My 73 Chevy needs some help

[RPOZ11]

Hereiis my truck...

Here is my issue:
This truck has the dealership add-on gas tank, a boxy steel unit, that resides under the cab/bed area on the drivers side.

This truck uses a toggle switch on the left lower side of the dash and is electric.

The gas guage wont read correctly the amount of fuel left in the tanks.
I took it to the repair shop that had history of working on it and it still has issues now after 3 months of him working on it.

Items replaced:
Later model side saddle tank seen in newer 75-up trucks with a new tank sender
Tank switching unit on the frame in the rear middle of the truck

Does anyone have any knowledge of these systems and could help direct me in fixing this situation?

I am wanting to fix this by reusing the original toggle switch.
The repair shop added an additionl switch so that the gas gauge could actually be read but I have to switch this toggle switch every time I switch the tanks.

Does anyone have any knowledge or experience with these dealership add-on gas tanks, the sending units in these specific tanks, etc etc...?

[75gmck25]

I had exactly the same dealer-added setup on my '75. I was able to get the dealer-added system working, but later switched to a stock GM gas tank and switch.

The most straightfoward solution is to get a dual gas tank wiring and solenoid setup from a junkyard truck. There are some variations in the wiring from year to year, but they all work the same. If you get it all from the same truck, it will make life much simpler.

Now for my long explanation - I also have the wiring diagrams from my '75, but its really not that complicated.

The steel gas lines (feed, return and vent) run from the fuel pump along the right side frame to a point near the front of the gas tank. At that point you will have either a 3 port or 6 port solenoid. If you have a gas return line, you need a 6 port solenoid. In my dealer-added solution they only used a 3 port solenoid, so I could only switch the feed line. The return line just had a T in it to connect to both tanks, but it always returned fuel to just one tank. That meant I was sometimes drawing fuel from one tank and returning it to another - very confusing.

The solenoid is grounded to the truck body, and only has one 12 volt power feed. With no power it feeds fuel from one tank (passenger side on a '73); apply power and it switches to the other tank.

The dual pole, dual throw dash switch has six terminals, but only five wires are connected.. For one half of the switch it connects the center pole (wire to the gas gauge) to either of the tank sending unit wires. The other half of the switch has 12 volts on the center pole, and it either sends 12 volts to the solenoid, or it switches to the open connection (no wire) and applies no voltage to the solenoid. Power for the 12 volt center pole comes from a line to an accessory connection on the fuse box.

The stock dual tank wiring is more complicated than necessary, apparently to make it easier to add the dual tank.
-- tan line from the gauge runs through the firewall with the other instrument panel wires, then down the right side frame rail to a point near the tank, and then has a junction. Note: On a single tank vehicle, you would just have a wire from this junction directly over to the sending unit.
Side 1 of the dash switch - gauge switching
-- at that junction on the frame the gauge wire connects to another tan line that goes across the truck under the bed, then back up the driver's side frame rail, and through a firewall plug to the center pole of one side of the dash switch. (in other words, the line to the gauge circles back up to the front of the truck and the dash switch)
-- from the tank switch there is another tan line and a tan line with a white stripe. These connect to the other two poles of the switch side 1. The tan wire feeds back down the driver's frame rail, across the truck and to the right side (main) tank sending unit, and the tan/white wire goes to the left side (aux) sending unit.
Side 2 of the dash switch - fuel solenoid
-- the center pole is connected to 12 volts. the wire from the aux side of the switch goes through the firewall plug, down the driver's side frame rail and over to the solenoid. When this wire has 12 volts applied, it switches to the aux tank (driver's side on a '73)


Bruce

[RPOZ11]

Ok, I will go see what is up on the frame for the switch.

Here is the frame switch.
I see now that this is a newer version:


Added images:





As for the dash unit switch, mine is the toggle type.

Here is the Toggle switch images:







On the latter part of your explanation, is this example defining the switch of a 6 or a 3 terminal, or my toggle or a dash unit installed near the ashtray?

Thanx so much for your reply, this will give me a place to start!!!

Guess thats why we fix our own trucks now!!!