Fuel sending units

[75gmck25]

I bought the 3 port tank sending units for my ‘75 GMC K25 from Autozone, and they work fine. Make sure the sending unit ground wire is securely screwed to the frame rail and the sending unit wire is connected.

fYI - My fuel pump also has three ports because it has a fuel return line to the tank.

Crawl under the truck and see what type of solenoid you have to connect the tanks. Mine used a 6 port solenoid (switches fuel feed and fuel return) and I have a 3 port tank sending unit ( fuel feed, fuel return, vent), but some use a 2 port sending unit and a 3 port solenoid that switches only the fuel feed.

A ‘76 should have the old type solenoid that only has one electrical terminal. With no power applied it will default to the passenger side, main tank. With 12 volts power applied it will switch to the driver’s side auxiliary tank. The solenoid grounds by being bolted to the frame.

I will see if I can find the write-up I did once before on how the dash switch is wired. It is much more complicated than you would expect, and the wires take a very inefficient path.

If neither unit is working, I would suggest looking at the gauge and wiring before you just replace both sending units. The dual tank switching wiring for the gauge is a bit convoluted and has quite a bit of areas where you could be having a poor connection that could cause it to not read right.

 

[SquareRoot]

@SquareRoot and anyone else interested. This is what a left side sending unit in a right side tank looks like. As you can see it is possible to that and have the rubber lines neat, organized and safe. Going on three years like this so far and hasn't been a problem.

Nice. Thanks for sharing!

 

[Doug Long]

Thanks we will try the wiring and gauge first. The rims are finished at powder coating so picking those up tomorrow.

 

[75gmck25]

This is the response I wrote a couple of years ago when someone was trying to figure out the wiring scheme. Useful, if you need it, but very boring to everyone else.
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This is the wiring on my '75, and I think it is set up that way to facilitate dealer-installed dual tanks, or to make it easier to add a factory aux (driver's side) tank.

Single Tank
On earlier trucks, if you have a single tank it would be on the passenger side, and there would be a tan wire running from the gauge, across behind the engine and down through the metal tube and the passenger side frame rail to the "main" tank. No other wiring is needed.

Dual Tank
When they add a 2nd tank they add a wiring loom that runs from the DPDT dash switch, through a plug in the firewall and down the driver's side frame rail. This loom has wires for solenoid power, left side sending wire (tan/white), right side sending wire (tan), and connection to the gauge (despite the fact that the dash gauge is only about a foot away from the switch location).

- After running it down the frame rail, at the driver's side tank the tan/white wire comes out of the loom and connects to the sending unit. The other wires continue across under the truck, supported by the bracket for the fuel lines.
- At the passenger side tank they pop apart the original wire connection to the sending unit and connect two wires in the switch so that the switch is now "in between" the tank sending unit wire and the sending unit itself (switch is now connected to the passenger side tank and to the gauge wire).
- They also connect the solenoid power wire wire to the solenoid (12 volts can be applied to the solenoid).

Dash Switch
The switch on the dash is a dual pole, dual throw switch, so it can switch two things between two sets of wires. It switches the sending unit wire, and also controls power to the solenoid.

- The first half of the switch has the center pole connected to the wire coming from the dash gauge. Remember that from the dash this wire runs all the way down the passenger side rail (stock on all trucks) , but is then extended with the wiring loom so it run across under the truck, then back up the driver's side rail to the switch. This is a really long wire.
The two side poles on this half of the switch have one side connected to the sending unit wire from the left tank and one side connected to the right tank. The switch just flips the sending unit/gauge connection from one tank to the other.

- The other half of the switch has the center pole connected to power from the fuse box, and one side connected to the wire that powers the solenoid. When switched to the aux side it provides power to the solenoid, and on the main position it is just open (no power).

This is not an easy wiring scheme to explain with words, and a simple diagram would probably make more sense. However, the GM wiring diagram does not have any overlay of the truck body itself, so even with the diagram the length of these wires and actual location is not obvious.

 

[Doug Long]

Thanks, this is great info. It sounds like it could take a minute to diagnose but we will try this first. A picture of the rims back from powder coating.

 

[Doug Long]

Over the weekend I traced the wiring and all looks good. The driver side reads just over a quarter of a tank and the passenger reads just under a quarter of a tank. The driver side should be about 3/4 full so I thing the tanks will have to be pulled.

 

[Doug Long]

Update. Both sending units are 3 port units. I have ordered new sending units and fuel tanks.

 

[SirRobyn0]

Update. Both sending units are 3 port units. I have ordered new sending units and fuel tanks.

Thanks for the update, just out of curiosity will you be using right and left sending units, or left side in both sides?

 

[Doug Long]

I purchased a right and a left sending unit. Spectra Premium sells a driver and passenger side unit. The FG05C is the passenger and the FG05D is the driver side.

 

[Doug Long]

All,
I need some more help. I have replaced both gas tanks and sending units. I pulled the gauge and it it works off two leads. I checked the polls the gauge plugs into on the dash and we are getting 11 volts. The gauge still does not work properly. it has partial reads and bounces around. We cleaned up the grounds off both tanks on the frame. What else are we missing. It has duel tanks and switch works changing tanks and the gauge will try to read when I switch tanks. The passenger side is full as of today and is reading 3/8's full when I switch to the drivers side the gauge drops to about an 1/8th of a tank.

 

[Randy and Easton]

Great truck Doug. We did my sons while bed was off (super easy) and got it from LMC with no issues. Agree with other comments above on checking switching first.

Randy and E

 

[BearKing]

I'd be curious as to what brand of fuel sending units the forum members have chosen?

Same here. I am going to replace mine and it would great to know which ones actually work.

 

[SirRobyn0]

All,
I need some more help. I have replaced both gas tanks and sending units. I pulled the gauge and it it works off two leads. I checked the polls the gauge plugs into on the dash and we are getting 11 volts. The gauge still does not work properly. it has partial reads and bounces around. We cleaned up the grounds off both tanks on the frame. What else are we missing. It has duel tanks and switch works changing tanks and the gauge will try to read when I switch tanks. The passenger side is full as of today and is reading 3/8's full when I switch to the drivers side the gauge drops to about an 1/8th of a tank.

Doug, how are the grounds for the sending units? Did you replace those or check or clean them up?

What about the resistor on the back of the gauge IDK exactly what it looks like but it is on the back of the gauge did your testing include that.

Well the next most likely thing would be the switch. Some trucks switch the gauge from tank to tank at the switch on the dash others do it at the actual valve. The easiest way to find out is to lay under the truck and look at the valve, it's on the passenger side frame rail under the cab. If there is one wire running into the valve then the dash switch switches the gauge. If there are multiple wires running into the valve then it switches the gauge at the valve. You'll need to look at a wiring diagram because I do not know the colors of the wires by heart, but you can unplug the connector and jump one wire at a time from the tank to the gauge and see it it reads correctly.

 

[Doug Long]

Thanks for the response Sir Robyn,
What a frustrating weekend! We will jump under the truck again this week. We cleaned up all grounds to the frame this weekend. I did not check the resistor on the back of the gauge so we will pull the dash apart again as well. Do you know what I should be reading at the resistor? I pulled the plug from the firewall and checked that. I was getting 5/6 volts off one side but nothing on the other side of the plug. I am assuming that one side is the passenger and the other side is the driver side. There are 4 posts in the plug. We got frustrated and took off the camper shell and cleaned up the bed yesterday. Before and after.

 

[SirRobyn0]

Thanks for the response Sir Robyn,
What a frustrating weekend! We will jump under the truck again this week. We cleaned up all grounds to the frame this weekend. I did not check the resistor on the back of the gauge so we will pull the dash apart again as well. Do you know what I should be reading at the resistor? I pulled the plug from the firewall and checked that. I was getting 5/6 volts off one side but nothing on the other side of the plug. I am assuming that one side is the passenger and the other side is the driver side. There are 4 posts in the plug. We got frustrated and took off the camper shell and cleaned up the bed yesterday. Before and after.

I have no idea what the resistance should be on the resistor, but one thing I will say in my experience that resistor, when it fails will generally cause the gauge not to read, or not to reach full on a fill up, and can be all the time or intermittent. But it should cause the same problem on both tanks. You get different readings on a full tank from side to side, so for that reason I'd check the switch first, rather than pulling the dash back apart.