79 3/4 ton Silverado Custom Deluxe - Dual Tank Fuel Gauge Issues

[pbj]

Hi, accidentally pulled a wire out from where it should be while getting rid of all the after market stereo equipment. Fuel gauge worked fine till this happened and have removed circuit board with gauges, floor mat etc iso far trying to fix with no luck yet.

Have dropped tank, replaced filter screen though now realize that was probably a waste as the fuel filter catches debris.

Tested with ohm meter the tank sending unit and working fine.  Truck has dual tanks and wires come to a 3 way toggle switch.

Have taken the center wire from switch and put it to where pink wire is on harness with speedo/gauge assembly removed and all that happens when powered and the gauge goes beyond full. (tank is only 1/2 full)

Any suggestions much appreciated as I've ran out of gas twice now!

Thanks

[Irish_Alley]

dont know if its from my lack of coffee but whats the issue again? is the switch working at all?

[pbj]

Switch works fine, gauge does not show correct fuel level or change when switch is flipped to other tank.

[VileZambonie]

So you accidentally pulled a wire off the switch when removing the radio wiring and that prompted you to drop the tank? lol I don't follow the logic?

Do you have the factory switch or a toggle switch? The trouble shooting and wiring diagram is in the tech section.

[pbj]

I wish it was as easy as "pulling wire off switch".

Wire that is in the center position of 3 way toggle switch came loose from under dash.

[VileZambonie]

Glad you figured it out.

[pbj]

Thanks but I didn't figure it out, that's what I hoping someone here would help with

[bd]

Check the wire connections to the toggle switch against the diagram from the 1979 Wiring Manual...

[pbj]

Thanks much BD

Feels like i'm getting closer to having it figured out!

Schematic is not so clear, does anyone know what the "62_____" with the 4 30's on it is and where it's located?

What is "bulkhead connector" and location?

*Circled here https://snag.gy/4lmNGM.jpg

Sincere thanks

[bd]

The four-cavity connector feeds through the firewall separately from the main bulkhead connector behind the fuse box.  Expect to find it on the driver side firewall in the vicinity of the brake booster and/or steering column.  The "62______" is an obsolete GM part number.  "Bulkhead Connector" is the main firewall plug behind the fuse box - circuit 30 from the gauge feeds through one of its cavities.

[pbj]

You rock!

This makes sense as a wire was loose that went to fuel gauge in the motor compartment by the back side of fuse panel.

Will explore more to try and figure out where this wire came loose from.

Thanks much for your time and wisdom and Happy New Year!

[75gmck25]

If your stock fuel gauge wiring is like my '75, it has a strange configuration.

- The tan gauge wire from the instrument panel goes through the firewall, then down the engine compartment and over to the main gas tank (this should be the left side on your truck).  Inside the frame rail there is a pull-apart connector in the gauge wire, and from there the wire runs over to the sending unit.
If you only had one tank this is all the wiring you need.

- When a second tank is installed they pull the connector apart inside the frame rail for the main tank and connect two additional wires that run back to the center post and "main" side of the fuel switch dash.  This puts the fuel switch in the middle, between the gauge and the tank sending unit. 
-  The sending unit wire from the right side tank (tan with white stripe) runs back to the aux side of the fuel switch on the dash.
- There is a solenoid activation wire running from the post on the 6 port solenoid back to the fuel switch on the dash.
-  There is a power wire running from the fuse block to the center post of fuel switch on the dash.

The bulkhead connector is a round plug where all the wires to the switch go through the firewall.

Bruce

[pbj]

Hi Bruce, I appreciate your assistance and it appears to me that the 75 may be wired different.

I've attached a few pictures below of what I'm seeing under the hood and don't see a "bulkhead connector" unless that's the square plastic piece with all the wires and what seems to be some kind of dark stuff on it  perhaps to help keep the connections clean with no rust etc.?

http://imgur.com/a/nzArk

[pbj]

Thanks BD, I added some pictures showing the square plastic thing behind fusebox (bulkhead connector?) and hope that helps.

I notice on the PDF you linked it says "1979_Chevrolet_Light_Truck_Wiring_CK_10_30_Only" my truck is a 20 but I think it may be the same because where the pink wire goes if you follow the lines behind instrument panel it goes to the fuel gauge.

I see "circuit 30 " is the pink wire. With the instrument panel removed, I slipped the sensor wire into where the wiring harness plugs into for gauges and then plugged harness connector back in and when I turn key gauge goes past full.

The four-cavity connector feeds through the firewall separately from the main bulkhead connector behind the fuse box.  Expect to find it on the driver side firewall in the vicinity of the brake booster and/or steering column.  The "62______" is an obsolete GM part number.  "Bulkhead Connector" is the main firewall plug behind the fuse box - circuit 30 from the gauge feeds through one of its cavities.

[bd]

I've attached a few pictures below of what I'm seeing under the hood and don't see a "bulkhead connector" unless that's the square plastic piece with all the wires and what seems to be some kind of dark stuff on it perhaps to help keep the connections clean with no rust etc.?

Correct.

I notice on the PDF you linked it says "1979_Chevrolet_Light_Truck_Wiring_CK_10_30_Only" my truck is a 20 but I think it may be the same because where the pink wire goes if you follow the lines behind instrument panel it goes to the fuel gauge.

Look again.  The linked wiring diagrams apply to 1979 C/K10 through 35.

I see "circuit 30 " is the pink wire. With the instrument panel removed, I slipped the sensor wire into where the wiring harness plugs into for gauges and then plugged harness connector back in and when I turn key gauge goes past full.

        Huh?       Fuel gauge swinging past Full to ~3:00 o'clock indicates an open sender wire.  Try jumping a temporary ground to that cavity and see whether the fuel gauge drops to Empty.