how to test if my parasitic draw is my fuel system?

[killerpiller]

To start - i have a 1980 K10 with dual saddle tanks. The selector valve is a 1-pin and the truck has the special fuel selector timer relay that allows fuel to be pulled form the aux tank during extended cranking. I am seeing a voltage drop of about 2.5v after 3 days of sitting. I used a multimeter to test amps and mA between the negative cable and the battery. At one point, i saw a sizable draw.. now i only see about 20mA which is acceptable. Before chasing the fuel selector system, i tested and removed all normal fuses. nothing changes the higher draw. However, my voltage is still dropping so there is still a draw. When i plug up the neg battery cable directly to the battery i could feel the special fuel timer relay trigger and the selector valve trigger. I do not hear the relay or selector every time but i do sometimes when i use the multimeter inline of the neg battery cable. Maybe the connection isnt good? i ordered alligator clips for the multimeter for a better connection.


1. fuel tank selector timer relay on the fuse box underdash (#14014527) - supposedly this was only used on a few 1980 trucks so replacements are hard to find
2. the fuel tank valve itself

After research the fuel tank selector timer relay has a +- 30min timer to enable fueling from aux tank during extended cranking. if i connect it to the battery, it triggers the relay and restarts the timer.. Currently, i removed the relay to see if i still experience the voltage drop after a day or two.

Any thoughts? Id like to verify dead component before ordering


General Notes:
1. RH is the main tank; LH is the aux tank
2. battery is about a month old


Fuel selector diagram

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unrelated side note - i dont have a horn in the center of the steering wheel but when i turned on my left turn signal today, it honked

 

[75gmck25]

I’ve been following squarebody forums for at 20+ years, and have never heard of that fuel system timer. It’s really an unusual option, and I wonder why it was needed.

I would have to study the wiring diagram for a while, but it seems you could disconnect the relay and just run a standard one-terminal solenoid. The relay is old, and maybe it’s sticking internally and staying on too long after you shut off your ignition.

Another thing to try is to disconnect the relay and just run off your main tank for a while. If the parasitic draw is resolved, you know it’s the relay.

 

[bucket]

I’ve been following squarebody forums for at 20+ years, and have never heard of that fuel system timer. It’s really an unusual option, and I wonder why it was needed.

I would have to study the wiring diagram for a while, but it seems you could disconnect the relay and just run a standard one-terminal solenoid. The relay is old, and maybe it’s sticking internally and staying on too long after you shut off your ignition.

Another thing to try is to disconnect the relay and just run off your main tank for a while. If the parasitic draw is resolved, you know it’s the relay.

Yep! I just learned something today too.

 

[killerpiller]

I’ve been following squarebody forums for at 20+ years, and have never heard of that fuel system timer. It’s really an unusual option, and I wonder why it was needed.

I would have to study the wiring diagram for a while, but it seems you could disconnect the relay and just run a standard one-terminal solenoid. The relay is old, and maybe it’s sticking internally and staying on too long after you shut off your ignition.

Another thing to try is to disconnect the relay and just run off your main tank for a while. If the parasitic draw is resolved, you know it’s the relay.

the relay is on select 1980s only so pretty limited use from GM. I removed the relay yesterday to monitor voltage and test the drop over the couple days. hopefully this will provide actionable results so i can buy an overpriced relay or a design a work around


Here is a photo of the relay (its the silver block on the right)

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[killerpiller]

Voltage Monitoring

Battery - DieHard 75FT-3 (700 CCA) - 1-2 months old
Conditions - I removed the fuel selector timer relay so the truck can only run on the primary tank (RH), battery is connected to both pos and neg
Result Expectations
- if the voltage stays around 12.5-12.7 after 3 days without driving, the fuel selector timer relay or switch is likely bad. Resolution is to buy new relay and switch. These switches are fragile and tend to break while removing so best to buy a new one before removal.
- if the voltage drops like before (12.86 to 10.7) after 3 days without driving, then there is another draw or the battery is bad. Resolution is to take battery in for testing and figure out why my multimeter isn't registering any high draws. It currently reads 20.9mA


Test Results
10-30-2025 @ 3:20pm - 13.10v (this is after driving so it had a higher surface charge)
10-31-2025 @ 9:45am - 12.75v (overnight low of 30*)
11-01-2025 @ 6:00pm - 12.49v (overnight low of 30*)
11-02-2025 @ 9:00am - 12.39v (overnight low of 30*)

I will update the test results as i test more over the next day or two

 

[killerpiller]

Based on the test results, removing the relay drastically reduced the draw. I found some merchants on ebay that had parts. I ordered 2 new relays so i have one as a backup. I found an extremely overprices 6port 1pin selector valve but i figured at this point a three hundred dollars of a necessary part that is discontinued is worth it if it works. So rolled the dice on that one.

Hopefully these replacement parts will solve the issues.

I may need to replace the sending unit in my RH tank because it reads way past full on my gauge.