Why Did My Constant and Switched Power Circuits Reverse After A Short?

Question

Question:

Hi Experts Im have a 1978 Buick century 2 Dr sedan ,not an RV I experienced an accidental short when I closed the car hood with the battery charger still attached it immediately blew the 30 amp mrbf on the OEM prim. battery pos. post this mrbf provides power to an aux. fuse panel that supplies 12v dc to various subsystems ,gauges ,relays ,and other monitoring devices . I also have an aux. battery that is connected in parallel via latch solenoids as switching devices .No fuses were blown other then the Mrbf ,no c/bs were tripped. Didnt smell any burnt devices ,when the mrbf was re-re with another 30 amp mrbf all is well,car starts and runs fine ,but the weird thing now is 2 subsystems very low power current used show reversal of power ,I.e. 1 device that turns on with switched power now turns on with const. power ,and the 2nd device that turns on with const. power now is with switched power,but the thing is these systems operate normally except for the power reversal relays trigger ok,status lights operate ok etc. I am slowly going through the troubleshooting process,all voltages on the OEM fuse panels,aux fuse panels ,and other power distribution panels show ok meaning switched and const. battery power is correctly functioning. Any technical advice as to why this is will be greatly appreciated. Thanks

asked by: Spiro

1

Helpful Expert Reply:

Hey Spiro, thanks for reaching out. The fact that the 30-amp MRBF blew and nothing else did tells me the fuse did exactly what it was supposed to do. Since everything else works and only two low-current circuits now seem to have their constant and switched power reversed, I don't think you've got a major wiring problem. I think you've got a backfeed through one of those two circuits, most likely a relay, diode, shared ground, or a splice that ties them together.

If it were mine, I'd unplug those two devices and pull their relays one at a time until the problem disappears. I'd also use a test light, not just a multimeter, to check the switched circuit. A meter will show voltage through a tiny backfeed that can't actually power anything. I think you'll find one relay, diode, or shared connection that's feeding one circuit into the other.

Give that a look and let me know your findings. I'm happy to help continue to troubleshoot if needed.

expert reply by: Jesse M

1

Spiro

8/6/2026

Hi Jesse regarding the issue I have narrowed it down to one possible circuit ,because it operates on both switched and const. battery power.The other circuits seem to only operate on either switched or const. powers,but jump wired the other circuits separate from the reversed power source and they operate as normal,I.e. circuit that needs switched ,and const. alone were o.k. If for some reason I can't locate the faulty power source(s) or if it is too hard to remove .I thought of running 2 new power lines a switched and a const. battery power from a known good power supply. If left alone and not fix this (but I will) what is the worst case scenario? Thanks again.

Jesse M.

8/6/2026

@Spiro It definitely sounds like you've isolated it to one circuit, which is good progress. I wouldn't run new power feeds until you find what's causing the backfeed. That would get the accessories working, but it leaves the original fault in the car, and that fault can cause some really strange electrical issues down the road. If it were mine, I'd keep tracing that one circuit. You're close, and I think you'll find a failed relay, diode, or a damaged wire where the short occurred.

Spiro

8/6/2026

@Spiro Hi Jesse Thanks for getting back to me very quick. So you still suggest tracing the faulty power sources even if I completely disconnect the faulty lines from the subsystems,and as I stated earlier to just run 2 new power lines from a known good source?

Jesse M.

8/6/2026

@Spiro Yes, I still recommend tracking down the original fault. Running new power wires will get those accessories working, but it won't eliminate the backfeed that's still somewhere in the harness.

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