Why did GM route Circuit Number 30 Fuel Gauge to Tank Unit as they did?

[Mark80K15]

Hi, I'm using ST_352_80_1980_Chevrolet_Light_Truck_Wiring_CK_10_30_Only.pdf (don't know the actual title... the front page is not included in the PDF). Page 11 includes the "AUXILIARY FUEL TANK" schematic, and it illustrates circuits 30 and 53. Please see attached snippet "aux_fuel_tank_schematic.JPG".

These circuits support the dual fuel tank system: dashboard gauge, L/R tank switch, and sender float sensors and grounds. I have a question about circuits 30 and 30A. Circuit 30 originates at the dashboard gauge and goes to the firewall bulkhead. On my truck this wire is then routed from the firewall bulkhead (in the engine bay) over to the passenger side, and then along the frame rail to a point near the sender at the top of the passenger tank.

At this point there is a junction, please see attached "pic_1.JPG" and "pic_2.JPG". According to the schematic, this junction is where Circuit 30 terminates and then Circuit 30A originates. Then Circuit 30A joins with Circuits 30C (passenger side sender) and 53B (fuel tank selector), goes across a cross member to the drivers side frame rail, joins Circuit 30B (driver side sender), and finally proceeds to a plug on the firewall, and from there to the tank switch and fuse block.

Why are circuits 30 and 30A routed from the firewall bulkhead, down the frame rail all the way back to both tanks, and then all the way back to the firewall? Why does Circuit 30 terminate at that junction at the passenger tank, and why does Circuit 30A originate there? What is the purpose of that junction? Couldn't they just be one continuous wire? Does this have something to do with how GM accommodated a single tank on the passenger side versus the option of 2 tanks, with minimal rewiring?

I ask because I hate how circuits 30 and 30A are routed, and if I'm going to improve that, I'd rather take a shortcut... Route Circuit 30 directly from the firewall bulkhead to that plug on the firewall.

 

[mtbadbob]

It must be done that way because it was the simplest if the truck was equipped with the "auxiliary" tank and so the fuel gauge worked on both tanks.

 

[RanchWelder]

You would be OK doing it as one, however if you need to pulll a tank and you do not have a male to female coupling, in the wire, then you'll just have to cut the wires. As soon as you cut the wire, you now have run A/B or 30A vs 30...

Keep in mind this harness is used for several vehicles, including deisel engines and different fuel tank configurations.
The harness numbering scheme has to account for the wires,wherever they originate or end, for multiple fuel systems.

It is the same circuit already. Do not over think it.
Great breakdown, by the way. Very detailed analysis!

GM does a lot of weird crap.

If you get bored, try and figure out the winshield wiper switch and motor wiring sequence!

$0.10 says you post another "Why Did GM Wire It This Way" thread...

 

[Mark80K15]

Thanks a lot, @mtbadbob and @RanchWelder . I appreciate the kind comments. So do I understand @RanchWelder correctly, that for a dual tank configuration, there's no reason for Circuit 30/30A to run all the way to the fuel tank area? And that's because the only function of Circuit 30 and Circuit 30A in a dual tank configuration is to connect the dashboard fuel gauge to the fuel switch? On my truck (and I suspect this is from the factory), there is no connection from anything else with 30/30A back at the fuel tank area.

If I'm correct, then the physical route for 30/30A could be anywhere, as long as it completes a connection between the dashboard fuel gauge and tank switch, right? Including in the cab... which would be best for protection, but I don't want to do that since I don't want to interfere with the firewall bulkhead connector or the firewall plug.

 

[RustyPile]

The simplest explanation I can give is GM didn't "rewire" for the second tank.. They simply designed an "add-on" harness for the second tank disconnected a couple plugs and plugged in the add-on harness, cut a hole in the dash for the switch added the switching valve and some plumbing, and called it good..