get it back tonight.
 

firebane

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I'm not sure if this will work but chengy can provide more feedback.

With your key on and not running you should be able to attach sender wire to fuel gauge wire and the gauge should move in accordance to how much fuel you have.
 

BigPoser

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I need to disconnect each wire from the switch for the corresponding tank and turn the key to see what the gauge does? Or can I ground it out with the test light and see what the gauge does?

Yeah, if that's the best you can do. At least that will let you observe proper operation of the gas gauge and also test the common wire that runs from the gauge to the switch.

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?



I'm not sure if this will work but chengy can provide more feedback.

With your key on and not running you should be able to attach sender wire to fuel gauge wire and the gauge should move in accordance to how much fuel you have.


Tried this as well and it didn't make the gauge move at all for either sender.
 

firebane

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Might be time to do a continuity test between the plug at the firewall to the tanks and make sure there isn't anything wrong with your wires (unless they are brand new)

Don't forget 73/74 trucks didn't use the switch we are trying to use. They originally used a push/pull switch so anything other has been added by a PO.

In case you haven't discovered it on the firewall near the steering column should be a 4 or 5 wire plug which is for the tanks and valve.
 
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BigPoser

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Might be time to do a continuity test between the plug at the firewall to the tanks and make sure there isn't anything wrong with your wires (unless they are brand new)



I was hoping I wouldn't have to do that. Funny you mention the plug at the firewall because I'm not getting power to my tail lights when I turn on my running lights. Everything else within the lights work, but the running lights. Right on the engine side of the plug, I get nothing. New wires will be on order soon, right after I figure this out.

Any idea which wire I need to look for / follow?
 

firebane

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I was hoping I wouldn't have to do that. Funny you mention the plug at the firewall because I'm not getting power to my tail lights when I turn on my running lights. Everything else within the lights work, but the running lights. Right on the engine side of the plug, I get nothing. New wires will be on order soon, right after I figure this out.

Any idea which wire I need to look for / follow?

Well start at say the drivers tank and take the wire for the sending unit and trace where it goes along the frame up to the firewall and do the same for the passenger side. It should lead to a plug and from that plug leads into the truck to the switch.

You need to confirm which wires are coming into the cab do what.
 

BigPoser

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Don't forget 73/74 trucks didn't use the switch we are trying to use. They originally used a push/pull switch so anything other has been added by a PO.

In case you haven't discovered it on the firewall near the steering column should be a 4 or 5 wire plug which is for the tanks and valve.

I have something like this for my switch:

http://www.amazon.com/gp/product/B00D833LU2?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00


I haven't noticed any plug near the column. All of my wires for the tanks go directly into said switch. Have a pic of the plug perhaps?
 

BigPoser

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Well start at say the drivers tank and take the wire for the sending unit and trace where it goes along the frame up to the firewall and do the same for the passenger side. It should lead to a plug and from that plug leads into the truck to the switch.

You need to confirm which wires are coming into the cab do what.


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.
 

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